Questions about 911sc rebuild

[stevebmd]

So as part of my buying process, I was planning on having a PPI by a reputable porsche specialist prior to my final offer on a 1978 911sc with about 50k miles on it. it recently had an extensive service performed including top end rebuild, head studs, cylinders honed, heads machined, and "valve job", reconditioning of 5 out of 12 rocker rocker arms, tensioners, new oil lines, cooler, etc.

The question has become whether new valves and guides should have been done while it was open, and why only 5 out of 12 rockersarms where done. The machine shop said the valves where fine lapped them and reset them. Should I be concerned about this if the PPI otherwise checks out...? Is this rebuild ultimately gonna need new valves in the near future if they weren't done, particularly the exhaust valves?

[Flat6pac]

In spec, is In spec
Bruce

[snbush67]

No you should not be concerned, in fact you should feel good in that at 50 k the engine was serviced. It is likely the motor was barely broken in and now you have fresh cylinders and in spec/resurfaced top end. Enjoy.

[stevebmd]

appreciate the input. BTW, nice avatar...Lundbergs car was for sale on the bay a few weeks back

[Gordo2]

Originally Posted by stevebmd

... it recently had an extensive service performed including top end rebuild, head studs, cylinders honed, heads machined, and "valve job", reconditioning of 5 out of 12 rocker rocker arms, tensioners, new oil lines, cooler, etc...

If you are confident that the rebuilder has a good reputation (i.e. not MM...) and the PPI turns out well - roll with it.

Good luck,

[The cap'n]

As a long time Porsche tech, I question the wisdom of honing 911 SC cylinders of either variety.

The Cap'n

[stevebmd]

Cap'n please educate me as to why?
Also, why lap and re set valves and not replace the springs guides etc while its open? Obvi its a cost issue but, the receipts are at 22k for all the work done. Which all seems high for a car that sat for 16 years, but maybe thats why..

[The cap'n]

Alusil cylinders are relatively soft and require special honing techniques. Even then, there appears to be a pretty long history of anecdotal reports of failure of the rings to seat, and in some cases even breaking (poor honing could be the cause of the latter). Pistons, which are plated with a ferrous material, are not available (as in, they never made them) in oversize. Nicasil cylinders are plated and pretty hard. The coating is pretty thin and easily penetrated. The pistons are relatively soft. There are places that can bore and plate both varieties with Nicasil, but you have to source pistons in oversize.

As for the guides, valves, and springs, if the valves are well inside the maximum allowable stem wear and the valve heads can safely be cut, I would reuse them. If the guides are also well within tolerances, I'd probably let them go, too. Some engine builders prefer an aftermarket seal, and for those you have to replace the guides. If the springs are good, I would reuse them, as well. Once they're set to the correct tension, they should be fine. There was a period in the mid 70s where we saw a lot of broken valve springs (I saw mostly intakes) and that might be the only reason I'd change them. Maybe Porsche got a bad batch, or changed the specs and it didn't work out, I dunno.

Last, but not least, I think $22K is awfully high for a rebuild without new pistons and cylinders and all new valve train components. I dunno what the labor rate is where the work was done, but it's about 55 hours plus machine shop services, and then parts. Where I worked, the prevailing labor rate was $100-125/hr. With machine shop charges the total labor would have been right around $8K. That leaves $14K for parts! Some bearings, a gasket set, couple of valves, a ring set, some hardware, oil, filter, and small stuff. Maybe even a clutch. Nice profit!

The Cap'n

[stevebmd]

appreciate the tutorial, and i figure it had to do with the metallurgy. I too figured the rebuild costs were high, but that included back date to SSIs, chain tensioners, new carrera cam, going over the fuel system (sat for 16 years), new dansk muffler, head studs, msd ignition, new oil line and oil cooler, brake booster, and going over the brakes r/r, but i believe it was 10k labor!! and the rest were parts and sent out stuff for injectors/new intake manifold

Lucky i didn't pay for it. He also did the heating and cooling system, with r134 conversion with new compressor etc..

Still needs targa refresh, but had new seals put on, albeit poorly and the RTV silicone didn't stick…

Again thanks

[snbush67]

22k is crazy high for a stock top end. Especially without new P&C's.