Hi All

I have my heads removed to try to find reason for low compression. Bank A ave 95psi and Bank B ave 115psi. All readings in each bank were consistent. The car is a 1972 roadster from US so low compression engine. 60K miles on clock. The heads had fairly worn exhaust valves and guides so might account for some loss of compression....(BTW Ive never managed to find out what compression this engine should measure)

The bores look good with honing marks visible and measure as v little wear.

Ive removed a piston and rings measure 4,03 mm wide ie v little wear.

However if I insert eg top ring halfway down bore Im measuring a piston ring gap of 27 thou (manual says gap should be within 14 - 20 thou)

This dosent make sense to me...

Any explanations anyone?!!

Thanks

David Offord

 

My 1st thoughts:

140ish is a bench reading for that engine, and most PreHE.

Lack of use, too much starting to shift the car, OR, to see if it runs OK, that will kill a V12 as quick as downing a Cold beer on a Hot day.

The liners a Super hard, as we know, and ring wear you mention is consistent with that "start and see" I mentioned above.

Worn valves and guides, again, that running is the prime issue, as the oil never really gets HOT, and into all the odd places it would when driven as designed, AKA, Redline regularly.

A set of valves, guides, rings, and drive it as mentioned.

All my PreHE are well over 400k Kms, with one at 860K, and the core engines are all as made in Coven657. Same with the He beasts.

 

Thanks Grant...interesting....I suspect that is exactly how the car has been used since 1981!

As the rings appear not to have worn....... is there any point in replacing them?

 

Exackery as Grant says . . .
AKA, on occasions, "Drive it like you STOLE IT!"
(perhaps that's the reason guru Grant has had so many of the big Cats?)

Bore hone marks and ring gaps "like new" are indicative of an engine short of work under load.
Seriously, these V12s are well balanced and work smoothly to the red line. This can place you in a further dilemma . . . finding a suitable road/track on which to unleash the beast. Keeping in mind that it can take longer than coolant to heat lubricants, take your time to get the engine oil hot. Acceleration can then be undertaken in lower gears. Similar in top gear can get hairy at top speed . . . but "work" is not about top speed . . . it's about hard acceleration.

Our preHE XJ-S FHC has now joined our Daimler V12 VDP (Series II) in going to a new home . . . but the love remains!
Best wishes with your beastie, Schrodinger.

 

Once piston rings, and bearings, have been disturbed, AKA removed, NEVER reuse them.

Bag of worms I know, but my Engine rebuilding training from the 60's, POUNDED that into us, and I will never change.

What price a set of rings and bearings, and other "stuff", compared to removing the engine and stripping and doing it again in 6 months time, those maths simply do not add up.