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Alex Roy · Posted: Sun Apr 18, 2004 3:27 pm Post subject: polishing valves

I am rebuilding the 931 engine from my parts car to go in my white '79 924. I got the head all taken apart. The valves were covered in carbon builid up, so I cleaned them off and then polished them to a mirror finish (both intake and exhaust) by putting them in the lathe and using 500 grit sand paper, then 1000 grit sand paper, then rouge and a polishing cloth. I then re-lapped them into the head for a good seal and they look better than new and the lapped area looks perfect with a good, even 1/8" sealing area evident on all valves. It was either I polish them to fix them up, or trash them.

I wanted to know if anybody thought this might be detremental to the valves, I.E. removing any kind of hardening they might have, if they do at all, or if this would be a good thing for performance since they will be less likely to build up carbon deposits in the future.

Let me know what you think. I'll post the before and after pictures when I get a chance.

Lizard · Posted: Mon Apr 19, 2004 3:43 am Post subject:

I doubt you would have caused a problem, but what happened to the electric idea? and what has happened to your BAE kit?
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3 928s,

Alex Roy · Posted: Mon Apr 19, 2004 5:43 am Post subject:

bae kit sold over a year ago. Electric is on hold for money reasons (need about $5000 more in parts). In the mean time, I got a 931 parts car without a title and a rear quarter panel pushed in, otherwise in great condition so I thought I'd make the '79 drivable with the 931 drivetrain.

Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

The only thing you have to worry about is to not get them too hot because this would change the temper.That being said however,as they are supposed to spend their lives in a hot cylinder I would say you've got nothing to worry about.If you have access to a glass bead cabinet I would blast them gently on the flat part,this induces artificial stress and actually helps them to not crack in the future.It's the same with piston,rods,etc.
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1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

Paul · Posted: Mon Apr 19, 2004 8:36 am Post subject:

I used to do valve jobs for a living. Nothing wrong with what you did, but next time you could just use a wire brush on a bench grinder. Just don't use it on the sealing surface or the part of the stem that comes in contact with the valve guide.