Checking main clearances

[boosted79]

I'm reading Wayne's book in prep for a rebuild on my 79. He doesn't plastigage the mains. I've never built any engine, except with roller cranks, w/o checking the main clearances. First rule is to "assume nothing and measure everything." Any reason why these are any different?

[Tippy]

I'd admit I didn't plastigauge my mains as I measured the crap out of the mains and then measured my new bearing shells to the ones that came out.

They were the same dimensionally and the crank mains were spot on, so I carried on.

Crank turned over smoothly, so it was a go.

[boosted79]

Seems like it would be easy just to drop in the crank dry and plastigage each main journal then drop on the other case half and then torque down the through bolts, remove and check. Simple. No need to torque the perimeter nuts.

[T77911S]

just a guess but might be because these engines are built to a tighter spec than say an american V8. if you sent an american V8 to a machine shop to be blueprinted it would come back at the tolerances of a porsche.
just based on stuff i have heard.

i was going to do a top end on a 3.0, a friend told me might as well put bearings in it. i asked about machine work and all that. he said just put them in. i sold the 3.0 and bought a 930.

[boosted79]

It doesn't matter how tight the specs are, w/o checking it you don't know what the actual clearance is. Even if you don't have the crank ground and it is within stock specs you are assuming the new inserts are made to the correct dimensions and the final assembled clearance will be within specs. Chances are they will be but the little bit of time it takes to determine the actual clearance is well worth it IMO. It's probably a half hour at most to plastigage it.

[ficke]

Plastic gauge is for people who do not have measuring tools, mic, calibers, bore gauge, etc.
It is a more difficult to use plastic gauge with a engine design like a Porsche 911 where the main bearing or the case half's and not individual piece that can be unbolted with 2 or 4 bolts.
I would go through the steps and use plastic gauge if I did not have a way to accurately measure stuff.
To much is riding to making assumptions.

[Tippy]

I don't 100% agree with Plastigauge being for those who don't have the measuring tools needed as variance in technique leads to different readings.

Plastigauge takes out a lot of the variability.

[boosted79]

The point is the book makes no mention of checking the main clearances and that is a critical factor in assembling any plain bearing engine IMO. It assumes the new main bearings are dimensionally correct for the journal size. It goes into detail in checking the rod bearing clearances but simply says to check the new main bearings to make sure they are the correct size size ie what is stamped on them? If you are sticking with the same bearing size then doing what Tippy did, measuring the thickness of the new bearings and compare to original will at least verify you have the right bearings. I like using plastigage to check the clearances, it minimizes the chances for error.

[ficke]

Yup, books have mistake in them, not just Wayne's.

[boosted79]

It is still a GREAT book, the info in the appendix is worth the price alone.

[jwasbury]

If you search these forums on this topic you will see there are at least a few threads on the topic. The consensus view is that if your crank measures in spec you're good to go. Perhaps the book focuses on rod bearing clearance check because the assumption is you will have the big ends of rods resized by the machine shop, and you need to check their work. This would also hold true if you had the case line bored which is recommended for the old mag cases.

Perhaps not a huge risk if you don't check it, but no doubt there is no way to be sure unless you verify with actual measurements. Given the quality problems that have been reported with the glyco bearings (rod bearings, mains supposedly ok) it's not a bad idea.

[boosted79]

Agree it is not a huge risk for a std. but it is so simple to check with plastigage it seems like a shame not to. It tells you where you are in the clearance range. If the crank has been ground I would never assemble the engine w/o checking the clearances.