Measuring/Sizing Main and Rod bearings!

[thedean]

I've read Peerless's "Golden rule" FAQ regarding measuring crank and rod clearances using plasti-gauge.

I've only built one motor before and I had a machine shop do the bearing sizing last time, but I figured it would be more fun to measure the clearances myself this time.

So my question is: Can I buy a micrometer and measure the crank journals directly to size my bearings? Or is it better practice to order standard bearings, use plasti-gauge, or a combo of a bore dial gauge and micrometer and then figure out if the standard will work or not?

Also does anyone know where I can get a reasonably priced set of micrometers and a bore dial gauge to do the job without breaking the bank?

For the record this will be being performed on a M52, but the procedure is EXACTLY the same.

Forgive me if my questions sound stupid, but i'd rather look like a fool for 5 minutes and ask a question that not ask a question and be a fool forever!

[jrdeamicis]

I bought my micrometers on amazon prime. They work great, but plastiguage works great and its very easy to read. There is also a lot of a learning curve of being accurate with micrometers. FFT>

[JTapper]

I went the plastigauge route. It was easy to use, and I lucked out and was able to use standard bearings. I figured I'd be fine considering the internal condition of the motor, no sludge, looked super clean, no scoring on any crank journals or abnormal bearing wear.

Plastigauge is cheap and pretty much idiot-proof if you can follow directions.

[reelizmpro]

I just measured and measured again with micrometer and ordered the appropriate size and color code from BMW. You have to know how to use a Mic properly though. I did not use platigauge to check my work. Plastigauge is a sure fire way to check ur clearances though.

[thedean]

Thanks for the advice guys. What do you mean by properly using a micrometer? Are you referring to them being zeroed out and making sure the mic is perfectly square to the journal?

[brianao34]

I would definitely go the Plastigage route. You should be able to order single bearings from BMW and test each one to see what size you need then order in bulk from there.
But from what I've read BMW cranks don't wear often so more than likely you'll be good with standard sizes.

[thedean]

I would definitely go the Plastigage route. You should be able to order single bearings from BMW and test each one to see what size you need then order in bulk from there.
But from what I've read BMW cranks don't wear often so more than likely you'll be good with standard sizes.

Standards are only $45 from pelicanparts. I may just purchase those, then check the tolerances with plastigauge. I figure thats cheaper than ordering a bunch of the custom size bearings you know?

[brianao34]

Yea, that's what I'd do. Just got done Plastigage-ing my mains, very easy

[RobertK]

I love Plastigauge but have yet to come across an engine that required anything more than stock bearings unless the crankshaft had been turned to repair scoring.

[thedean]

I love Plastigauge but have yet to come across an engine that required anything more than stock bearings unless the crankshaft had been turned to repair scoring.

Ya the cranks are nitride hardened so they are pretty tough, might as well buy a new crank if it isn't standard. The cost of the non-standard bearings will be more than another crank lol.