Critical Crankcase Tolerance
 

Hi guys just had a call from the machine shop regarding my crankcase bearing journal specs. The journals are only 2 thou of an inch out of spec which really in my opinion is not a very large gap. I have to decide whether to now go ahead and have the cases line bored oversize or to put up with this tolerance? It is going to be a very expensive exercise if I go the line bore route.

Can the machine shop resize the case? This is done by machining the faces of the case flat a few thou, then re-lineboring. This is usually more cost effective than buying oversized bearings.

Not sure on the critical diemnsions, but you may be able to coat the back side of new bearings to tighten up the tolerances, as well.

Steve Weiner, where art thou?

2 thou. out of round or out of alignment? What kind of case? Mag or Al. What is the shop recommending and who are they?

The case is aluminium. They are not out of round they are oversize by 2 thou. The problem is that the shop has never done a 911 case line bore. I am in Cape Town South Africa and there is one shop who are willing to look at the job not even taking it on. My only other alternative is shipping the cases to Johannesburg and having a reputable shop there do the work, the cost for this however will be in the region of $1200-1400 which is a lot considering the amount out of spec.
Considering that 2 thou is only 0.05mm is this really worth it and oversize bearing step up in 0.125mm increments unless I am mistaken?
Also the crank is within tolerance and from looking at Waynes book the clearance range is 0.010- 0.072mm between bore and shaft I assume this is with bearings installed?
Oh and just to add there seemed to be absolutely no play in the crank prior to tearing down.

Davy,

JMHO, but these engines are not for the inexperienced,....:) :)

When they measured the main bearing bores, did they use the #8 nose bearing as one of the reference points?

If they are all .002 oversize, you should line-bore the case properly for oversized bearings unless you can source a case in better condition. Oversized bearings are about $1250/set so finding another case would be cheaper,...:)

Steve yes they used the #8 nose bearing as a reference point. I appreciate that 911 motors need experience to work on and thats why I am searching for the best local and of course experienced forum members best advice.

That will account for the discrepancies that they saw,....:)

To date, I've rarely ever seen the need to line-bore a late aluminium case, even in the 700+HP 935's. :) :)

Steve well then that will account for it I believe. I have called them and asked to re-do all the main bearing journal measurements and record them so that I can take further advise.

The only instance I've ever heard of where a late Al case was oversize was caused by a spun main bearing. As Steve says, these are rare. I would have the shop remeasure each bore at 90 deg intervals and look to see how much variability there is in the measurements. The measurement variability should be less than 20% of the wear tolerance to ensure reliable data.

If they are using micrometers to measure, get two measurements as suggested, one at 0 deg, second at 90 deg.

The other option is to use plastigage. Install the bearings in the case, install a strip of plastigage on each journal, then install the upper half of the case per torque pattern and torque spec. Then remove upper case and compare readings.

Well happy to report they re-measured at 45 deg intervals as specified by myself and the variance on each journal is negligible to say the least. The greatest variance is 2 thou/0.05mm and that measured in line with thrust trajectory of rod to mid cylinder bore as can be expected , the rest of the circumference ranges between 0 and 1 thou/0- 0.03mm. Given this information I am happy to replace main bearings and call it good. This re-build is purely for street use and if I can get another 50,000 miles from it then I am happy.

Update: I eventually got my case back. I was still sceptical of the measurements they gave me so when I got it home I checked myself with my own micrometer and the results below speak for themselves everything is within specification. I reckon the dude who measured could not read and only put 1 zero in the reading:

(http://forums.pelicanparts.com/uploa...1274887080.jpg