The above numbered head is currently being rebuilt at the machine shop. I got a call from there stating that my old exhaust valves measure 1.530" across the face of the valves and the new replacements I gave him measure 1.430" across the face. Which should be correct? Does it sound like the old ones were upgraded with something else in the past? A running production change? And if these are non-stock, where could I source replacements in that 1.530" size?
Not having a good feeling about this. That 1.530" measurement translates as 1 17/32". A Google search under that head size pulls up references to 1930s Nash through 1960s Dodge. Nothing Jag. Could it be the valve seats got worn and the local garage went up a size with something domestic? So close now to having all of the machine work done and now I’m thrown this curve ball. Arrrggh.
It looks like a previous owner replaced the original size exhaust valves with a later type such as from an XJ6 then turned and faced them down to 1.530". Interesting. I wonder what effect on performance it had since I know bigger valves don’t always equate with better performance within the context of a chamber and passages designed for something different. I also don’t have a bench mark for comparison since it’s never run during my ownership. Anyway, for those still reading, my machinist has advised to stick with the mod because undoing it by messing with the seats and throats would make for complications. So we shall see.
I measured the valves in a standard A head and get right around 1.430 although a difficult measurement with the valves still in the head.
I measured the valves removed from a C head and get 1.625" diameter.
BTW the intake valves in both are 1.750"
I would feel uncomfortable turning and facing valves to a smaller diameter. Aren’t they case hardened?
Maybe try the XK Engine forum, maybe somebody with a newer engine knows where those 1.530s came from.
XK Engine forum. Thanks, I’ll try that. This sounds like a poor-man’s attempt at an upgrade in lieu of having a C-type or later head from back-in-the-day?? And I wonder where the previous owner or rebuilder got the idea for this – perhaps a 60s/ 70s magazine article from when these were just older cars and not collectible yet? Everything else about the valves looks Jaguar: correct length, stem diameter, collet profile, etc. The head and non-numbers matching engine is stock too. Rob, I thought modern exhaust valves were some stainless steel variant and not case hardened? My rebuilder – Lonnie Jensen up in San Francisco – told me to order the exhaust valves found on the XJ’s: 1.625" ones. Maybe he wants to go bigger with the exhaust seats. Anyway, I’ll swing by there by early next week and see if this still sounds do-able.