Block prep for 350P turbo build?
 

Hi,

Did a search, couldn't find apples to apples info.

I am going to take my block down to the machine shop to get cleaned and bored. I wanted to know if there is anything substantially different about a turbo'd build that would pertain to the machine work, like looser clearances, etc.

It's a std bore '72 350, getting punched out 040, with stock N crank.

I have Probe SRS forged pistons, and 4130 Tomahawk rods. I think these will be fine for the job based on my hp goals.

My goal is for around 500 hp max on mild boost (maybe 10-12psi) from a single turbo. Nothin' crazy. I plan on building a stout 300-350hp NA engine first, and then toss on the hairdryer. I've only ever played in the world of mild street motors.

Anything weird I need to worry about, like extra cylinder wall clearances beyond what the piston manufaturer recommends?
What about ring gaps compared to normal?
Would hard-block filler be necessary?
Any special fasteners?
Additional input welcome.

I don't plan on spinning the motor over 5500. I am hoping I don't have to get too weird beyond standard performance engine building practices.

Thanks!

Here's what I would do, for whatever that's worth.

Give the block a good deburr. No need for anything terribly fancy, just clean up any flash and sharp stuff that could be a starting point for cracks, especially around the main webs.

I would give the block an extra little bit of piston to wall clearance. Call the piston mfg, and tell them what you're doing with it. A lot depends on what alloy the pistons are made of.

The piston ring end gaps need to be a little bit looser than a naturally aspirated engine. How much looser depends on your ring choice. What's the bore size? What piston ring pack is the piston set up for?

At this power level, bearing clearances should be close to whatever you would run with a NA engine.

I wouldn't worry too much about a block fill.

Good fasteners are always a good investment. I prefer head studs, regardless of power level, but if you can't swing that, at least use a good head bolt and main bolt.

500 hp should be no problem at all. At 10-12 psi, I would not be surprised at all with 600 hp or more.

Thanks, Travis.

I want to use the stock windage tray. ARP makes main studs, with 200K yield strength, but no worky with windage tray. Milodon makes studs rated for 190k, but says it works with windage tray. Any big deal in the 10k difference?

I'll have to call Probe. They included specs that got looser as one stepped up from their street level pistons to their race pistons, but never had any setup differences for boost.

Concerning rings: Anything to stay away from? I've read in some threads where some people said "Stay away from moly", but I'm not sure if they were playing in a "bigger sandbox" than I will be playing in.

Thanks again!

I checked with Probe's website and found some ring gap info. It had a formula for gap based on bore size. They also suggested adding .005 to those numbers for boosted apps.

Suggested piston clearances was .0025 minimum to .0065 max. They said minimum was OK for NO street motor, but go looser for boosted or race apps.

I guess I have to figure out how loose to go. .0040, maybe?

I wouldn't be at all afraid of the Milodon studs, I have not used them, but they are bound to be a great improvement over a stock bolt.

I would think .004-.005 would work just fine on piston clearance.

I have used a moly ring in some pretty high power stuff. I really wouldn't be afraid of it at all; a fair bit of the negative stuff you hear about moly comes from the early versions of the rings that had some issues. Then it gets repeated/regurgitated forever. Is it a 1/16" ring or a 5/64" ring? You definitely want to stay away from the stainless and tool steel rings, they are pretty hard on the cylinder bores.

I'm no engine builder, but hope this helps anyway.

Squid,the moly ring talk you mention is most likely in reference to the nitrous guys.

Anything over say 175 to 200 hp shot and it's not @ all uncommon for them to find it'll knock the moly coating off the face of the ring,and then you tend to lose ring seal,and oil + nitrous = very bad things.

Boost & nitrous do indeed share some traits,but from my understanding that is'nt one of them,nitrous applications often have a very high/quick initial "shock" to the engine in terms of cylinder pressure,and that can be hard on things like moly rings & headgaskets and such,where-as boost tends to be applied in a more "linear" fashion.

Though like a nitrous engine,I'd probably still shy away from low tension oil rings for an application like this.

One other thing I'd mention here is this,if you plan to use the Cometic headgaskets,be sure the machinist understands and can supply a proper RA finish to the decks & heads for those gaskets,as they have some specific recommendations for those MLS gaskets.

Good luck with this!

Bret P.

I wouldnt run the factory windage tray... they are prone to cracking. Under 6000 rpm I dont think I'd bother... but a higher volume pan would take priority... Canton has some nice pans and sandwich mount windage trays...

Careful on your cam pick... you dont need much. It's not uncommon for tq to peak @ 500 rpm sooner and hp to extend @ 800-1000 rpm later over NA. Not optimum but an 068 works ok on a 400 with a turbo but an 067 might be better on a smaller motor

A custom grind would be far better. Travis can set you right with that better than I.

I measured the ring lands, and came up with something like 1/16, 1/16, and 3/16.

I talked with the shop, and they were leaning toward .005-.006, but I told them that I didn't want to go straight for the upper end of the recommended clearance. I wanted .004-.005.

Does anyone have a brand and product line they would recommend?

And Bret - I haven't yet thought about head gaskets. I know that 350 options are limited, so I guess I have to look into that. I appreciate the heads up since I would hate to haul an engine back to the shop for putting the right finish on the deck.

As of now,the only decent/functional 350 headgasket choices are the Cometics,and the Innovative Seal (formerly ROL IIRC).

There have been some discussion about that in the street forum not too long ago,a search should bring up those threads.

All the other headgaskets are made for the larger bore engines,and thus have too much "dead space" around the top of the bores.

One of the main reasons some of us are looking at the 350 for a boost appl. is because the smaller bores should allow for slightly more headgasket material between the bores which hopefully may add some % of strength to a known weak spot in the larger bore applications,this would be especially important with regards to the center cylinders with the siamesed exhaust ports (3-5 & 4-6) as that's where pontiac headgaskets are most likely to fail.

HTH

Bret P.

OK, thanks. I looked at the Cometic. Just over a bill - definitely not your run of the mill "Mr. Gasket" stuff.;)

I've searched for Total Seal rings, with 3.915 bore and 1/16-1/16-3/16 rings. Found a couple of places. Is it better to get something a tiny bit bigger and file back to the proper gap, or does ordering the exact size usually get ya in the ballpark with just a little bit of tweaking needed?

Sorry about the late reply...

Yeah,cometics aint cheap,but they are very good gaskets,you pay for what you get & vice versa.

Anyhow,on to the main question.

I dunno if I'd say it's "better" per-se,but yeah some ring sets are sold .005"-.010" over size to allow for "file fitting" to the given bore size allowing more precise final gap spec.

IIRC these end up being the same size rings that a 283/307 SBC uses,so they come in a good variety of oversizes and such.

HTH

Bret P.

I plan to also build a turbo 350 Pontiac after I find a good core block. clean it, block fill it, stud it. I would like to get between 750 to 850 hp from it.

SC, I looked into the sbc stuff. Most of the 283/307 stuff has 5/64 compression rings. But total seal makes 3.915 std and file fit versions with the 1/16, 1/16, and 3/16 setup. My machine shop says he found them locally (one of the few benefits of the big city). Gonna go with TS rings and cometics.

Good news... crank was in great shape. He said it was easily at standard, and should be good with a micro polishing. Not bad for spending a month or more submerged in rainwater! Prev owner let it sit with valley cover off out in his yard, and AZ DOES get some rain.;)

I wish you the best. It was a little challenging finding forged pistons. I think I got lucky. Head gaskets are also a bit dicey. It seems like the aftermarket parts people pulled the plug on 350P's shortly after all the hobbyists pulled the plug on wanting to build them. Your goals are more aggressive than mine. I am shooting for a smooth streetable 500 or so with my foot in it.

I want to start conservatively on the boost and power levels. I am sure I will find a weak link farther down the road, either with tranny (TH350) or rear end (8.2). But I want to make sure that I build a strong enough motor to keep me content as I step it up. I currently have a 72 block, 40 over, forged pistons, forged rods, stock N crank. I will be using ARP main studs. I want to do a little bit of "weird factor", especially if I can turn in some good timeslips at a Pontiac Heaven event with my redheaded stepchild motor.;)

Squid. I'm in Glendale. Who did you find to do your engine work? l had my engine built by Paul down in Tucson but I would like someone closer to take the little stuff to.

Mark L

Baileys in mesa. Bob's a good guy, been doing it forever. He's never done me wrong, but I've never done a build before at this level. We'll see how their work holds together!

Actually, I just got back from there. They were checking the balance with my probe pistons, tomahawk rods, and my stock 72 350 crank. I guess it was way out by somewhere around 200 to 300 grams? Anyways, he had to make some calls. We were in the classic discussion of what exactly is internal, external, and neutral balanced on a Pontiac assembly. So... balancer is neutral, flywheel is not? The flywheel has extra holes on one side, so I would assume that the flywheel has an external balance?

Sorry. Some of this stuff may be basic to some of the key players in this forum, but I am in newer territory here.

Yes Pontiac used a externally balanced rotating assembly. They put some into the flex plate/ flywheel. Whenever I built one of my Pontiacs, I would internally balance it. Then I used zero balance flywheels or flex plates. Not a big thing but it's easier to balance the motor and change parts later.

Mark

Sound like you're going to have some fun!

Was wondering what size turbo you're going to use, mm/AR ?

Are you going blow through or injection.

I bet 500 hp for you will be under 10 psi.

My 4 cyl made over 500hp @ 15 psi.

Good luck and I hope you keep us posted on you progress....

I got the call today everything is done. Rotating assembly balanced nicely. It turns out that there was a balancing equipment setup error that was discovered when they went back and rechecked again why it was so far off. After that they said it balanced pretty easy. Check twice, cut once...;)

I had them balance it with the factory flexplate, so there is going to be some external balance involved, I guess.

Going to go blow-thru carb. So far I am thinking about 70mm, maybe about a .8 AR to get some quicker spooling at the expense of some top-end ceiling. But I have more research on that stuff. I still have to figure out cam. I want a decent mannered cam that will still function well with the boost when I get on it. I might have to call someone and get a custom grind since this won't be an all-out boost engine.

4 cyl? Half a V8? I like that. That definitely takes the weird factor a step further!:D

Do you have a thread on your build details?

Yeah. Heres a pic for ya.

Weird? Not to me anymore. But to most everyone else i meet it is. Lol..

I do have a thread going on it.

Hey, I want to say, if you need any help with figuring this build of yours out , I'd be glad to help. I have gone through a big learning curve with tuning from 5 psi all the way to 25 psi.

So feel free to PM me if you want just in case you get stuck on something..

God knows iv had some real head scratchers on this stuff...

Good luck...

Yep,they make various widths for that given bore size,but most all of them are still listed under the SBC application "umbrella",as the piston bore diameter mostly only fits those SBC engines,and as a random occurence it also happens to fit our "small bore" pontiac 350 as well,so yeah everyone knows which engine they're going to list those rings for then,right? :rolleyes:

The thicker rings represent the OE ring widths (say OE cast pistons),the thinner rings represent the aftermarket ring widths (say aftermarket forged pistons).

I dont look for rings under a given application,I look for them by bore size mostly.

What they were originally used for is mostly unimportant to me,that's more of a footnote than anything else.

Anyhow,I'm glad they got the balancing deal all worked out for you.

Good luck with this project!

:cool:

Bret P.

It was pretty cool to learn of the sbc stuff. When I started looking it up, I started seeing some info about custom setups, rod lengths, deck heights, and coming up with various off-the-wall rod/piston/stroke combos. I did some number crunches trying to devise some combos of off the shelf pistons and rods that might work. It was some fun mental exercise. No magic formula yet, but it was great learning.

It will give me some ammo for the planning of my next 350 (I still have a couple to play with). I will make that one a mid level street motor, maybe a 69 H.O.-ish motor, so no need for the more expensive goodies that I am throwing at my current project.

Motor is ready for pickup, so I go tomorrow to check it out. I still have some pieces to pick up for the bottom end, like some main studs and a BOP rear seal.

Out of curiosity, with the cometics gasket it needs a certain RA finish. Any idea of how to achieve that? Should a machinist just know off hand how to "finish it for an RA of xx?"

Thanks again!

I would hope most machinists would know how to achieve such,but I guess that's really only an answer they can give,what I say would pretty much be meaningless.

I could see that certain machinery could be needed,or a different methodology used for a given type of machinery,I dunno really,that's one of those things we often have to take with a bit of blind faith.

It's much the same as what one faces with regards to bore honing for a given type or ring material,as RA is very important in regards to bore finish as well.

Yeah,I'm still tracking down a 350 block for my stroker 350/383 build,and have'nt really decided which way to go with it yet (NA/Nitrous/Boost),so I hear ya' on some of this stuff,I'm just glad I got a set of those forged stroker pistons from Ace's garage/GooB sale when I did,as that really set the direction for my "small bore" project.

That's why a lot of this info is fresh in my mind these days,LOL,but I'm still a ways off right now,especially being winter right now.

And I'm not 100% sure if my next build will be the 350/383 stroker -or- a fresh 460/EFI for my '88 F350 DRW tow vehicle?

Sometimes decisions get made for us,,,not by us... :o

Bret P.

Blind faith... that's one of those things I struggle with when it comes to outsourcing my work. The cynic in me keeps me awake at night wondering if I'm getting some kind of mushroom treatment.;)

And the decision making part? That line's almost good enough to get sig'd! The only part missing is how often we fool ourselves into thinking we do the latter instead of the former!:D

I have a shuffle of engines to play with. I technically have more engines than space!

As always, thanks for the advice. I will have that conversation with the machine shop this week when I check out/pick up the motor.

I picked up the block/crank last week. When I checked the pistons/rods, the rods were hung backwards, which would have put the rod bevels facing toward each other instead of toward the crank cheek. I pointed this out, and the machinist agreed. Simple enough mistake, easy fix, no big deal.

I went yesterday to pick up the pistons/rods, and all seemed well until I got home. I went to test fit the pistons to the block, and pistons 2-8 fit just fine. Piston #1 wouldn't go in hole #1, or any hole for that matter. It got hung up about 1/2" from the bottom of the skirt when stuck in the cylinder head first. Closer inspection looks like the piston deformed slightly around the skirt when the pin was pressed.

I called the shop, and they want me to bring them back in. When my schedule allows I will.

My question: What are the possible corrective actions? I'm guessing that a) piston could be reformed through careful massaging, b) some material could be buffed off the skirt area that bulged out, or c) nothing, piston is trashed.

These pistons were a bit of a rarity - forged 350 pistons aren't an off the shelf item. I considered myself lucky to find them. I called Probe, and they said they only do runs on special order of 200+ units.

I am at the stage of "expect the worst, hope for the best". Worst case? I change build direction and go with Sealed Power or KB cast 3.915 pistons, and I might not boost this motor. I don't have the experience to know if cast pistons will stand up to my boosted intentions. If it comes to that, then the shop will have to buy me a new set of pistons, return my Total Seal rings for a set that fits the new pistons, and rebalance the rotating assembly.

I've been experiencing a range of emotions since yesterday, and it's probably a good thing that I don't have the schedule availability to go down there at least until Friday afternoon. I'll have time to get some advice, and calm down to handle things in the most rational manner.

Thanks for any advice, and for abiding my vent.

Squidward, I din't read every one of the posts , but, & I'm not sure if you can do this. But have you considered O-ringing the block to help in keeping it sealed up. If you use the cometic head gaskets & use the o-ringed block will that be enough to keep it sealed under the pressures while you put the boost to it???? I don't get into the higher end stuff for the all out racing but it seems that o-ringing the block would be a big help to keep it sealed. Again I don't get into the high end stuff so I don't know if that o-ringing the block is even an option..

Squidward, I just read your last post !! WOW, first don't even consider using cast pistons for anything, I wouldn't even use them for a stock rebuild anymore because of the junk gas we have now. A cast piston won't even stand up to some harsh street use any more. So fro that I can say without a doubt ,,, DON'T USE ANY CAST PISTONS EVER !!!!!!!

Pics of the warpage. Note the wavy edge line that is straight at the other 3 "corners".

Rex, oringing the block won't be an option. If I can't make things seal with some top quality head gaskets then it wasn't meant to be.

Cast pistons on a boost build won't be an option either, unless I do some sort of "token" turbo setup of just a few pounds so I can say I joined the boost club. But turbos are an expensive option. For a very low power adder option I'd be money ahead to get a 50-100 shot of nitrous and call it done.

Bret, when I get the chance Monday I will give some phone calls to some of the manufacturers you listed and pursue some die hard availability for replacements. That will assist in the negotiations/remedies.

I hope the two of you can heal your bad blood. We are all on the same team here, and I think for the most part we try to add our best. I appreciate all of the inputs, whether they demonstrate heartfelt enthusiasm, partially related personal experiences, or laser precise technical info.

Just so you know if you are running factory two bolt main caps and you remove the factory bolts and replace the bolts with studs you are going to increase the cap walk exponentially once under boost. At 10 lbs of boost the cap will start to fret to the block.

Really? Please elaborate. Are you saying I should retain factory bolts? Or something else?

Rexs73gto,

Did I say something to offend you? Please elaborate or explain your attack or who the attack is directed too.

I do not understand. If I made a typo or created a grammatical error again I am sorry.

This gentleman is having problems and I thought I would share our experience and knowledge working directly with the factory two bolt main blocks.

Squidward,

The main bolts that were designed by the factory engineers have a shoulder under the hex head of the fastener. This diameter creates a tighter fit at the top of the main cap compared to the diameter of the main stud.

A couple other items that you will need to keep in mind are the engine rpm and the type of oil pump pickup you use, the oil temperature (oil cooler is a must) and your bearing clearances.

Depending on the camshaft profile and type and the related spring pressures you might consider our grooved cam bearings. They supply oil in three spots compared to the factory's single oil hole.

Again my apologize to the OP or anyone else we are only trying to provide real world experience with the two bolt main blocks and boost.

Rexs73gto,

Please except my apology I made the mistake of not reading the couple of long posts earlier in the thread.

I see that the post was not at least to my understanding directed at us.

I do hope you and SC can air out your differences and keep this subject moving forward.

That is all.

If it helps I believe we are located between the two of you I can set out the beers and try to defrost the grill in the back yard and make burgers. It's going to be 26 F for a couple hours today...LOL

Thanks, Tom. Yup, shouldered bolt. So are you saying that factory main bolt is superior to the aftermarket studs? I am in a weird place now...:eek:;

I will be spinning maybe to 5500, Idk yet. I planned on doing an 80lb Melling style pump. Cam is yet to be determined. I am looking for a grind that is not obnoxious when out of the pedal. I also don't want to go with a pure turbo cam because I want to be able to drive the car naturally aspirated until I get to the turbo bolt on phase. I was thinking about a single pattern grind with minimal overlap, but didn't want a reverse dual pattern like some stuff I've read. Maybe you have some insight?

Are there any 283/307 SBC pistons or other brands with bores close to this range that would come close in pin height? LS motors? A smaller pin size could work with a bushed rod end.
Dependin on cam and valve diameter/chamber depth, the valve notching may/may not need to be changed/corrected.
Could take a rod length change or some head/deck milling, but maybe not?Turbo motors dont neccessarily need to run at zero deck... many high end turbo builds run significantly below... May/may not be a can of worms goin this route but I figured I would throw this option out here for discussion.

(When I first got my 59 motor it had ancient set of JE domed pistons with NO notches and the little 1.88/160 valves cleared just fine with the old Pontiac/Isky E2 solid.Later checked with a 286 dur .48x lift Engle hydraulic which cleared but very close.)

FWIW seen a few turbo builds on YB where guys ran hypereutectic(not in my spellcheck) cast pistons at moderate boost levels and used good combustion control techniques like water/meth and intercoolers to keep charge temps in check...

Hypers (properly installed and clearanced) are certainy a way better choice than say a Badger stock replacement. Real Pontiac castings are sturdier than std cast replacements from what I've seen... not sure what they'd handle for boost. I've rattled stockers hard(na) more than a few times and hurt other components first.(maybe not hard enough?)

BTW the material forged pistons are made from matters as well. 2618 is better suited to boost than 4032. That said I've seen some boosted builds that lived just fine with the 4032 material. MUCH depends on prep and actual stresses(heat/rpm) you put them to. AFAIK TRW std replacements were 4032 and the domed race pistons were 2618 and I'm not sure if that carried over with the SpeedPro brand.(think so)

Does Pontiac Dude still offer custom forged pistons from BRC?

I wouldnt bother with a custom grind for what you want. At least not till you have it up and drivng and sorted through.

Fwiw I know of one turbo build (400) that worked well with an 068 cam.(Lookup Firechicken's posts) PontiacJeff has kind words for the std performance factory U cam with a turbo (ran it in a 400 in his Suburban). The 068 might be a bit to soft for a 350 when just cruisin, but Firechickens bird(79 or 80 iirc) weighed over 4000 lbs had all its accessories and he never complained of poor street manners. Car ran into the 10's.

Gross overlap vs actual overlap window the motor sees... numbers can be misleading.

With a turbo a stock type cam tends to extend power peak rpm 800-1000 rpm and lower pk tq rpm about 500 or so.

Better than stock valve springs are more important for what you plan to do. You definately need more seat pressure than stock to account for the extra pounds per square inch above atmosphere being applied to the valve.

With a factory type cam I'd probably uses a Crower '404 or '405 and add seat pressure.

Curious: Why are there two listings for pistons(68-74)(74-77) for the 350? What changed?

In the October 2005 issue of HPP magazine there was a story about using 318 Mopar pistons in the 350. These are some pictures of the story.

JC455, I wondered that myself.

Meanone, thanks for the article. I have been doing some searching for some other brand motor components. I am looking at SBC and some other stuff, and playing around with various compression heights and rod lengths. I haven't found the magic one yet.

I'll be making some phone calls Monday to see if I can get a forged set from one of piston manufacturers for a reasonable price. I will also be taking my pistons/rods down to the shop to see what we will have to work out. Wish me luck.

Without seeing them, and a guess on my part, I'd say the piston top outer edges are chamfered to lower the cr more.

The factory ones were like that I believe.

:)

No problem Squidward. I remembered seeing that article and thought it would be an option. Good luck with your search.

Update...

I talked to the shop today. I had dropped off the pistons/rods two weeks ago for them to figure out how to make this right. The shop owner is getting me a new set of slugs from Venolia. It sounds like he is going to take care of this properly. He actually sent a piston and pin to Venolia and said "do a set of these for my guy", with approriate specs (bore, ring sizes, etc.).

Now... The crank was already balanced using the previous rod/piston setup. How many grams of difference between old and new pistons would require a rebalance?

His error caused the piston swap so IMO that creates the need to rebalance if needed... at his cost not yours. One or two grams is livable 1/2 gram very good... a performance engine.... as close to perfect as you can get applies to everything.

You have a nice project going here... try to keep your compromises to a minimum... I'll chose durability/longevity almost always. Dont always need the very strongest part but you want at least a bit more than you need to get by. You are asking questions and researching... thats the best "expense" to invest in. Hope we get you the best answers.

Thanks, Bruce. We had the balance discussion yesterday. Since the same rods will be used, we are hoping the new pistons will arrive at comparable weights to the "old" ones that were used during balancing. We'll have to see how things go over the next week or two once the slugs arrive.

If the new pistons are lighter than the old ones I wouldn't worry about it(in fact I would be happy about it). This would make your rotating assembly overbalanced,in other words the crank weight is slightly more than the piston/pin/rings/etc bobweight. It's beneficial for a boosted engine(and high rpm engine) and I've built my last two engines with an overbalance of 2-3% with good results.

Thanks, Taff, I'll keep ya posted. Is it because boost pressure creates some sort artificial effective weight, acting like the pistons/rods are heavier? Curious...