Bearing Clearance, Plasticgauge

[sdb999]

Generic Engine build question.

Last time I built engines I'd use plastigauge on mains and rods. Just a gut check to confirm measurements from machine shop. It's been a while so I don't remember if the plastiguage measurement represents 1x clearance or 2x?

Part of me wants to say with the engine on a stand upside down (with no oil pressure) the crank would settle into the block and I'd be looking at 2x under the cap. Maybe not, looking for thoughts. I guess the same would hold for the rods but they can float up as the cap is tightened so maybe a different behavior.

Thanks, S.

 

[bentos1]

Generic Engine build question.

Last time I built engines I'd use plastigauge on mains and rods. Just a gut check to confirm measurements from machine shop. It's been a while so I don't remember if the plastiguage measurement represents 1x clearance or 2x?

Part of me wants to say with the engine on a stand upside down (with no oil pressure) the crank would settle into the block and I'd be looking at 2x under the cap. Maybe not, looking for thoughts. I guess the same would hold for the rods but they can float up as the cap is tightened so maybe a different behavior.

Thanks, S.

Sorry but I am not following. When I had used plastigauge it was take your reading and see if it is within spec per the FSM and use the chart on the plastigauge.

 

[sdb999]

Right, that's what I remember but I don't recall making an allowance for no oil pressure at assembly. Crankshafts do not contact the bearings while running and maintain a more or less constant clearance all the way around only at the pleasure of oil pressure. Remove that and the crank will rest on the main bearing, no longer concentric all the clearance will be on one side. Maybe I'm over thinking it, maybe the crank always floats right in the center of the bearings regardless of oil pressure, maybe the plastigauge takes this in to account. Just building a bottom end for the first time since the early nineties but I didn't really pay close attention then, maybe got lucky.

 

[KidVermicious]

I think you're overthinking it. FSM clearances are surely given under the assumption that you're not going to have oil pressure while you're taking them?

 

[bentos1]

Right, that's what I remember but I don't recall making an allowance for no oil pressure at assembly. Crankshafts do not contact the bearings while running and maintain a more or less constant clearance all the way around only at the pleasure of oil pressure. Remove that and the crank will rest on the main bearing, no longer concentric all the clearance will be on one side. Maybe I'm over thinking it, maybe the crank always floats right in the center of the bearings regardless of oil pressure, maybe the plastigauge takes this in to account. Just building a bottom end for the first time since the early nineties but I didn't really pay close attention then, maybe got lucky.

I agree with KidVermicious, you are over thinking this too much.
Just put your plastigauge on, torque to spec and see if you are in the FSM tolerances, done.

Get rid of the plastigauge if you are worried about it not working for you. Get a proper micrometer and vernier caliper and measure the crank in detail. I would be most done bother with this route unless its a very high end engine but I could be wrong.

 

[sdb999]

Yep, picked up the block yesterday and machine shop confirmed what has been said here. Thanks All.