930 rebuild, Questions after measurement

[Htegenbo]

Hello, I'm rebuilding a 930/66 engine, and just have measurements results. There are two things worrying me, and in all the data I can find in books they all referring to factory spec.
1st is the piston to cylinder clearance, this is in all 6 cases around 0.1 mm, the ovality of the cylinders is worst case 0.023mm. specific measurements are in table below.
Cylinder vertical horizontal ovaliteit
1 97.060 97.083 0.023
2 97.075 97.080 0.005
3 97.073 97.082 0.009
4 97.075 97.070 0.005
5 97.068 97.080 0.012 has a scratch due to broken piston ring
6 97.065 97.075 0.010

Pistons are:
1-96.980
2-96.980
3-96.970
4-96.970
5-96.972
6-96.978
all measurements in mm-ers

What to do best, let it get a new layer of nikasil and re-hone? or leave it as is?

2nd is the crank-case bearing bores.
1 Horizontaal 65.025 0.020
(flywheel) Verticaal 65.045
2 Horizontaal 65.040 0.015
Verticaal 65.025
3 Horizontaal 65.040 0.020
Verticaal 65.020
4 Horizontaal 65.038 0.028
Verticaal 65.010
5 Horizontaal 65.015 0.085
Verticaal 65.100
6 Horizontaal 65.005 0.085
Verticaal 65.090
7 Horizontaal 65.000 0.120
Verticaal 65.120
8 Horizontaal 65.140 0.100
pouliezijde Verticaal 65.240

Is the only way to solve this to re-bore and use over sized bearings? I read in "how to rebuild and modify a 911 engine" that that was not advised and to look for another case... I'm a bit confused here.

Is there any-one with good advise? thanks in advance,

Harold

[stownsen914]

Hi,

For the crank bores in the case, if you find you have to line bore, you may have another option besides oversize bearings. Some machinists can cut down the mating faces of the case, and then line bore back to standard (so no oversize bearings). If any clearances are way out, the case can be welded before the line bore to give it some extra meat. I had this fix done many moons ago by Walt at CE on an aluminum 2.0L case that is in my 2.7L 914/6 track car. I've never had any issues with it. The machining wasn't cheap, but it was probably cheaper than a pristine crankcase would have been.

Scott

[Eagledriver]

For the pistons, .1mm is at the limit for wear. How are the ring lands and ring end gaps? You might have to get a different set or custom pistons. If it is just the clearance that is a problem you could try an anti friction skirt coating. That will close up the clearance by about .05mm. Don't know if that coating will be a long term solution. I've used it on my race motor with good result for 2 years now.

-Andy

[Htegenbo]

thanks guys, today I have the cylinders re-measured on a 3D Zeiss cmm. I want to have a second opinion, and the previous measurement was done in 30 deg C... (I just found out).
Still don't have a clue about the case. Lapping the case halves does not give me enough meat to re-bore and I'm skeptic about "welding" meat on. Because of the stresses introduced. For now the only option seems to be the over-sized main bearings. Wayne's book says this is not preferable, what is the disadvantage of these bearings besides price and availability? does anyone know? thanks again