Crankshaft clearance check

JohnJL · 09-16-2005, 05:08 PM

I'm following "the good book's" instructions and have confirmed the rod bearing clearances using plastigage.

When the machine shop was polishing the crank journals, they said it was "on the lower end of the range" and recommended checking the crank bearing clearances with plastigage also. I noticed Wayne's book doesn't show that procedure (ie, oil pump in or out, etc.)

What's the correct procedure for checking crankshaft/case bearing clearances? won't it be a little off is I assemble the whole bottom end 'dry' and check the clearances? Won't a string of plastigage in each bearing throw off the measurements?

thanks

JohnJL · 09-17-2005, 02:50 AM

Ok, I went ahead and laid down green plastigage down on all the bearings, dry assembled teh case doth all the throught bolts and perimeter nuts to their final torques.

I disassembled everything and got clearances of .06 to .074 on all the D1 (main bearing) measurements. Wayne's book calls for .01 to .072.

What's the risk of going ahead with this bottom end? This will be a track-only car and so will never see another 50K, so what are the downside risks of running a crank with a main just low of the given tolerances?

I know the right thing to do for a daily driver/long-mileage rebuild is to have the crank ground and go with +1 bearings, but it hardly seems worth it if its so close and I'm not trying to get big-time mileage.

Thanks all

edbaus · edbaus's Garage

Hi,

You did not say what motor that it was for. A loose motor will actually be a little faster.

That is on the good side.

On the down side, the motor will not last as long. In theory. And, the oil pressure that you see at idle will be very low after the motor is hot.

This will be exagerated on a mag case motor. (becasue all mag case motors flex over time causing oil loss in the system.)

To put this in perspective. I had a mag case 2.2L race motor that after it was hot read 0 on the oil pressure gage. I worried over this for 3 years before I built my next motor. Ran the motor to redline every chance I got durring the races (7200 rpm) and never had a problem.

YMMV

Thanks

Ed

jpnovak · 09-17-2005, 07:51 AM

there is a spec. Your motor meets it. I would use it as-is.

JohnJL · 09-17-2005, 02:12 PM

Thanks guys.

Sorry, you're right, not a lot of info.
This is a 3.0 83 alu case, no mods to the case but carrera tensioners.

john

Eagledriver · 09-17-2005, 07:45 PM

This clearance sounds good to me. It is about .003 inches. I can't imagine that would be too much. I'm not sure about how to do these measurements. I'm thinking that if you put the plastigage on one side of the journal it will offset the crank to zero tolerance on the other side. I assume this means that if the crank were centered in the bore that the clearance would be half of what you measured. Maybe someone can verify this or correct me.

-Andy

JohnJL · 09-18-2005, 02:08 AM

I'd be surprised if the spec were quoted 1/2 off tho. I am looking at Wayne's book and it says ".076MM" as the max clareance. Not much contoversy there.

So what are the risks of running at that clearance, or less?

Eagledriver · 09-18-2005, 08:10 PM

My point was that maybe you are running at .03 or so MM. If you center the crank in the bore you'd measure .03mm all around instead of offsetting it with the plastigage to get .06 on one side. I don't know how these measurements are supposed to be done (one side or all around).

Since either way this is within spec I'd run it and not worry. The bottom of the SC motor lasts so long that I doubt you'd notice any difference in motor life or even oil pressure with these numbers.

-Andy

Wayne 962 · Wayne 962's Garage

The reason why I don't mention doing this in the book, is because it's very difficult to get accurate readings with the Plastigage and the crankcase. The crank should be measured with a micrometer, and the case should be measured with a dial bore indicator, when tightened down.

If it's in spec, then go with it...

-Wayne