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Main Bearing Clearances Too loose after Crank Machining

Practical Engine Building

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B6ZE (mx5 1.6) 10.6:1 comp Naturally Aspirated

I just measured my Main bearing oil clearance and I'm a bit concerned:

Service manual states oil clearances should be: 0.018 - 0.036 (mm) | 0.0007 - 0.0014 (inch)

Local shop that has built these engines aim for: ~0.05 mm |0.0019"

Max Wear that the manual states: 0.10 mm | 0.004"

Measures oil Clearance: 0.08 - 0.09 (mm) | 0.0031 - 0.0035 (inch)


The crank has been ground and polished 0.02"

paired with this I have ACL .50 bearings

seems like the crank was slightly ground off too much

from conversing with mx5 mania, 0.55 bearings should move my oil clearance to 0.03 - 0.04 (mm) | 0.0012 - 0.0016 (inch) Which would be perf

ACL doesn't specifically sell that bearing size for my engine but is there a way of getting a custom size, or maybe another company that can offer what I need?

Alternativly if I were to build the engine with this clearance should I be using a hella thick oil (that's a real mesurement) to compensate?

My only concern being the piston to bore clearance is 0.09mm | 0.0035" Which when the machine shop gave my block back said " these engines like to be tight" So I'm presuming this is a tight piston to bore clearance for a forged piston. I did ask the shop to pick clearances for me since they had extensive experience with my engine family.

So having looser bearing clearances and tighter piston to bore clearances seems not ideal since we can only have one viscosity of oil.

Also I can't really measure Big End oil Clearances as I need to use plasti gauge since my bore gauge doesn't goe small enough (small displasement lyfe)

Would it be worth assembeling just so I can measure Big End Oil Clearance? also would I need to put the pistons in to do this? just would rather put the pistons in only once to avoid increased risk of damaging the rings

I would prefer to not re-machine new parts as a lot of money has already been spent.

Thanks again in advance! I have no idea what I would be doing without this forum!

If you do plasti-gauge the main bearing clearances, and you find they are incorrect or not what you are looking for, in my experience, many bearing manufacturers or retailers for bearing manufacturers will not return the bearings once the package has been opened.

On another note, i have come across engine builders who prefer main bearing clearances in the range of .003" for high performance applications, and a 50w oil. I have assembled a few engines at .003" on the mains and a few with .002". So far so good. I do not personally have any data on which clearance is better, but Andre has mentioned a general rule of thumb based on journal diameter for calculating bearing clearance.

Most bearing manufacturers can do custom undersize bearings, but $$$s, I know King bearings do offer undersized in some sizes for reducing clearances - might even have something suitable off the shelves.

With the big end bearings, rather than opening another set of shells, carefully measure the crankshaft journal and work from there - if it is on the mark, you should be fine.

TBH, though, unless really pushing for the optimum, I would happily run it as is provided the shells were of a suitable spec' if i couldn't get undersized ones - it may be slightly larger than desirable, but still well within service limits if the manual's wear figure is of the journals rather than clearance.

Personally for the sake of a new set of mains, I would buy some undersize ones. This will solve a potential problem further down the line which could end up being far more expensive.

For the same reason, measure the big ends.

What is the manufacturers spec for the pistons?