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Old 10-21-2023, 01:40 PM
tekuhn tekuhn is offline
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Default Main Bearing Clearance

Hello,

I am building an engine using an NOS RA-IV block and Scat 4" stroke forged crank. I'm using Mahle-Clevite MS-496P-STD main bearings and my clearance is coming in at about .0015" which is too tight. As the crank is brand new and the block is NOS, I'd really like to avoid taking it to a machine shop. Are there any bearing manufacturers that offer oversize, or is one manufacturer more likely to run on the larger size than another?

Any other tricks that might gain me a thousandth or so? Would running a 0.001" stainless steel shim under the caps be out of the question?

Thank you for any advice!

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Last edited by tekuhn; 10-21-2023 at 01:42 PM. Reason: Spel
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Old 10-21-2023, 02:29 PM
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Use .001 extra bearings. They're out there. Start your search with King.

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Old 10-21-2023, 03:31 PM
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Sealed Power 4040M1, Summit says they have them with a ship date of Nov 17th.

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Old 10-21-2023, 03:33 PM
tekuhn tekuhn is offline
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Quote:
Originally Posted by AG View Post
Sealed Power 4040M1, Summit says they have them with a ship date of Nov 17th.
Unfortunately, I believe those are .001" undersized and I need oversized.

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Old 10-21-2023, 03:36 PM
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Ooops.

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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule.
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Old 10-21-2023, 03:39 PM
tekuhn tekuhn is offline
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It looks like King MB5511XPSTDX are .001" oversized, but they are non-grooved lower and I have a straight-shot oiling crank, so I need fully grooved. Their MB5523 is fully grooved, but not offered in STDX. Kings catalog is very difficult to read through, but it looks like MB5523 is made using MB5511 uppers. I guess I could by 2 sets of MB5511XPSTDX to get a fully grooved set. Expensive, but still cheaper than a machine shop.

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Old 10-21-2023, 03:39 PM
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Quote:
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Ooops.
I appreciate the effort

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Old 10-21-2023, 05:08 PM
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Do not put any shims under your main caps !

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Old 10-21-2023, 05:12 PM
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Do not put any shims under your main caps !
Nope. I will pursue the oversized bearing solution.

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Old 10-21-2023, 05:27 PM
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It seems that new crankshafts are being ground right at the high limit more and more these last couple years. The extra clearance bearings are the easiest solution if you can get them. We had a 440 crankshaft in class we just couldn't get extra clearance bearings for. It had .0008"-.0012" clearance on the mains. We had to send the crankshaft to Shaftech in Fostoria, OH to have it ground .0015". It came back very nice. But a 6 week wait and not very many shops can remove such a small amount of material properly. You really have to know what your doing to make that happen. The average crankshaft grinder and operator would probably turn the crank into scrap.

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Old 10-21-2023, 05:34 PM
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Quote:
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You really have to know what youíre doing to make that happen. The average crankshaft grinder and operator would probably turn the crank into scrap.
Thatís my fear. I do not have any experience with the local machine shops and hate the thought of letting any of them touch it. If a .001 oversize bearing will get me to .0025 clearance, I can live with than on a street motor.

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Old 10-22-2023, 08:58 AM
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Quote:
Originally Posted by tekuhn View Post
I have a straight-shot oiling crank, so I need fully grooved.
I was under the impression that half groove mains are recommended for high performance applications even with a non-cross drilled crank. The non grooved lower provides a wider hydro-dynamic wedge for better crank protection and the rods still receive plenty of oil. Maybe someone with more knowledge in this area can shed some light on this.

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Old 10-22-2023, 09:05 AM
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This will be interesting. I hate the thought of no pressure to the rod bearings for 50% of their rotation, although there is constant centrifugal force on the oil in the passage, so I guess there is always some pressure, and more so at higher RPM.

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Old 10-22-2023, 01:33 PM
chiefbigb chiefbigb is offline
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I was thinking the same thing about the lower bearings.

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Old 10-22-2023, 02:42 PM
Dragncar Dragncar is offline
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Cross drilled cranks are wanting compared to a straight shot oiled crank. I put half grooved FM race bearings in my new engine and did not give it a second thought( Scat 4340 crank).
Cross drilled cranks are very old school, the oil has to overcome centrifugal force. I believe there is some sort of a do not take a cross drilled crank past 7500 RPM deal.

But IMO here is the best deal you can do for your Pontiac oiling system and costs you nothing The oil hole in the middle of the main bearing, even the "slot" in FM Race bearings do not.
The shot of oil in you Pontiac block does not come from the middle hole. Where the 3 holes trisect , come together the oil comes from the hole towards the "tab" side of the bearing.
So if you have say a Clevite main bearing the oil shot has to hit the back of the bearing, move over and go on to the crank.
Problem is, your engines rod bearings get starved at high RPM.
So what I do is drill a hole right through the main bearing that lines up with the hole on the slot side. You take a Sharpie and mark the edge of that hole, snap your bearing in and there is enough of a transfer to mark your drill point.
Put the bearing in a vice, wrapped with rags or use copper soft jaw adapters and drill a 1/16" from the back side and work your way up one drill size at a time until it matches the hole in your block. Then take a jewelers half round file and de bur the inside of the bearing.
This is something I have always done. I blueprint my own oil pumps too. The last one I enlarged the low pressure-pickup side and made my own enlarged pickup with a proper cone shaped screened pickup 3/8" off the bottom of the pan.
Engines have great oil pressure. 60 -75 lbs going down the track and the lowest I will see is 45lbs at a fast idle.
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Old 10-22-2023, 02:43 PM
tekuhn tekuhn is offline
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No question the non-grooved lower bearing would be stronger, but it's going to take a pretty strong argument to convince me that outweighs the need for the rod bearings to have constant oil pressure.

I just got back from a trip to Summit racing where I purchased 2 sets of bearings so I can piece together a fully grooved set. I plan to wait a day or so until I get feedback on this before I open the second set.

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Old 10-22-2023, 03:10 PM
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There is this block mod done by some of the big boy racers.
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Old 10-22-2023, 03:17 PM
Dragncar Dragncar is offline
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There is a bearing mod some are doing also.
A pic of the before and after results on the same bearing location.
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Old 10-22-2023, 04:12 PM
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I installed the King MB5511XPSTDX and measured .0030" to .0035" clearance. Apparently they are sized slightly looser than the Clevite. I know the "rule" is .001" per inch of diameter, so I'm now on the loose side of acceptable. The crank is on the small side of acceptable at 2.9990. I could get a set of STD and close that up to about .0025" to .003". Suggestions?

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Old 10-22-2023, 04:31 PM
Dragncar Dragncar is offline
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Run it.
You could give them a .0005 coating or just run them, after drilling them holes.

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