help with polishing bearing surface

[hillelson1]

hi i had a rod bearing failure and it scratched my crank they are not very deep i dialed the crank in and it is within spec. low mile engine. how should i polish the crank? should i use a wet sand paper like 1000grit or so? i like to do all the work on the engine myself because i am anal like that. my friend has a good selection of tools that he would let me use

[afterburn 549]

Even so you need to send it to a proper place for polish....not expensive and they can evaluate the damage....check the engine board there is a like question there....

[hillelson1]

how come it is so important to let a shop do it. it can't be that hard. i have done it on high horsepower outboards and had no problem? i have to tool that you put around the lobe so the force is even. i just want to know what polishing material i should use

[911pcars]

Can you chuck the crank in a lathe and polish the journals? The journal OD must be evenly treated.

If these are rod journals, can you chuck the crank in a lathe to rotate around the rod journal axis?

If the goal of polishing is to remove the scratches, then you're removing material from the journal, however minute. Verify you'll still be within clearance spec. or else you'll have to regrind and polish to the next standard undersize or reweld the crank to bring it up to standard dimensions.

If you're okay with the above, then proceed.

The machine shop guys will probably have more to say about this.

Sherwood

[davidppp]

Hello there.

Even if within spec, I'm afraid the jouranl may well be oval..much worse than a round journal a bit too small..
Check very very carefully..youll be polishing for evr with 1000..id start with 400 at most, and work up..

What tool do you have?

I'd like one..

Kind regards
David

[hillelson1]

dave thanks for the input now i checked the lobe from three different points with a micrometer and it all read fine. with the polishing take much of the metal away if i use 400 grit. and should i use paper or should i use a cloth backed sand paper. the tool i have i have no idea where my buddy got it it looks kindy like a nut cracker and puts equal pressure around the whole surface

[davidppp]

Hello there.

I would not use anything like that if I were you..

You need a lapping tool which rotates and can be moved alomg the journal.

Or, use the normal polishing belt the crank shops use..

Kind regard
David

[911pcars]

I realize you're probably doing this "in the back yard", but please don't resort to "back yard" machining techniques. Or maybe you think this procedure is perfectly okay and "good enough", a relative term, btw.

I suppose you might do what you planned to do anyway, but since you posted the question ..... don't.

MHO,
Sherwood

[Nine9six]

Grind undersized, hard chrome plate, grind back to spec size.

[hillelson1]

how much does that cost and who can do it?

[afterburn 549]

polish is cheap !!

[hillelson1]

how much is it to grind then chrome plate then grind to spec?

[911pcars]

"how much does that cost and who can do it?"

Have you ever used a search engine like Google.com? Type in these keys words, "crankshaft service" and look for someone in your neighborhood, then call and see if they can handle it. There may be Porsche specialists in your area too, so add the word, "Porsche" to the search.

BTW, upon the advice of many on this board, you may want to avoid the first company on the hit list.

Sherwood

[Jeff Alton]

If it only needs a polish, it is CHEAP. I get all mine inspected, measured, polished and final cleaned for around 85 CDN. Even if it was 5 times that much I would never attempt without the proper tools.

Cheers

[ianc]

polish is cheap !!

Porsche crankshafts are not.

ianc