924Board.org

[gla924sem]

Going to start putting the short block back together after getting it back from the "REBUILDER".

Want to check his work so there's no surprises......I hope......so I am going to "Plastigage" the mains and con rod bearing clearences.

Any one have what the spec should be for stock bearings?

[924RACR]

Offhand, I seem to recall that the clearance for "new" bearings is 0.0008-0.001", and the wear limit is 0.003-0.004".

OK, from the manual...

Con rod radial clearance, .02-.07mm new, wear limit .1mm (.0008-.003" new, .004" wear limit)

Main crank bearing radial clearance, .02-.08mm new, wear limit 0.16mm (0.006" wear limit).
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[gla924sem]

Thanks......knew either you or Dan would respond.
Am hoping those guys in Warren did me no wrong.

[ideola]

Fingers crossed for you, Gary.
_________________
erstwhile owner of just about every 924 variant ever made

[924RACR]

As confused as they might get, I would be very surprised.

Feel free to post back w/ your measurements, for feedback...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[gla924sem]

Will do.......got to find Plastigage first.
Hear NAPA carries it

[924RACR]

Yeah, I think I got mine at Murray's... may be a bit of a PITA to spot, but they should have it...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Martijnus]

Did they grind the crankshaft? Since there isn't really a direct oversize available for bearings I doubt that...

If they didn't do anything to the journals, they're totally depending on the quality of the supplied bearings.

If they did grind the crankshaft, I sure hope they all have the same clearance, because that's how I'd do it. Measure bearings, determine desired journal size per journal, grind journals.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Rasta Monsta]

[gla924sem]

Before checking on bearings......they reground the crank 10 under.....had to scrap that crank as none are available. They still billed me $110 for the regrind eventhough the crank was now junk. Would not budge even though it was their mistake!!!!!

Thanks to Ideola....he had a spare we pulled from one of his engines. Its been straighten and polished.

[924RACR]

Well, they'd have had to pay someone else to do the grind, so I can understand why they'd be unwilling to budge on that.

Then again, they scrapped your part thanks to not checking such things in advance...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[gla924sem]

Ideloa had a similar experience.....estimate vs actual cost at the end.

Did get some checking done last night on the block.......checked the new rings in the cylinder bore for end gap. All four came in well within spec.
Minimal wear on the bores even after I would say is 170K miles on that block.

Did not get to the mains or con rods last night. Looks like Thursday night for that task.

FYI-O'Rielly's/Murrays does carry PlastGage. They had it in two sizes.....red & blue.

Anyone interested in a 10 under crank?????
Willing to make you a good offer.....

[ideola]

[TJC]

Herein lies the problem with machine shops and their customer's (and certainly no offense meant to the OP or anybody else) who rely on building an engine to "Tolerance" vs "Specification"

I'll limit my comments (maybe) to rod, rod bushing, and main bearing clearances, but many others apply.

Every engine whether it be built for "street" use or "race" use has an optimum specification within the manufacturer's tolerances to which the clearances should be held to. This even varies, for instances in a race engine, if the engine is to be used as a "qualifier", set up for "sprint" distances or "endurance" purposes. Again, for the proposed purpose the theory remains the same.

To achieve this result requires extra time and expense, but the results are enormously beneficial.

A short list of what should be done is as follows:

The machine shop should be provided with (at minimum) the block with main caps, the crankshaft, the rods, the piston pins, the rod and main bearings, the rod bushings and the specifications to perform the proper machining for the intended purpose...although I would personally shy away from a shop that did not have this knowledge for the given make, Porsche in this case, myself! And ALL components must be checked for viability in a rebuild!!!

Starting with the crank, it should be magnafuxed, checked for straightness, scoring, dimensions, etc. If it needs to be ground, that should be determined prior to buying any parts!

Then with the block, the main bearing bores must be checked to see if they are within specification, NOT tolerance, and be corrected by align honing or align boring.

The connecting rods will probably need re-sizing and re-bushing. This also should be done to specification, NOT tolerance, and the rods should be corrected to achieve equal center to center lengths as well at this time.

The bearings for the mains and rods should be installed, DRY, into the block and rods and the bore diameter should be measured...preferrably with a quality dial bore gauge.

The crankshaft, after polishing (if needed), should be again measured and then those rod and main diameters may be subtracted from the previously measured bore diameters to determine the bearing clearances.

If adjustments need to made, (albiet in the slightest amount) the rods may require additional resizing as well as the main bearing bores. This is required if measurements (preassembly) had not been done prior to machining initially.

There is actually a lot more work that is involved if attention to detail is important to you, but what I briefly explained is a small part of the blueprinting process that takes place in building a quality engine.

Also, I personally would chuck the plastigage idea, it may be okay for a Briggs and Stratton lawnmower engine, but for an engine that may see 6000+ RPM I think it is useless. If nothing else is available get yourself (or borrow) a good quality dial caliper that reads to the 0.0000" and measure your bores (with bearings installed of course) and crank diameters and get a truer reading of your bearing clearance.

Let me stress that these type of procedures are not only important on race engines, but street and tow vehicle engines as well if you desire longevity and decent performance. We've all heard about the factory built engine in a particular vehicle went forever and performed just great (no matter what the make) and that it must have been built "midweek" or was in the "sweet spot". This is one way that you can achieve this when rebuilding your own engine...no rocket science or magic required!

One quick question since your're going through this whole rebuilding deal...did you have the assembly balanced?

Oh, I should add that of course that the rod and main bearing caps must be torqued to spec when taking your measurements. The rods should be torqued while being in held in a rod vise preferrably, or if that is not available one can use a normal vise, with soft jaws, if it is large enough to grip the rod without interference from the vice "screw". The rod should be clamped very near the parting line. It is not generally advisable to torque the rod cap to the rod without the vise as it may be possible to twist the rod if excessive force is exerted upon it.
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'02 Porsche Boxster..."Sunbeam"
'04 BMW X3..."Xander"
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www.ttrs1.com

[Rasta Monsta]

^great post
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• WeiBe (1987 924S 2.5t) - 931 S3