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Old 12-10-2020, 08:31 AM
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charlie66 charlie66 is offline
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.0025 Is way too tight for a 3.25" main bearing. Even with a 3" main i wouldnt do it. especially for a boosted engine. I use to get caught up in the perfect number for crearances, but over the years of trying different numbers i realized that for the mains between .003" - .0045" you will have no issues.. One summer i broke 3 crankshafts. so each time i had new one made to fit my block it had different clearance numbers because the bearings are never consistent. This use to drive me crazy. So id call my crank grinder and try to have him grind it to get an extra .0001" off it if it was too tight . If it was loose i would get a different brand of bearing and try to fix it that way. Point is I found my self having to settle for a clearnce i didnt want and it never really made a critical difference as long as i stayed over .003" and under .0045" for the mains. I even payed close attention with the oil pressure at idle when hot . I thought it would be alot lower at .0045" then .003" But it turns out that it really wasnt.

Good luck with your build...

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Old 12-10-2020, 09:26 PM
AIR RAM AIR RAM is offline
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.0025 Is way too tight for a 3.25" main bearing. Even with a 3" main i wouldnt do it. especially for a boosted engine. I use to get caught up in the perfect number for crearances, but over the years of trying different numbers i realized that for the mains between .003" - .0045" you will have no issues.. One summer i broke 3 crankshafts. so each time i had new one made to fit my block it had different clearance numbers because the bearings are never consistent. This use to drive me crazy. So id call my crank grinder and try to have him grind it to get an extra .0001" off it if it was too tight . If it was loose i would get a different brand of bearing and try to fix it that way. Point is I found my self having to settle for a clearnce i didnt want and it never really made a critical difference as long as i stayed over .003" and under .0045" for the mains. I even payed close attention with the oil pressure at idle when hot . I thought it would be alot lower at .0045" then .003" But it turns out that it really wasnt.

Good luck with your build...
Ok... I'm all ears on this. In your opinion, what would be the down side to running a .0045 on a daily driver if any? What would be the best oil pressure with such clearances? It just sounds awfully loose to me... Would I need to prime the oil system every time i start the engine?

Thanks for the input.

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  #23  
Old 12-11-2020, 08:19 AM
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Ok... I'm all ears on this. In your opinion, what would be the down side to running a .0045 on a daily driver if any? What would be the best oil pressure with such clearances? It just sounds awfully loose to me... Would I need to prime the oil system every time i start the engine?

Thanks for the input.

SPEED SAFE, NICK
I dont think there is a down side to it. No you would not have to prime the engine every time you start it .
If you think about it and put it into perspective, a human hair mearsures .004" . Thats not very big at all. Now split that hair in half and its .002" right. So when the crank journal is floating in the bearing shell the space between the two is half of the thickness of that hair. Thats hardly any kind of big space at all. The oil is the only thing keeping the two surfaces from grinding together . Thats why the big horsepower engines need lager clearances .. Just for example , My engine now that i drive all over the place is a street car. It makes over 4hp per cubic inch. This is one of those times where i settled for a clearnce that i wasnt happy with just so you know. The front #1 main clearance is .0042" #2 .0032" # 3 .0032" #4 .0035 and #5 .0045. I would of been happy if the front was .0032" but after checking it like 10 times and it stayed the same i just left it. That night i didnt sleep that well . Was thinking that maybe i shouldnt of left it like that . But the next day i continued putting it together. Long story short when i got it running and broke it in it wasnt any different then it ever had been. Oil pressure was 70-80lbs at idle cold and hot was 35-40lbs . So my main point about this is if i knew all of this when i was building it i never would have lost sleep over it . lol Oh and this is a 3" main journal crank..

A bigger journal needs more clearance . So think of it this way. In your case with my clearances it would be in a more tighter relationship with the manufacturers bearings recommendation..

So you're good if you ask me man..

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Old 12-11-2020, 08:44 AM
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The old rule of thumb was .001" clearance per inch of journal diameter, so in your case a minimum of .00325" . I've run .003" -.0035" clearances on a n/a factory cast crank/block combo to 7000rpm, and .0035" clearances on my Scat 3.00" journal crank/factory block turbo combo to 7000rpm , bearings always looked good on strip down. Cold oil pressure was always around 80-100, hot it drops down to 20-30 at idle, Valvo VR1 20/50. Besides the extra clearance allowing more oil to pass through the bearings keeping them cool, it also allows a margin of safety when the crank flexes (and they do) at high power/rpm, so that the crank doesn't nip the bearing. JMO

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Old 12-11-2020, 11:19 AM
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I have under .003 main clearances and only 60 lbs oil pressure but Iím under 7,000 rpm .......no problems!


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  #26  
Old 12-11-2020, 04:48 PM
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Yeah when i take it out for the first 10 minutes of driving if not careful with the throttle the pressure will exceed 100 psi it will go to 110 or so. So im always weary of that because of the polymer dist gear. I might go bronze for that reason. Not sure yet..

I just got back from a run in the car . Made a full pass and the oil pressure was 90 psi . This build has all summer of real hard passes on it. So, yeah .0045" clearances on my mains no problem with the oil pressure..

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Old 12-11-2020, 07:52 PM
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Thanks for all the input, I will look into setting the mains for .0035-.0040...

With that said, is .0025 tight for the Rods? I'm looking to target 6,000-6,500RPM... I really dont know if I will attempt more RPM from it even though I really think the cam will easily do. Again, the goal is to be able to do 500+ miles a day towing a small trailer, run 9.xx down the 1320, pack up and drive another 500+ miles... To me, surviving will be considered as winning... I am not looking for any trophy's, just enjoying the experience with a reliable "Street driven Pontiac".

Thank you again for the constructive feedback/tips.

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  #28  
Old 12-11-2020, 07:53 PM
AIR RAM AIR RAM is offline
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Yeah when i take it out for the first 10 minutes of driving if not careful with the throttle the pressure will exceed 100 psi it will go to 110 or so. So im always weary of that because of the polymer dist gear. I might go bronze for that reason. Not sure yet..

I just got back from a run in the car . Made a full pass and the oil pressure was 90 psi . This build has all summer of real hard passes on it. So, yeah .0045" clearances on my mains no problem with the oil pressure..
This is all good information... thanks for the reassurance.

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  #29  
Old 12-11-2020, 08:31 PM
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This is all good information... thanks for the reassurance.

SPEED SAFE, NICK
No problem man..

I think you're on the right track now..

.0025" is what i always shoot for. Its not always what i windup with though unless i take my rods with the bearing shells torked in and my crank down to my crank grinder and have him cut the crank to fit the clearance i want. So for me , No tighter then .002" and no looser then .003" on a 2.200" journal. Im not saying that you can't go looser but thats where i draw the line..

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Old 12-11-2020, 09:38 PM
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I would shoot for .0025-.003 max on steel rods.

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  #31  
Old 04-06-2021, 07:28 PM
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I would still check deck height on whatever aftermarket block even after squaring up. When I swapped mine from a MR1 to an IA my pistons were 0.010 out and had to get a thicker head gasket.

Most aftermarket blocks even Chevy etc there is going to be some work involved. Casting flashing to remove(check cam tunnels) possibly coolant holes to drill. Mike cam bearing holes too.
Just wanted to confirm a similar encounter.

My target CR is 9:1cr. Since I am running a 72cc head, I had to go with a 40cc dish piston and this was to be used with a .041 gasket. This combined with setting my pistons .020 in the hole was supposed to put me in the 9:1cr ball park. I intended to fine tune with block decking as necessary.

A monkey wrench was thrown into it after Luhn Perf talked me into purchasing their 5 layer head gaskets for boost. Now this will drop my CR below 9:1 and I did not want to go below.

Since the block does need to be decked, I thought I could just deck the block and get some CR back. So I tossed the crank, piston and rod into the block to check and see what I had... well lets just say to my surprise, the deck is .010-.011 shorter than expected.

This is not such horrible news to me, it actually works out in my favor... however, If I had not checked and thought I had an advertised 10.25 deck height and had the machine shop take off .015 to put me .005 in the hole... that would have made me peaking out of the top about .05....

The moral of the story... no matter how new your parts are, verify the dimensions... this could have gotten ugly had I not verify before cutting.

SPEED SAFE, NICK

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  #32  
Old 04-14-2021, 11:29 AM
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Measure twice cut once! lol.

Just to throw it out there, I had my motor set up for 8.75 to 1 and later found out that it was really 8.4 to 1. It always ran great and made real good power. Then further down the line I changed the pistons to flat tops to raise the ratio up . Got it to 9.5 to 1 and it didn't give any better response down low. It did make more power with less boost for sure though.. I myself like the lower ratio 100%.. Just throwing out my real world experience because I know how these kind of things really make for sleepless nights trying to make the right move on things.. And also , the higher I had the ratio the more tuning challenges I had .. I'm building a new twin build and im shooting for 8.75 to1. If it goes lower I wont even care..

Good luck ....

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Old 04-14-2021, 09:30 PM
AIR RAM AIR RAM is offline
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Measure twice cut once! lol.

Just to throw it out there, I had my motor set up for 8.75 to 1 and later found out that it was really 8.4 to 1. It always ran great and made real good power. Then further down the line I changed the pistons to flat tops to raise the ratio up . Got it to 9.5 to 1 and it didn't give any better response down low. It did make more power with less boost for sure though.. I myself like the lower ratio 100%.. Just throwing out my real world experience because I know how these kind of things really make for sleepless nights trying to make the right move on things.. And also , the higher I had the ratio the more tuning challenges I had .. I'm building a new twin build and im shooting for 8.75 to1. If it goes lower I wont even care..

Good luck ....
Yeah, this is going to be regularly street driven on pump gas(Street) / e-85 (Track). I initially chose 9:1cr as the target, however to achieve the 9:1cr part of that equation was to sit the pistons .020 in the hole. The new head gaskets change all that.

My fear is having to much quench. With the 72cc heads, 40cc dish, .041 head gasket that was going to land me around 9:1cr... but with a .061 quench.

By stepping up to the thicker head gasket (.060 - just measured them) , that would have put my quench in the .080... making 8.8cr. The high quench being the bigger issue for me.

So, I was going to have to deck the block anyway to bring things down to a more efficient quench. So now our target is .005 in the hole which still puts it at .065 quench (Still high) and right at 9.01cr.

I'm not really sure how this is going to effect the engine... Some say the quench is not so important with boosted engines... Some say any more than .060 can lead to slow combustion and ultimately detonation. All of which I would like to avoid.

However, with a 40cc dish pistons... the whole "quench" thought may be irrelevant.

So it really is a pleasant surprise that the block was already .010 +/- .005 shorter than advertised. Just need to remove .005 to clean it up.

Good times.

SPEED SAFE, NICK

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Last edited by AIR RAM; 04-14-2021 at 09:42 PM.
  #34  
Old 04-15-2021, 03:40 PM
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What does the top of the piston look like . If there's no quench pad then it wont matter.. Sounds like a lot of dish, so unless its a deep dish 40cc's spread out could be quite big taking up area for the pad.... at 65 I think you would be ok .. Jmo..

I'm sure you will figure it out. Good luck...

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Old Yesterday, 12:43 PM
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If its not a problem post a picture of the top of the piston. I'm curious what it looks like .. I never seen a 40cc dish..

Thanks

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Old Yesterday, 04:18 PM
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If its not a problem post a picture of the top of the piston. I'm curious what it looks like .. I never seen a 40cc dish..

Thanks
They are prerdy!!! lol

The more I think about it... I honestly dont feel I need to worry about quench at all... With the dish... there is no quench! LOL (Just more room for boost)

So many small factors to take into consideration... its easy to get caught up on things that may or may not have an effect on the performance of an engine.
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Last edited by AIR RAM; Yesterday at 04:25 PM.
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