I have a damaged bore that needs boring to remove gouges from a broken valve. I have read in the Porsche workshop manual "Individual cylinder bores can be remachined as required, as the oversize pistons are of the same weight as the standard pistons"
I read this as that if needed only one bore can be done with a new piston - correct or otherwise? And has anyone done this?
Thanks

 

The problem is finding oversized pistons assuming you find someone who KNOWS how to bore and hone the block

 

Done and done, but can you just do the one damaged cylinder?

 

 

The oversize pistons are somewhat available, you just need to have the machinist bore and hone the cylinder to the correct tolerance group. Well, the group you can find available.

You only have 0.5mm oversize to work with as far as the oversized cylinders go. So if your cylinders are scratched beyond this I would look for a new block.

 

There are 2 oversize pistons available, 97.5 and 98.0.
​​​​​​​yes they are hard to get hold of, but some are still out there.
so that is a yes that you can bore just one cylinder?

 

or install a new alusil sleeve in the damaged cylinder.
Åke
http://www.pierburg-service.de/ximag...199_en_web.pdf
http://www.w124performance.com/docs/...nditioning.pdf

 

I had no idea that was offered. Hopefully they don't have the same faults that steel sleeves have and expand at the same rate as everything else.

 

I have a factory exchange S4 engine where all eight cylinders are alusil re-sleeved. The quality of the job is exactly the same a for an all new engine block. Same material, same expansion rate.
Åke

 

The factory workshop manual say it is OK to bore just one cylinder.
The broken valve, is that a stock valve or something else? How much damage did it do to the cylinder head?
I have fixed many cylinder heads damaged from broken valves, needs welding, grinding/machining and new valve seat inserts installed.
Åke

 

Get yourself a used short block or a used motor and call it a day. You will have many more things to address in addition to the bores. If a used motor is not an option, strip down the motor and split the block, then take the block, pistons, crank shaft, rods etc to a machine shop that is experienced with 928's and have them measure everything against the specs/wear limits. Until you know what type of wear you are dealing with it's all guessing, you could end up with bores that have excessive wear and may not even be salvageable.

If you do not have a machine shop that knows and has done 928 blocks, then your back to buying a used motor IMO.

 

That valve looks new. Was the head recently rebuilt?

 

BTDT,
Please consider the GREAT AND AWESOME option of getting a replacement short block.
Please.
But, if you do buy a used block in good condition, make sure before purchase that the
pistons are the same tolerance group as the bores.

LOL ask me how I know $$these$$ $$things$$.

 

Just throwing this out there.

I don't see where you give the engine dispacment but I'm assuming since you say oversized pistons are are 97.5/98 that the engine is a 4.5l. However, even if it's a 4.7l, since you have someone to do the work, if the gouge is too deep for an oversized piston why not get a piston from a 4.7/5.0 and bore your block for that. Just make sure there's enough meat on the cylinder and have the piston weight adjusted and the assembly balanced to match existing?

Another thought, my understanding is some parts (rings?) for the 4.5's are hard to come by so you may just want to get a new short block? Maybe cheaper in the long run? Check with Roger.

 

OP's questions:

Yes, you can safely bore one cylinder only to match the available piston you find. the WSM shares some of the details for the work on the bore, including the special honing requirements for the alusil surfaces. If your machine shop has done 944 and 968 blocks, they are familiar with the requirements. Know that more than a few good pistons have trashed and been trashed by poorly prepared bores.

That valve is truly a bit of art in itself. Looks like a keeper or retainer let go, allowing the valve to drop way down in the cylinder. Perhaps a bit of carelessness when working on or "assembling" a head whilst still bolted to the block? More typical 928 valve damage is from a timing belt mishap, leaving just the big end of the valve slightly bent over. The guide supports the rest of the stem, so it all stays straight except for he slightly cocked head.