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Crankshaft polishing and main bearings

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Hello all,

I dropped off my block to get bored, cleaned, decked, crank polished, rings ground and lower end bearing clearances checked.

Machinist told me MAINS tolerances are good with the standard sized bearings that I provided. I checked when I got the block back and the bearings I provided show a very large clearance, more clearance than standard size. Turns out the bearings I thought were standard were +0.001.

Main Bearing clearances:

Service manual Specification: 0.0007-0.0013" (0.0039" MAX)

Measured with new (oversize) bearings: 0.0045"

Measured with Standard bearings (the ones that came out of the block at 230K km): 0.0025"

NOTE: All measurements taken with micrometer (referencing polished main journals) and bore gauge on main cap girdle fastened to 60ftlbs with ARP studs.

My questions:

1. How much does polishing the crank journals typically remove? (There was virtually no scoring on the crank originally), does this procedure typically require different sized bearings?

2. Do you think my machinist would save time by assuming the bearings would be "Fine"? (I don't see any scoring or scratching indicating their installation/measurements)

3. The bearings I have are +0.001", I can get standard size, -0.001" or -0.010".

I imagine the best solution is to get the 1 step undersized (-0.001") and accept the 0.0015" even though it is out of recommended specification. Otherwise I can take it to the machinist to grind down to the next size bearing.

What do you guys think?

This is a classic example of why we never make assumptions that the machinist has done their job properly! Good that you picked this up!

1. Typically polishing a crank journal should make almost no measurable difference to the journal diameter, particularly if there's no obvious scoring to rectify.

2. Hard to say but this is a reasonable assumption.

3. It's a little tricky to advise as I don't know what the engine or application are. It's quite normal when building a performance engine that will see more rpm and more power than stock to purposely build with slightly looser clearances than factory so what you've got is not out of the ordinary at all.

Andre, I really appreciate the quick reply! I now see why rebuilding an engine is typically seen as inferior to the original assembly.

My application is just a reliable daily driver with a slight boost in power. This is why I was hoping to stay within specification.

With the bearing sizes available, I can either be slightly out of specification or need machining done to the journals and install larger bearings. The machinist told me to just put standard sized bearings at this point.

Logically, I don't see why I would go undersized bearings if the crank wasn't machined down. On the other hand, what I am measuring tells me that I need undersized. Thoughts on this?

Given the application isn't extreme and the fact you're only marginally outside the factory spec but still within the max spec I'd probably run with the undersize bearing shells.

Thanks Andre.

Why does the FSM indicate a clearance range and a maximum thats 3 times greater than the upper tolerance? (Is the maximum to allow special cases to go out of specification?)

I got the standard bearings now (As per machinist recommendations) and I am getting an oil clearance of 0.003". I am a little uneasy putting this assembly back together as I am not sure if this close to the maximum specification is safe. I am looking for longevity more than anything else.

My logic was, crank and bores weren't machined, so I wouldn't need to undersize from standard? Is there a flaw in this logic?

I got another specialists opinion and he torqued the main studs to 70 ftlb (instead of 60) and measured the clearances anywhere between 0.0019-0.0022. (outside caps were tighter).

He only measured 1 journal with the micrometer though.

What are the adverse effects of going past the ARP recommended torque spec of 60?

Basically the manufacturer offers the range that the clearance should fall within and then a maximum service limit beyond which part replacement would be recommended. I can't tell you why the service limit is set so far beyond the expected range though and I certainly wouldn't be comfortable there and would work to get my clearances closer to spec.

You're right in so much as if the crank and the main bearing tunnel haven't been touched then fitting a new set of the same bearings that came out should achieve the exact same clearances you had been running and this may well be the case. What you do need to be mindful of is that aftermarket 'STD' bearing shells may differ marginally to an OEM shell thickness which could affect your clearance. One other question I have is did you always have the ARP stud kit or is this new? The reason i ask is that these will usually deform the main bearing tunnel and could easily change the clearances compared to what you previously had.

You can definitely manipulate the ARP torque spec (within reason) as it is set to 75% of the yield strength. If you go past the yield point then the fastener will be permanently deformed.

The ARP main studs are new to the engine, unfortunately I don't have any choice at this point as the bearings I need will not be stocked until next year. So 2 thou clearance will have to do (with thicker oil).

On the topic of deforming with studs, my machinist bored the block 0.5mm over to fix an out of round, but I received my head studs back in the sealed box the same way I gave him the parts. Is there any concern if different fasteners were used or if the block was machined without the torque plate?

I really wouldn't be concerned at 2 thou and I'm sure you'll be fine. Ideally you use the same fasteners for the torque plate as this ensures the distortion is as close as you can get to the same under running conditions. Are you 100% sure that a torque plate was used?

I am glad you asked. I requested it to be, but seeing as they were in a hurry to get my engine done, I would imagine a torque plate was not used on this block.