warped 1972 magnesium case

[robmog]

starting the rebuild, disassembled the engine.
most things in spec needed a head for broken fins, several cylinders were pitted.
mostly small stuff and i'm planning on doing the suggested upgrades
checked the case for warping with a straight edge and found it was .002" out at the bearing surface.
does the need to be flattened and align bored?
or (no flaming) would the case bolts pull it together?
i'm planning on sending it out for casesavers and the oil pump upgrade anyhow.
thanks,
bob

[Steve@Rennsport]

Bob:

The BEST thing you can do with that case is have the case halves machined flat and the main bearing centerline bored back to standard.

This is VERY common with those things and the only way to fix it.

Depending on displacement, cams, and usage, I would have it shuffle-pinned as well.

[robmog]

thanks steve.
i'm doing a stock rebuild, so was not planning the shuffle pins.
bob

[Henry Schmidt]

Steve is correct about the best way to proceed but my question might be "how did you measure the case?" Laying a straight edge in the saddles of an open case will hardly give you an accurate measure of main saddle straightness.
The case must be bolted together and properly torqued to give you an accurate measurement.
If you do this and only have a .002 variance, run it and enjoy.

[robmog]

henry,
the case is split and a long straight edge went from end to end.
the saddles were .002" shy of the edge.
thanks,
bob

[michel richard]

This may be too obvious, but how certain are you that the straight edge really is straight ? Not that I know how one would check, mind you.

[tom1394racing]

Henry or Steve

Couldn't one check the case flatness by putting plastigage on the bearing saddles, torquing the case down and then checking the plastigage thickness for variation from saddle to saddle? Wouldn't this, combined with bore gage checks on the case bores and checking the crank rotation resistance with the bearings in place, be a good rough check for case flatness and line bore?

[911pcars]

Originally Posted by tom1394racing

Henry or Steve

Couldn't one check the case flatness by putting plastigage on the bearing saddles, torquing the case down and then checking the plastigage thickness for variation from saddle to saddle? Wouldn't this, combined with bore gage checks on the case bores and checking the crank rotation resistance with the bearings in place, be a good rough check for case flatness and line bore?

Tom,
That would be an indication. However, if the crank is straight, fine, but one would have to confirm that first. Then you'd have to discern whether any difference is due to improper oil clearance, case warpage or main bearing journal irregularity. Better a machined straight edge or a dial bore gauge inserted on-axis.

Sherwood

[robmog]

i used a machinists' straight edge, true to 0.0001".
i measured diagonals and straight across.
i was surprized at the consistency and wondered if the case came from the factory that way.
if you think about it, undersize at the saddles would make the perimeter "tighter".
would this decrease the chance of an oil leak?
just thinking out loud, still going to send it out.
bob

[69BahamaYellow]

I had a similar problem with a 1971 magnesium 911T case. Ground and polished the crank to perfection, fitted new oversize brgs and all was fine until the last 10 ft-lb's were applied to the main brg bolts; then, the crank locked tight. I tore it apart and re-assembled with plasti-guage to find a few brgs. (.0005") tight. I found it hard to believe that only that much made such a difference. I took everything out of the case, bolted it back together at specified torque settings, and took it to a NASCAR engine shop in NC, where they did a line-hone on the case for $300. This proceedure solved my problem, but the shiny surface pattern left by the hone on the dull grey bearing saddles surprised me. The 2 tight bearings had very little marking, while others had circular patterns at various locations. I think hundreds (or thousands) of heat-cool cycles on these old magnesium cases causes the metal to creep in strange ways, but probably not enough to justify milling the case halfs and line boring. Moral of the story: If you can find someone who does line-honing, make it a standard proceedure when rebuilding a magnesium case motor. The process removes so little metal, you can use standard bearing sizes.

69BahamaYellow

[jsbc]

if you machine the case halves not only will it have to be line bored but the oil pump drive shaft will also need line boring go with bh yellow if you have a problem i put mine back together without even checking goes as good as new with its new bearings etc etc