Felix

What's RA?

On my recent rebuild the cylinders showed the original factory hone as subtle cross hatching with only a visual indication of the top and bottom edges of the piston travel area; nothing could be felt by touch. I would guess honing the Nikasil is not easy as it's very very hard.

autobonrun

I think he's referring to Roughness Average. There are several methods to measure the relative smoothness or roughness of a surface. RA is the average value of the peaks and valleys along a straight line across a surface. There are instruments that will measure this value. Typically you see the measurement in microinches, micrometers, or nanometers. Another method that was once used was the RMS (root mean square) method. I think there are a few other methods as well. I've always specified roughness requirements in terms of RA. If this is not the RA Geoffrey's speaking of hopefully he'll chime in with the right answer.

jimhsu

is top end rebuild a one-time thing or is it a major service that will be required every XX,XXX miles on all 993 Porsches?

If re-rebuild is needed every so XX,XXX miles, what is generally the figure for the "XX,XXX" miles?

And how would one know from daily driving when a top end rebuild or a re-rebuild is needed?

So many questions. I know....Searched for it and saw no info on the re-rebuilding issue.

vjd3

On the 993, top-ends generally get done earlier because the original factory valve guides are quite soft and wear prematurely, also contributing to the clogged SAI passages that can result in a top-end to pass emissions, even when oil consumption is tolerable.

The consensus seems to be to use a harder valve guide material when doing the top end and the motor will last a long, long time barring any other problems.

Aside from a check engine light due to the SAI passages, the usual rule of thumb is to go by oil consumption and/or leakdown and compression. The oil consumption can vary wildly from car to car, but if you're using more than a quart every 1000 miles, you've got a problem. (Factory, I believe, says 700 miles per quart is okay). Most folks get well more than 1000 per quart, although I had a 95 C4 that used a quart every 1000 miles but had perfect leakdown and ran beautifully.

Texas993

BTW, I started this string. As you can see in my avatar, I bought the car. It has 84k miles with a fresh top end rebuild with the stronger valves. Compression and leakdown numbers were very strong. I love it, thanks for your your help!

Slow Guy

vjd3 answered your question indirectly, there really is no xx,xxx mileage anyone can give that any particular car will need a top-end rebuild. I've read of cars that had it done at 15k and others haven't been touched w/ 100k +. My car is just under 70k, has not been opened (as far as I know) and judging from my oil consumption won't need it for quite some time.

To put it simply, there is no mileage that you can say you'll need a rebuild. There are too many variables.