A and B pistons

Guy I know is rebuilding his HE V12. He realized after the fact that he put B pistons in A bank, and A pistons in B bank. The top of pistons actually have an A or B stamp.

We assume it does not matter. But why would Jaguar distinguish A from B???

Are you sure “A”-“B” refers to bank rather than piston size? Pistons are graded by size and sometimes identified as A,B,C

If we leave that possibilty off the table and assume that the designation refers if A-bank and B-bank, the problem would most likely be in the connecting rods and their orientation. It’s critical that they face the correct direction on many (all?) engines.

Someone who actually knows something about Jag V12 pistons will chime in soon, I’m sure :slight_smile:


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I hope so, I’d like to alert him before he puts the heads back on next week!


From the ROM

Connecting rods must be fitted to the pistons in such a way that when installed in the engine the word FRONT faces the front of the engine and the chamfer on the big-end eye faces the crank pin radius. [italics mine]

Did he remove the pistons from the rods and then reassemble them? Did he number the pistons and rods?


No idea…i think the pistons and sleeves are from a donor engine.

This just came up on @Craig_Balzer E-Type build.

A and B denote the machining variance between piston and cylinder liner at the initial build, which varied 0.0001."

In Craig’s case, he had new pistons and new liners, so the ‘A’ and ‘B’ designation was not applicable.

In your buddy’s situation, if using used / original pistons and liners, you want to be sure that you’re marrying A pistons with A liners (NOT original liners that have been over-bored and new oversized pistons fitted as there again, they would then be ‘custom sized’ appropriately).

I found most the engines in my shed were all either A or B, for the whole motor, but I did find one mix and match block that doesn’t appear to have ever been opened.

Hope that helps!


It does not: @Jeff_Schroeder snd @Craig_Balzer settled this a few weeks ago.

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BTW, I hope to get a photo from him one day. He’s bought three V12s for parts. One of the heads, there was an indentation of a screw in the head!!! Owner must have dropped it down there, piston slammed it against the head, and it eventually broke down and disappeared.

Jaguar graded their pistons. So instead of seeing STD for “standard bore” or “+.010” “etc. for overbore pistons to denote the bore size they would grade them with letters. If the pistons or liners were worn they wouldn’t overbore the engine’s liners but simply replace the piston and liner with another pair. So an engine can have many differently graded pistons but you just need to make sure the individual piston and liner are the same grade as one another.