Bearing Clearances

Ok, I understand that this is probably pretty basic and something most on this board know.

What is (are) the determining factors for bearing clearances? I always read that these aren't Chevy engines and you have to set them up differently. But what is it that makes them different?

Is it the inadequate oiling system?

Is it your compression that determines clearance?

Is it the type of crank you have, stock or billet or forged? If stock and it's been turned .10 or .20 or it's been stroked?

Is it your intended usage?

Is it the power level that determines?

Are mains and rods set at the same clearances?

Again, this is probably Engine Building 101, but I never took that course.
Thanks for your time and answers, I really appreciate it!

 

Without writing a novel here, let me tell you what works, not based on theory, but experience. I counted the other day, and I have built 104 BBB since 2003, and 4 350's..

from a 340 HP stock 430 to a boosted 465 that made over 1100 at the stock crank..

The answer is .002 to .0025 mains and rods.

Enough room to accept block and crank distortion, tight enough that with a few oil mods, pressure is maintained.

Oil flow is just as important as pressure, and anything tighter hurts bearing cooling in HD applications. IN fact the factory had two modifications in their original "HD use" bulletin, designed for those racing with new stock engines.

Raise the oil pressure via the relief spring.

Increase the rod clearance via special bearings that gave .001 more clearance.

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What I have found over the years is that lifter bore clearance is the x factor in idle oil pressure. Spin the pump fast enough, and it will fill up everything just fine, use baffles and windage trays to control the oil. But lifter bores are something we have done nothing about.. until recently.

We have now purchase the BHJ lifter tru tooling, as well as fixtures to measure the cam bore location in the block. The very first one we checked, the lifters were ok in location, but the oil clearance was .003 (twice the ideal clearance) and a whole bunch of them were not 90* square to the cam.

Further, we found that the cam was .005 low in the rear, and .001 high in the front, based on the crank centerline. So we have all kinds of ugly forces here, robbing HP, potentially hurting parts, and bleeding oil pressure.

The solution is to Bore/hone the cam tunnel straight with the right equipment, and use oversize cam bearings to correct the cam tunnel placement, and make it perfectly square, based on the crank centerline.. and then to oversize the lifter bores to a .875 lifter and size for .0015 clearance, while insuring they are 90* to the camshaft. You can't re-locate them much, and the BHJ tooling was actually purchased for the Tomahawk blocks, doing lifter re-sizing and indexing, but we wanted to learn on the iron blocks, if it was an issue. Seems that unwanted tilt from the desired centerline and sizing is a bigger issue.

Of course the lifter bores are out of tolerance.. does anyone think that the lifter bores did not wear, while the rest of the engine did? I have seen some better, and some worst, but never one where I would prefer it to be.

Correcting the lifters, and the cam tunnel, are about a $700 investment here.. small change in the light of a 10K plus engine build.

And now we have left "no stone unturned" in our engine program.

JW

 

Great info Jim! I just printed it to keep in my paper files. I had been looking at the lifter bores of blocks that had been tumbled (wheelabrator) for the initial block cleaning. Looks like they need to be addressed as there is a definite visual surface change due that process. Though I haven't gotten around to actually measure them yet. One would think that a machine shop would have a way to plug the lifter bores during that process, unlike main bores and cylinder walls that will be final honed anyway. Any thoughts?

Larry

 

Yes larry,I just ran into that on my newly tumbled block.had to hone the lifter bores for smooth operation, clearance.

 

Foam stuffed in the bores, when we used the wheelabrator and weren't intending on remachining lifter bores.
Or make lifter bore machining mandatory as a part of your finished product.

 

Ok, so the optimal clearance for both Rod and Main bearings is .002---.0025.

And that optimal clearance is obtained by.... Bolt torquing ???
Do you use plasti-gauge and adjust the amount of torque on the rod bolts and main bearing caps? So if the factory specs call for 100 ft/lbs on the main bearing caps to the block, do you assemble with the plasti-gauge and torque to 100 ft/lbs and then disassemble and measure the width of the plasti-gauge marker?
If the tab measures too thin then you need more torque, and if it's too wide you reduce torque??

Are we looking at a few ft/lbs or only a few inch/lbs? Obviously you can't adjust the torque values up or down too much or you would be out of spec?

I know TA sells bearings in Standard, .010, .020, .030 and .040. Are these choices enough to mix and match to get to the desired clearances? This would be my guess because I can't imagine you want to vary the bolt torque values enough to get to your desired clearance...

Maybe it isn't so obvious after all, or maybe my comprehension is failing in direct relation to the number of gray hairs that appear....

As always, thanks for the help and the willingness to share!!

Philip T

 

Philip,

No, bolts are torqued to a preset value, to properly stretch them for torque retention.

With the mains on a 455 Buick, you can fuss around with the torque and pick up a little clearance, but that is not the preferred way to do it.

Main and rod jornals have a size spec. That spec typically has .001 tolerance window... so on a stock Buick crank the main size is 3.2490 to 3.2500. You specifiy the exact size to your crank grinder, based on your measurments in your block to obtain your desired clearance.. Most crank shops use the Hi, Mid, Low" system.

ON a .010 under 455 Buick main, the book size is 3.2390 to 3.2400

"hi" is 3.2400

"mid" is 3.2395

Low is 3.2390

Shops can vary that a couple tenths one way or the other, but that's the gist of it.

Now, the same tolerance specs exist for the rod and main housings.. believe it or not, as you make the housing bigger, the finished bearing clearance will get bigger, at nearly a 1-1 ratio. The main housing has a full thousandth in the spec (3.438 to 3.439) and the rods have half a thou allowable variation..

Example:

For the 455, we do the main housings to the middle.. 3.4385. With a Clevite 77 main bearing, when torque up, it will measure 3.2425 or so, at the .010 under size. This is where it gets tricky.. you have to use the exact same lube, and same torque, as the machinist used when he did the machine work on the housings... your re-creating that pull on the block, and lube and torque can make a huge difference in finished size.

YOU MUST use exactly the lube and torque spec your machinist used.

We use CMD#3 at 100ft lbs on the mains, and I prefer the stock bolts on anything that does not have a girdle on it.

We then have the crank ground to the HI spec.. which is 3.2400

That yields .0023 to .0027 clearance, and that is my spec window .0025 +/- .0002

JW

 

Ya Larry, I won't wheelabrate a block.. just beats it up too badly.

We use hot tank with caustic Soda, and glass bead blasting. Takes more time and costs more money, but i prefer the finish of the bead, vs the wheelabrator.

Cliff Studaker once visited my shop.. for those of you that don't know, he was the guy who designed the 400/430/455, and literally "wrote the book" on it. See the sticky in this forum.

Cliff, walked by one of my blocks in process, looked down at it, and said "Where on earth did you get a brand new engine block".. I smiled and said "that's not new, just clean"



JW

 

I can only share my experience from a customer standpoint. I'm no engine builder. Back in the early 90s when I started bracket racing my Buick with second hand stuff I became friends with a race family that ran off brand power (Oldsmobile) and they always seemed to be in the money and didn't seem to have trouble with round to round reliability. They also built engines for people on the side and everyone I knew that ran their stuff had good reliable results.

So when it came time for my first "build" I chose to have them handle it. I did all the oil mods to the block and timing cover that I knew of. These were outlined in the GS newsletter at the time which was pre-internet for me.

After that my builder handled the machine work and assembly.It was stock crank with reconditioned rods and no girdle. I was ignorant to clearances at the time. This engine ended up being a mid 10 second performer at 3400lbs +/- and turned 6400 rpm and had maybe 65 lbs of oil pressure at the finish line with 20w/50 oil.

Ultimately I sold this engine with the car it was in and it changed hands a second time all the while racking up several hundred runs before it expired (Rod failure I believe).

When I built my current car and the first engine it had, The same people handled the machining and assembly. This time a girdled stock block with a billet crank and aluminum rods that one turns 7100 rpm and has run 9,40s at 3400 lbs. and carries about 60 lbs of oil pressure at the finish line albeit with a belt driven external pump.

It was somewhere towards the end of owning the first engine and establishing the second that started to "catch up" on bearing clearance. My builders used the .001 per journal diameter which meant my stuff was "loose" by the standards of most of the Buick builders. I remember them joking at the time that they didn't want to tell me what the clearances were because of the contradiction to what I was learning from the Buick community.

My 505 has .0025 +/- on the 2.100 rod journal and the 3.250 mains are .0035 +/- and the bearings have always looked fine.

My point being... Is before you get to bearing clearance on a Buick 455 race engine, there are many other details that I believe contribute the success of the engine.

You (OP) mentioned the "inadequate oiling system" . I think that is a great place to start. Along with the other details in the lifter bore size as JW mentioned.

I only share my experience and would never make a "recommendation" ... That is between you and the builder you choose as he is the one that will lose sleep or feel some accountability in the event that your engine fails.

 

Let me add my thoughts on wider main clearance, from a builder perspective.

The wide clearances on the mains is adopted by some builders, to account for block or crank distortion. What kills the bearings, is closing up the oil clearance, and/or actual contact between journal and bearing at high rpm.

Some builders have adopted wide clearances, in the .003plus range to avoid this issue. I understand that in a motor with no fill and no girdle or a halo only, and if I were to build such a thing I can assure you the mains would be north of .003..

But, with a filled block, a properly installed full TA 1105 girdle, and especially with a Billet crank, you have minimized the deflection concerns tremendously. So adding another .001 to your main clearance won't matter at that point, and I prefer to keep the larger load area on the bottom bearing half, that the tighter clearance provides, considering our puny width main bearings.

Bottom line, if your going to "let it all hang out", and build north of 650 Hp and spin the motor, with no girdle, then by all means, open up the mains, stuff is going to be moving.. but if your building a more dedicated, properly prepared piece, my experience has shown me that there is no requirement to open up the clearances.

It's not that the bigger main clearance won't work, it's that the wider contact patch with the tighter clearance gives more margin for safety against mishaps and tuning errors.

A couple dozen HD/Race iron block based BBB's have proven that now, from 700 to past 1000 HP for a boosted one, and regular trips past 6500 rpm.

Of course, light parts, good balancing, sufficient oil pressure and volume and how it's taken care of all factor into this.

BTW... On the first Tomahawk built 8 years ago (wow, time flies) I ran the mains at .002 since everyone was freaking out about alum expansion and big clearances when hot.. I have found that those motors can run the same main clearances as the iron ones, they simply need higher viscosity oil and better oil pumps to keep the pressure at idle.. again, once the pump is spinning, oil pressure is not an issue, regardless of the clearances.

But I would not build a 'hawk with the .0035 mains.. might be tough to have any idle oil pressure, even at 1000+rpm, when everything is hot..



JW

 

Thanks again for your wisdom JW and as always, thanks for being an open book about it!

Gary I appreciate your input and experience too. Now maybe it's different with a Tomahawk, but conventional wisdom would say your oil pressure is a little on the low side going through the traps. Using the 10 psi per 100 rpm guideline anyway. Do you think your trap oil pressure is directly related to your "looser" main bearing clearance? Your also using an external pump which probably flows plenty of gpm to feed everything so you have no worries. I bet with an "inadequate stock system" you could run in to problems with that oil pressure!?!?
Are you still using 20W50 in the Tomahawk?

 

Phillip,

Let me clarify ... My Tomahawk has not been down the track yet, so I omitted that from my reply until I can speak from experience.

Also, My 505 that ultimately ended up with the external pump did start life with a more typical internal pick up and later was changed to a "Scavenger" system with an external pick up.
I only added the belt driven pump when I made the front cover that replaced the stock style front cover.

My oil pressure is low by some standards but inline with other race engines. I could turn it up at the pump but I've never felt the need.

My Tomahawk has 40wt break in oil for intial dyno pulls and then I will change to 30wt Redline synthetic for the final pulls and then hopefully going down the track.

 

Oil flow and oil pressure are not the same. But are connected. You have to have flow to make pressure. Oil flowing across the bearing keeps them cool. Oil pressure provides the cushion parts ride on.

I've heard many builders ( which I'm not ) are more worried about flow than pressure. High pressure also takes hp to generate, and you only truly need pressure to get the job done. Anything more is wasted power to produce, wasted power to overcome. The question becomes what is the psi need to do the job? What the answer to that us ever changing and effect by loads, temps, oil viscosity, on and on and on. So no set in stone answer, just a compromise of conditions. But thicker oils will need more clearance to flow.

 

Just a little more real world experience to confirm JW's observations.
Back in the late 70's early 80's when we began running Thumper we didn't use a girdle,don't even know if they were available, but we ran a 462 with al. rods,light weight venolia pistons,stock main bolts,and got nearly 1000 runs out of her before a rod bolt expired,plus some street miles in my sons car. After a season we determined that .002" on the rods was right but had to open up the mains to .0025".Ran the KB hi volume pump to keep up too, Even after opening up the mains, every winter I had to replace #2 and #4 main bearing due to funky wear from the block moving around so much.I was only making around 590hp @ 6000rpm with a KBmarkIV cam and iron stage2's.
Once we started installing girdles and sometimes filling we were able to step up the hp levels and go back to .002-.0023" main clearances with out any problems,and also remove the hi volume pump. Contrary to some others I have no problem building motors with this tight of main clearance,it works for me and I've rebuilt motors that other guys had failures with machine shops that not only used .003+ mains but .0025+rods as well.Maybe if we were spinning them over 7000rpm and used an external pump the bigger clearances are necessary, but 6500rpm is usually our max.

 

Good stuff......

 

Yes lots of great info shared, some learned the very hard $$$ way

 

"I've rebuilt motors that other guys had failures with machine shops that not only used .003+ mains but .0025+rods as well.Maybe if we were spinning them over 7000rpm and used an external pumpthe biggerclearances are necessary,but 6500rpm is usually our max."

Correct me if I'm wrong, but the way this reads is that your conclusion was these clearances were the reason for the engines failures?
Can you share the path to that conclusion?
Curious since the 464 that was in my green car that Josh and Emerson bought and ultimately ended up in Cincinnati was loose on clearance with no girdle, stock rods and crank and lived a long life with a stock pump and 6500 rpm ceiling.

 

Yes Gar, the excessive clearance implied that hemorrhaging of pressure and volume occurred with these clearances to cause #7 and 8 rod bearings to fail. Also, there was material in the pan that was from the mains since some were worn down to the copper.I also keep the rpm below 6500 for non girdled blocks. I'm not saying that it would do this in all situations, just that I've seen it and never go with more clearance than .0025".Thanks for the question.
gary

 

So these messurements being off, is this an issue that's been with said motor from birth being machined wrong, or is it more caused in the life of the block from being used?

With the cam being off, all be it only a little bit, I can see things like pushrod length and preload also being altered a little.