I am trying to determine the CR on my 1971 5.3L V12 engine and am not sure what it is.
It is engine number 7S1801-SA and the pistons are dished as shown in the pic below.
The CDRom lists it as 9:1.
The head is a flat head design with no dish at all other than the light recessed areas for the valves.
Any ideas? Is it truly 9:1 or one of the lower CR versions?
The pistons do have a part number on them; however, I didn’t get a picture of them yet.
I’m pretty sure all 71 v12s were 9:1 but I suspect others here will know how to confirm. By 73, I think they were all 7.8:1, which unfortunately is what I have. My brother had a 71…there is a noticeable difference in power.
We really need a collection of photos of piston crowns from the pre-H.E. As I understand it, you can visually tell which is which by the diameter of that recessed area, with the lowest compression pistons having a recess that nearly covers the entire crown.
This is what SC parts shows as a 9:1 piston:
Do we trust a parts house photo? Too many of those places just show a generic picture of a part.
And if that’s a 9:1 piston, the one shown in the engine above must be a 10:1.
Not sure we should trust a parts house photo…but also not sure that I’ve ever seen one like the OP shows.
I have some from my engine that cleaned up nicely. I will take some measurements and post a better image this evening.
Having a image database of sorts makes sense.
The OP’s photo looks exactly like the 9:1 pistons from my 71 E Type. As I understand it, 71 and 72 had 9:1 and 73 and 74 had 7.8:1. The only factory 10:1 pistons were in the XJS for the 1980 model year.
Here is one of the pistons that has been cleaned up. These are all rough measurements.
There is a bevel right at the start of the dished area. It is approximately 2.650 across the top of the dish (Pay no attention to where I placed the line)
The bottom of the dish is approximately 1.9" across.
The dish is approximately 9/32" deep.
That dish looks a LOT deeper and greater than this one from a Euro/Rest of World car:
The dish is about 0.31" deep at the centre and the part number is C39908S which I can’t find on Google or Barratts or XKs. It may be the unobtanium 10:1 1980 flavour.
in this video, at 1:11 it shows what purports to be a 9:1 piston and 7.8:1 piston, side by side. Hard to tell the visual difference.
The picture below of the used piston is from my 1974 V12. The bright piston in the liner is sold as a 9:1 by Welsh. I do not have the measurements, but my mechanic said the center was definitely less scooped than my Original for the US market.
Citation from: http://www.jag-lovers.org/xk-lovers/library/engine_numbers.htm
Note that for V12s 1971 through 1979, LA and LB suffix was 7.8:1 CR and SA and SB suffix was 9:1 CR.
Regards, Manfred
Aaaaargh. Maybe we need pix of OEM pistons and aftermarket pistons!
Here’s another idea: Perhaps people with pistons in hand could generate a profile of the recess, maybe using one of those profile copy devices for woodwork. Then we could post those profiles as images, complete with a ruler in the photo for scale. Print it out, cut it out with scissors, and hold it against your piston to confirm it’s the same shape recess.
9/32" is 0.28", so the dish in the piston above isn’t as deep as yours at 0.31"!
Good idea. I’ll snag one of these and post an image of it on a scaled grid.
I’m thinking that these probably are 9:1.
OK here is a pic of the 1st factory pistons V12, 10.6-1 !
notice the deep valve reliefs and piston raised edge comes all the way up, about .050 thou from top , head gasket gives around .045 squish area, seems most other pistons do not come all the way up!
Welsh supplied me with set
9-1 pistons for Euro engine, they only come up to .150 thou. from top.also there is a good pic in Allen Scott’s book Jaguar Group A , of an 11-1 piston.
i think OP has 9-1 pistons.
ron
Correct. I knew that it wasn’t quite right unless I specified “…at the centre”. But who besides Americans talks in fractions? 
I specified the centre as the only place thar dimension exists because it tapers to flush around the circumference.
The really amazing thing to me is that you can normally only buy tape measures with ONLY inches marked. It’s like there’s a battle for minds going on and somebody is scared in case people gradually learn metric dimensions by ‘osmosis’ from exposure to foreign commie numbers on the same tape as good ole’ boy regular numbers that God uses. At least have them metric on one edge and imperial on the other, so awareness gradually builds. It’s the same as forecasters giving the weather temps now as only F numberd, then as F/C numbers, then after ten years or so they gradually switch to C/F and eventually drop the F Values.
Pete
PS: The UK has had that arrangement for decades and now nobody cares that technically you can’t buy 8’x4’ panels any more - they know them as the metric sizes. Then in, say, another hundred years the country would be brave enough to take the last empty seat on the metric bus…
