E36 325 rebuild questions

[e36its]

I'm having a local machine shop work over the bottom end of a motor for my IT car. This motor was overrev'd, but continued to run until it dropped a cylinder about 2 hours into an enduro. The head was pretty well beat up, but we've gotten that back into shape with a bucket of new parts and a bit of machining. The machine shop's evaluation of the bottom end is more surprising.

Appparently the crank is within spec, but the mains come out with only 0.0005" clearance -- apparently *smaller* than the spec range. The machinst says he'd much rather see 0.0015" to 0.0020" clearance for an enduro motor. He's recommending align honing the mains to open up the clearance. I suppose an equivalent move would be to turn down the crank, but I haven't asked him about that.

I'm puzzled by the tight (but consistent) clearances on the mains. I'm surprised the clearances would be that far under spec on a used motor that was running well and didn't seem to have any oiling issues (at least that were apparent in the teardown).

Any insight would be appreciated. I thought the head would be the costly part and the bottom end would be a routine cleanup, measure, and reassembly with new bearings. Figures I'd be 180 deg off on that!

tom

[dj10]

I'm having a local machine shop work over the bottom end of a motor for my IT car. This motor was overrev'd, but continued to run until it dropped a cylinder about 2 hours into an enduro. The head was pretty well beat up, but we've gotten that back into shape with a bucket of new parts and a bit of machining. The machine shop's evaluation of the bottom end is more surprising.

Appparently the crank is within spec, but the mains come out with only 0.0005" clearance -- apparently *smaller* than the spec range. The machinst says he'd much rather see 0.0015" to 0.0020" clearance for an enduro motor. He's recommending align honing the mains to open up the clearance. I suppose an equivalent move would be to turn down the crank, but I haven't asked him about that.

I'm puzzled by the tight (but consistent) clearances on the mains. I'm surprised the clearances would be that far under spec on a used motor that was running well and didn't seem to have any oiling issues (at least that were apparent in the teardown).

Any insight would be appreciated. I thought the head would be the costly part and the bottom end would be a routine cleanup, measure, and reassembly with new bearings. Figures I'd be 180 deg off on that!

tom [/b]



I'm not a engine builder but (I did stay in a holiday inn express :D ) without knowing the history of your engine I'm sure it would be hard to say. I know that BMW bottom ends are usually bullet proof.

[pfcs49]

The main brg clearance sounds pretty unbelievable. I'd get a second opinion (ie: get someone else to measure it) before signing on for line boring/honing. I wouldn't say half a thou used mains were a possibility until I saw a corroborating 2nd measurement.

[Z3_GoCar]

I'm having a local machine shop work over the bottom end of a motor for my IT car. This motor was overrev'd, but continued to run until it dropped a cylinder about 2 hours into an enduro. The head was pretty well beat up, but we've gotten that back into shape with a bucket of new parts and a bit of machining. The machine shop's evaluation of the bottom end is more surprising.

Appparently the crank is within spec, but the mains come out with only 0.0005" clearance -- apparently *smaller* than the spec range. The machinst says he'd much rather see 0.0015" to 0.0020" clearance for an enduro motor. He's recommending align honing the mains to open up the clearance. I suppose an equivalent move would be to turn down the crank, but I haven't asked him about that.

I'm puzzled by the tight (but consistent) clearances on the mains. I'm surprised the clearances would be that far under spec on a used motor that was running well and didn't seem to have any oiling issues (at least that were apparent in the teardown).

Any insight would be appreciated. I thought the head would be the costly part and the bottom end would be a routine cleanup, measure, and reassembly with new bearings. Figures I'd be 180 deg off on that!

tom
[/b]

Another thought is to have him double check the numbers on the bearings. You'd think that with modern sorting technology, bearings would always be packaged right, but you'd be wrong. We once had a set of Big-Block Chevrolet bearings without an oil hole in the rear main. Actually that was a lot problem as multiple sets of the same lot had that problem, double check the part numbers printed on the new shell with the book number along with measuring one of the old shells close to the parting line and the same spot on the new one. Secondly, what kind of gauge is he using? He'd better have the main caps torqued and using a dial-bore gauge. Lastly, when was the last time he calibrated his micrometer? They'll always say, "Last Friday" or "Monday morning," but did they really?? Stand there and watch them check the mic on the standards at both ends. There's a reason why metrology is a department at the DOE labs. Don't do anything to either the block or the crank untill this is straight. What does the book give as the correct clearance stock? Use that. Good luck.

James

[steve s]

don't use too much main clearance or you'll lose oil pressure really bad.
i would have the shop install the crank no oil and test spin the crank. it should spin freely if not you'll need the line bore.
or you check the clearance with the old bearings and then the new bearing for accuracy.
we usually do head only work after the overrev. once we had piston failure.check the valve spring retainers they usually cracks open on the underside where the keepers go.and then the valve drops causing the piston failure.
ps. i usually sleep at a holiday inn express b4 any major engine rebuilds,actually carlos sends me for the whole week to make sure i get the edge b4 the rebuilds or any major work on his car. :D