[tonyk]

Its been a few years since I've done a motor, so Im paying for it now. I didn't ask the machinist to blueprint the rods/crank journal dimensions and now Im paying for it. I Need a little help with the dimension I see on my rod clearances on a 1968 pontiac 400 and Im wondering what my options are.

Crank turned 010/010
Bearings are +.010
Trouble is I had the rods resized and my oil clearance is measured at .003. This is a street engine .060 over, 2 bolt mains, Comp XR276HR, nothing fancy. Maybe some spirited driving once in a while.

Wouldn’t it be best to see .0020 or 0025 on the rod clearances?
I asked the machinist but he seemed to have a problem with resizing them to my preference since his Mahle manual says .030 is suitable. (He said its hard to just breathe on them).

So what are my options:
Can I get .020 over bearings and replace 1 of the 2 (1 @.010, 1 @ .020)??
Should I sand the rod cap mating surface with 600 grit on a piece of glass?
Should I do nothing? Am I just splitting atoms?

Any help is appreciated

[HWYSTR455]

King bearings make +.005 bearings, but honestly, resizing the rods are a better option.

Not sure what manual he's looking at but that would be a red flag for me. That would make me question everything that shop did.

My opinion is 30 is too big, and would shoot for 25. Some would say tighter than 25, but it depends who you ask. All the Pontiac-specific builders shoot for @ 25, so...

Have to ask though, how are you measuring?

Don't sand the rod cap, it is not an acceptable method to clearance rods.

Another 'have to' is to comment on the cam, that profile is not a favorable profile, and very few if any here would recommend it.


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[Schurkey]

Originally Posted by tonyk

Crank turned 010/010
Bearings are +.010

Your bearings are almost certainly -.010, not +.010.

"Ten Under" bearings, not "Ten Over".

Originally Posted by tonyk

I had the rods resized and my oil clearance is measured at .003.
Wouldn’t it be best to see .0020 or 0025 on the rod clearances?
Should I do nothing? Am I just splitting atoms?

HOW was the oil clearance measured? Professional measuring tools done by an experienced hand? Plastigage? Chinese micrometers, no "standards" to calibrate them with, and used by a novice?

Some folks would say .003 was preferable.

IF the real, live, genuine clearance is .003...I wouldn't worry about it.

[steve25]

I hope that when your rods where resized that a big chamfer was not made at the bearing parting line to deburr the cut on the cap and rod?

[Sirrotica]

.003 is on the loose side, so you need to keep an oil wedge in between the crank, and the bearing. This would be an occasion that an 80# oil pump would be a viable option. .003 is only a problem if you can't maintain an oil wedge, and the bearing starts riding on the journal, then the rod bearing spins

The other option is swapping .010 under rod bearings from different manufacturers to see if one companys bearings might be a tiny bit thicker to take out at least some of the clearance. This however gets expensive in a hurry as once you scratch the inside bearing surface, you pretty much own them, suggest buying just one from each available manufacturer to try.

I know it's been said that some Pontiac engine bearings at present are scarce, so finding bearings from different manufacturers may be somewhat difficult at present.

I would do whatever you need to to keep from turning the crank again, especially if it was the same machinist doing the work.

Sanding the parting line on the caps would probably be a last resort, but I have seen it done, to race engines that just needed to be tightened a tiny bit. Those engine lived fine in an oval track car for at least a couple seasons. You have to realize that the bearings, and the big ends will be slightly ovaled if you resort to this. then you have to weigh if that suits you personally, or not.

Lots of ways to cheat clearances, some people say it's wrong, you have to be onboard with the decision you make.

If you know anything about antique poured babbit engines, that the bearings were scraped by hand to the approprite sizes for the journals, sanding the parting line looks high tech. I've also seen shim stock between the back of the bearings and the block/rods, to tighten up clearances, not high tech for a slow revving industrial engine, but it worked. I'm not suggesting that for your engine, but I've deffinitely seen that method used.

[steve25]

If your dealing with the cast rods here there will be no problem with .003” rod clearance as long as the side clearance is no more then .016”, and this is even with a 60 psi oil pump.

What you need more then oil pressure in a case like this and in street usage is constant volume, so the main bearings must be half groove or better yet fully grooved .

This is assuming a non RA crank that is not cross drilled .

In a motor that is not making more then 550 Hp the load carrying ability of the fully grooved bearing suffices just fine.

[Mr Twister]

Run it. Don't over think this.. Update us later. 20w50 to start.....

[tonyk]

I appreciate the input. Yeah I thought the measuring could have been in error, too. The measurements were done with reliable bore guage and caliper and then I verified with another set to be sure. I measured with the bearings seated and torqued and again unassembled, same results. The main bearings are half groove and the pump is a melling 10542 so there should be plenty of oil.

[Gary H]

.003 is a little on the loose side for a strictly street engine IMO. I'd try a different brand bearing to see if it's a little thicker to get you closer to .002 or .0025. In my experience, Clevite and King bearings run thicker and Sealed Power and Speed Pro run thinner. Just mic'd a set of Sealed power rod bearings today for a customer's engine I'm building, and they are .0005 thinner per side than Clevites. In other words, Clevite's are .001 thicker than Sealed Power bearings.

[72projectbird]

If it were me I'd just run it. Wouldn't think twice, and would beat the bag outta it.

[mgarblik]

I have to admit I am a little confused with some of the information in your 1st post. I will just clarify what I know. The housing bores in the big end of the connecting rod need to be to specs. +- .0002" and straight and round. That size for a Pontiac rod would be 2.3745". Making the big end housing bores smaller only crushes the bearing more and makes it more out of round. Yes it will tighten up the vertical oil clearance about .0002" at the minimum tolerance of 2.3740", but this is not the best way to change oil clearance. If the housing bores are to specs, the best way to tighten oil clearance is with different bearings as mentioned. If your crank and rods had been machined to BBC journal size, they make -.009, -.010, -.011 bearings. Then you can mix and match to get what you want. Honestly, if your at .003" measured correctly, it will be just fine.

[hurryinhoosier62]

Originally Posted by Schurkey

Your bearings are almost certainly -.010, not +.010.

"Ten Under" bearings, not "Ten Over".


HOW was the oil clearance measured? Professional measuring tools done by an experienced hand? Plastigage? Chinese micrometers, no "standards" to calibrate them with, and used by a novice?

Some folks would say .003 was preferable.

IF the real, live, genuine clearance is .003...I wouldn't worry about it.

Actually, there are +.010 bearings. I have machined for and installed many +.010 OD cam bearing in two stroke Detroits; we had +.010/-.010, +.010/STD, +.010/.+010 cam bearings as well as main and rod bearings. +.010 OD cam bearings are also available for Pontiac V-8s. As far as rod bearing clearance I’ll go with what Ken Crocie told me years ago: .002/.002 for street Pontiacs. .003 wouldn’t hurt anything, though.

[Tim Corcoran]

Why not just get a +.0002 bearing and call it good.

[Dragncar]

Send you bearings out and get them coated. You can get a full .001 coating or half that if you want.
Done.
My engine came in at .003 on the mains and .003 on the rods. Coated bearing gave me .025 on the rods.
Although .003 would have been fine with what I am going to do with this engine.

[steve25]

Hey gang let’s drop the notion that a anti friction coating can be used to tighten up a bearing clearance issue, it’s not made for that purpose!

The RA4 motors had a rod bearing clearance spec range of up to .003” like you say you have, so as others have said just run it.

If you feel you’ed like some added protection then run a Qt of Lucus oil supplement, it good practice in general anyway at least during the Summer months if you do not have a oil cooler

[chiphead]

Mgarblik gave the critical answer. If the rod big-ends are out of spec, you'll have a bearing failure due to insufficient or excessive crush between the rod bore and the bearing insert, even if the bearing to journal clearance looks good. If the rod big-end bores are in spec, leave them alone. The correct way to tighten up clearance is to use tighter bearings or have the crank re-cut to a size that will give the desired clearance.

The ole Clevite catalog specified no tighter than .00075 per inch of journal in any application, and recommended .001" per inch of journal for performance applications. You usually want to go a bit looser on the rods, so the extremes of the range for a 2.225" rod journal are .0018" to .0026". .003" is a bit out of that range, but the only penalty will be slightly lower oil pressure at low RPMs hot. Rod side-clearance has just as much to do with oil pressure than the bearing clearance.

I don't suggest crutching it back with thicker oil. You have to consider ring pack, other engine clearances, ambient temp range, RPM range, oil control methods, etc when choosing an oil. It's not just about crutching the largest bearing clearance. The best way to control oil pressure is to blueprint a good pump, control ALL the clearances, use good oil control methods and keep the oil clean and cool.

[HWYSTR455]

Rod side clearance doesn't impact oil psi, just to say.

Technically, you should assemble and torque the rod on a journal, remove, torque the cap back on, then measure. That will give you the proper crush and provide an as-assembled reading.

You need a cap 'splitter' to take it back apart though.

No telling how long friction coatings last, and most won't disassemble to find out.

If the OP is concerned about the clearance, the options are still the same, resize the rod or use different bearings. It's that simple.


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[Nobuddy]

I know opinions vary but I don't think calipers are reliable to check finer than .001. Use a decent .0001 micrometer. Or just plastigage to backup your bore gage/caliper indication. If a caliper says .003 you really don't know if it is .0025 or .0035.

[Tim Corcoran]

I had my bearings coated last time I freshened up the engine, after years of use and many trips down the 1/4 mile when I tore the engine down the coating was still there and the rod bearings looked like they hadn't been installed yet.

[hurryinhoosier62]

Originally Posted by Tim Corcoran

Why not just get a +.0002 bearing and call it good.

Didn’t know they produce .0002 bearings ( two ten thousandths). I believe you mean .002.