Kandlhoft

Hello fellow Rennlisters,

I've acquired a 4.7L 16v engine that overheated and warped the block and heads and am trying to determine if it's worth rebuilding or not. I've removed the pistons and the engine has been torn down to the bare block. Here are my findings:

A) Coolant was found mixed in the oil.

B) With a straightedge and feeler gauges, I found the block and cylinder both heads to be warped up to 0.15mm. (WSM tolerance says max distortion allowance 0.05mm, so all of these surfaces will need to be machined).

C) I measured each bore for roundness. All of the cylinders measured within this spec both midway and to the bottom of the cylinder bores. However, several of the tops of the cylinders measured as much as 97.155mm. (WSM for a 4.7L US spec motor, the bore should measure 97.0mm. Cylinder bore "out-of-true" to be 0.010 and a wear limit of 0.020). 97.155mm would put the engine bores "out of true" but just within the wear limit. Does this mean reboring will be required if I want to rebuild this engine?

D) All the bores surfaces look smooth with the exception of one small nick that my finger nail catches on. Is this nick big enough for concern?



F) All of the pistons and rings look more-or-less like this-


Thanks in advance for your expertise and opinions,
- Jake

The Forgotten On

All 928 engines require a rebuild if they haven't already been.

That engine does seem savable, the question is, will it be cheaper to get another engine and rebuild it or do the one you have?

You need to check and see if the bores for the cams and crank are out of true.

That will determine if it is beyond saving or not because right now it just seems to need normal machine work that any 928 block will need at this point.

Worst case you would need a couple of oversized pistons , a bore and a hone on those cylinders (Porsche states you can run a mix of 97.0 and 97.5 pistons in the same engine).

Mrmerlin

I would look for another engine,
whatever your next engine is you will still be pulling the heads to do HGs,
unless you find a good engine thats already had this performed.

NOTE the machine work required to get the block fixed and buy new pistons could easily pay for another engine that will run.
( and first you must know a great machinist that works on 928 engine blocks)

Take the engine you have and get it powder coated make a coffee table out of it and sell it on BaT.

Shirah

Maybe rebuild something like this............ no affiliation
and insert in your car.

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