[PontiacLars]

Molnar 4.5" forged crank
Molnar H-rods
FM 7200CH bearings

I get 0.0030 rod bearing clerance. I guess I need 0.001 undersize bearings or will that be too tight? Is this normal with new parts from Molnar? Crank messures 55.87mm wich is 2.1996".

[Joe's Garage]

We have a Butler-built 541 that will be used about 70% on the street and 30% on the strip. Originally intended to run in the Silver State Classic Challenge - a 100 mile plus timed road race on a closed public highway here in Nevada.

Butler set the rod bearing clearance at .0027.

I've always tried to get to bearing clearances a little (.0002 to .0003) on the loose side for better oil flow to cool the bearings. Not sure if that's scientific or not, but it's worked for me.

Might be worth a call or an email with Tom Molnar to get his input.

Good luck!

[HWYSTR455]

Try different bearings, like King, see if it's any better.

I've heard chatter that the FM bearings are hit or miss, worth a try to use something else.

Were the crank and rods prepped? Or straight from Molnar out-of-box?

How are you measuring? And at what temp, shop, parts, tools? Stuff takes time to stabilize, 70 degrees is roughly the minimum temps.

Measure the main bores too, make sure dowels aren't hanging up and check for crap that could get in the way.

You sure the crank is coming in at 2.1996?

Spec for the journal is 2.9895 +/- .0005. Would think it would be a lot bigger of a clearance at 2.1996.

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[HWYSTR455]

Duh, was thinking mains, I see now it's rods...


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[AG]

You can get the rod bearings coated to tighten up the clearance. I used Calico with great success, I coat all my bearings now.

[hurryinhoosier62]

In your application a rod bearing clearance of .003 is doable.

[PontiacLars]

I will try a set of Clevite and see if they are a bit tighter.
CB743HN. Anyone with experience with these bearings?

[HWYSTR455]

I may have used those at one point, but know right now, the King bearings are the best out there, and closer in tolerances. You can ask around for confirmation.

I'm still curious as to how you're measuring.


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[PontiacLars]

Local supplyer has Clevite in stock, so will try them first.
I use micrometer and set it with the journal, then zero the dial bore gauge on the micrometer and messure the bearings. Shop temp is stable 70 fahrenheit.

[HWYSTR455]

That should work!

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[Dragncar]

Like AG said, Calico.
I had the same exact issue with Molnar rods and Scat crank.
Calico coated bearings and it came in at .025, perfect.

[steve25]

I have no idea what the OP is taking about here!
Whatever he has measured he states to be be 2.1996” yet a 3” main should be 2.995” + or minus .0005”.

A rod journal should be 2.492” + or minus .0005”

[b-man]

He’s talking about a Molnar crank with 2.200” BBC rod journals, not an uncommon size for Pontiac aftermarket and reworked factory cranks.

[steve25]

Then that makes sense!

[HWYSTR455]

Yeah, I got a little lost at first too!


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[PontiacLars]

Installed the Clevite bearings today. I now have 0.0027. Very happy with this clerance.

[mgarblik]

Just for general verification, look at the box of your FM bearings. If the box says Made in Mexico, those tend to run a little big. .0003-.0005" is common. The one that say Made in USA are about the same as Clevite. Pretty sure all the FM bearings are made in Mexico now.

[PontiacLars]

They are made in Mexico.

[steve25]

This is right out of the FM catalog.

If you have .0027” then I would say your good.

[mgarblik]

Originally Posted by steve25

This is right out of the FM catalog.

If you have .0027” then I would say your good.

I agree he is great with .0027". The clearance ranges in the FM and Clevite catalogs are a little frightening though. We never let anything go together without measuring at least the middle of the range. Any tighter can spell disaster in a performance application. Some of the new automotive engines run extremely tight clearances, especially Honda. They run 0.000-.0003" piston to wall clearance. They let the skirt coating wear in a tiny clearance. Bearing clearances of .0003"-.0005". But 0W-5 oil and very precisely machined parts.