Tolerance on Chevy 2" journals

[Jonny Grigg]

Hi,

I need to get the rod journals re- polished on my crank. Can anyone tell me the dimensional tolerances of a Chevy journal so I can get the machine shop to check it is still good to go?

Also please could anyone let me know the min/max oil clearance on the bearings?

For reference I am also using CB rods and clevite bearings.

Thanks

Jonny.

[Jonny Grigg]

Ok, I have found the tech sheet on the clevite website for the CB610P bearing. I would still be interested to know whether anyone runs oil clearances different to this for whatever reason.

Alternatively if I need to re-grind the rod journals, what bearing should I use? Has anyone got any recommendations?  I am hoping the scoring will polish out, but if not I need to look at my options.

Thanks in advance.

[Garrick Clark]

I also would like to no this.
I have scuffed bearings for what ever reason. not too sure why mines done this. Low mileage motor.
My thinking is that the crank journal is a lot tougher than the  bearing material and the
scuffing on the bearing will be worse than on the crank journal.
I'm going to oil soak some 1000 grit paper and polish the journals.
get some new   shells and give it a go.
Type in
popular hot rodding, bearing clearance control.

[Jonny Grigg]

Thanks for the reply.

Does no-one else here measure their crank journals? I would appreciate any comments or advice if possible please.

Thanks,

Jonny.

[leec]

Hi Jonny,
Probably not a lot of help but when I was messing around with Polo G40's I bought a cup car engine with a big end knock.

The crank needed grinding but my local machine shop knew the answers to all your questions. Are you over thinking it?
Lee

[Jonny Grigg]

Hi Jonny,
Probably not a lot of help but when I was messing around with Polo G40's I bought a cup car engine with a big end knock.

The crank needed grinding but my local machine shop knew the answers to all your questions. Are you over thinking it?
Lee

Probably! Having read the Clevite website, it appears the suggested "starting point" for a racing engine with 2.000" journals is 0.0025", but this is also a function of the oil viscosity- seems logical.

I took the crank to my local machine shop and he asked me whether I knew the journal tolerance before he polished it. I thought as this place full of 'free" advice I'd see what other people have measured crank journals to as I can't be the first person in the world to need a crank polished.

I think I'll just get it back, plastigage it  myself and see what margin I have. Seems the most sensible option in the absence of any other thoughts :-/

[leec]

I know nowhere near enough about this stuff. Have you thought about giving Matt Keene a call?
Lee

[Jonny Grigg]

I know nowhere near enough about this stuff. Have you thought about giving Matt Keene a call?
Lee

Yeah, I hadn't really thought about it that much until the other day either. I think there are a few guys I can have a quick chat with about this... thanks anyway!

[Udo]

0.0025",  should work well . i have this in my autocraft engine , but some cranks have a little smaller diameter and works also. 0,03 mm less in diameter

Udo

[Mike Lawless]

Udo's on the money here.

I like a journal diameter of 1.9985 to 1.999. Any bigger and I'll polish it a bit more. If the clearance is too loose, you COULD have the connecting rod big end resized on the tighter side of its tolerances, which will close up the bearing clearances. I wouldn't want to loosen up the rod big end to gain clearance because it will loosen the rods "hold" on the bearing. Better to reduce the journal diameter.

I've run quite a bit more clearance than Udo's .0025" before without incident. As much as .005" after polishing a hurt journal just to get the thing running. Hot oil pressure was low, but that was it.

Now, if you have journal to bearing metal to metal contact, the clearances may be too tight, but the most likely cause is oil starvation. VW motors are notorious for this and most try to blame it on the bearings, crank, or for some other reason. The bottom line is that 99.9% of the time, the oil supply is being interrupted. Usually it's the sump being sucked dry with most of the oil ending up in the heads....not oiling the bearings. Fix that and the bearings will live.  

[1946vw]

I like .003 to .0035 for an engine at 10,000

[Udo]

We are talking about diameters ...   in mm i like 0,06-0,09 . But mike is right a little more than 0,06 is better , that is what you must have

Udo

[Mike Lawless]

I like .003 to .0035 for an engine at 10,000

But everyone know you're crazy B!

[1946vw]

I use more at 12,500

[Jonny Grigg]

Thanks for the advice- crank looks ok and will polish up within the tolerances suggested by Mike

Much appreciated.

[spanners]

All the above quoted running clearances are sound, the minimum factory value for a stocker  is 0007", too tight for a racer, the upper wear limit is 006" , again ok for a stocker but would show low oil pressure for racing use, I've only seen values near that show up as ovality, a reading 90* removed showed 0045" , the motor had a huge mileage log, road use and racing, it had an 86 mm stroke which would have had an influence on it no doubt.