Dear friends.
I have a Jaguar XK 4.2 engine and I can't decipher the engine number. It's "8L129 *** - HZ" (* Hides serial number)
I have searched the internet and found some clues... But no answer to the final question.
8 seems to be a later block (S2 forward) (some also say that the 7 and 8 prefix is compression number)
L =long bolts (also read that it simply means it's a 4.2)
Above 90,000 serial number, it seems likely that it is Series 3 (it have the vertical lines on the block sides)
After the "-" in the engine number, it should maybe be an S, but it's not. It says -HZ.
H= in one forum thread it says that H means High compression.
Z= no clue found at all.
Can someone please help me and explain the HZ suffix?
The number is very clearly stamped so no reading error 😁
The car is not matching numbers with the engine so can't use the chassis number as a clue.

The car, the engine is going into, is a 1978 XJ6. The stock engine had a problem and was taken out. The work was never finished so the car has been parked indoors since the summer of 1984 (!) and it's in amazing condition.

 

8L means that it is a 4.2 used in both Series 2 and 3 XJ6
I've not heard of an HZ suffix. Usual suffixes are either S, L or H. Depending on the year, they were either stamped S for standard compression ( which meant European 8.7:1) or Low compression (7.8). Other years had High and Low. Earlier engines ( E Type, Mark 2 etc) has the compression ratio stamped on the head following the engine number as -8 or -9.

If you look on the right rear side, just below the top of the block you might see a casting date for the block there.