Tolerances and wear limits

[BK911]

Gentlemen!

In an effort to learn as much as I can (and pay a machine shop as little as possible!) I would like to learn how to inspect my engine components. The tech spec books have some useful info, but I am not sure how to use it. For instance, there's a chart for measuring the crankshaft. This chart gives Bore and Shaft measurements which are self explanatory. But there are also clearance and preload numbers. What do these numbers represent? Are they the difference between the actual Bore and Shaft dimensions for a particular bearing? Also, how can I verify my crankshaft is straight? Are high dollar special tools required? How do you check to see if a line bore is necessary? Can anybody recommend a good book or manual for any of this?

Thanks!
BK

[rvanderpyl]

Clearance is checked with plastigauge which is basically a putty like substance of a specific thickness. You put in on the bearing face, install the crank, install and tigten all bolts to specs and then disassemble. You then measure the width of the squashed plastigauge and a table you get with it tells you what your clearance was. You can do this with the crank, conn rod big end and layshaft bearings. Save your old conn rod bolts for this test since you don't want to ruin your new ones.

You can check your crankshaft by making a holder which basically cradles the end crank bearing journals. Then you put a dial gauge on the centre journal and rotate the crank. That will give you the runout. Use something smooth and hard. You don't need to create a curve to match the journal, just make a V to cradle it and try to make sure you get them both the same size and as level as possible so the crank doesn't move back and forth. That will let you know if the crank is bent. * NOTE you have to make sure that your journals are in spec first (round) or this won't work.

You can do the same for camshafts and any other rotating object which has more than two bearing surfaces or at least three smooth surfaces which are supposed to all be in the same plane and round.

[911pcars]

The setup you need to check runout on a shaft is a dial indicator and V-blocks. While a dial indicator might be in your tool box, V-blocks are usually found in a machine shop. The purchase price isn't that much ($30-$50), but unless you're doing many, the guy who's going to do the machine work might check them for you for a few bucks.

Sherwood Lee

[Roland Kunz]

Hello

The plastigauge in the used bearings will proof nothing except there war.

On Porsche engines ( Or general on aluminium engines ) you have to messure the absolute bore and not a realatet to other bore ( chainmessurment.

The thing is if your bore is out of round it still can be within the wear limit.

If you don´t have a chance to messur working with plastigauge is better then nothing also preassemble the mainbearings and the crank in the case an "Feel" how good it will rotate while yourtourque down the case.
Many 2,7 engine rebuilders will know what I´m talking about.

Grüsse

[RoninLB]

IMHO-if the engine has over 100,000 miles you need new P&C and other machine work. There are a few good Porsche machine men in US. They don't want to hussle you. Find one, I did.

[rcwaldo]

If you dont want to purchase V-blocks, use one of your crankcase halfs with the #1 and #7 main bearing installed (only 1 & 7), set the crankshaft in and use a dial indicator on the center most journal to measure runout.

Chris

[BK911]

Gentleman!

Thanks for all the input. I found a decent book on engine rebuilding that covers V-8's. Not Porsche specific, but a lot of the same principles apply.

Thanks!
BK

[Jim Sims]

I strongly urge you to obtain some additional references; at a minimum: Bruce Anderson's 911 performance book, a Haynes 911 manual, the technical specification book for your year and model and Waynes book. There are many issues and important steps in doing a quality 911 engine rebuild. It's very different from a cast iron V-8. Ruin some parts with the wrong prodedures and you'll easily spend more than the cost of buying the above books. Jim

[rvanderpyl]

I assumed that it would be understood that you had to use the new bearings.

I don't know of a way to measure the straightness of the crank bore with home tools so the bearing measurement is a pre-install, after line bore to make sure that you do not have an out of spec set of bearings that don't give the correct clearance.

Maybe Porsche bearings are always well enough made this is required but I have certainly run into American bearings that were out of spec.

Any suggestion son how to measure the bore??

[BK911]

Jim!

I hear what your saying! I do have the Haynes, BA's book, the tech manuals and the factory workshop manuals. I've done several engine rebuilds already but I always send out the case and crank for inspection. I am trying to learn how to do this procedure myself. The V-8 rebuild book describes some inspection procedures that could be utilized for any type of engine. I think I am going to start hanging out at a machine shop to see what tools and methods they use.

All: Thanks for all the help and advice!
BK