Valve dimensions

I am about to rebuild the head of a 1989 3.6 XJ40. While an (also old and tired) donor head is getting the car run, I took the valves off the original one and I’m a bit lost at the valves data, trying to determine which ones to keep and how many to buy (two exhaust ones are burned). In anycase, looking at the old valves, the intake ones have less margin than the exhaust onew. Would anyone know of sizing of the new ones, and the limits of usability? I am specifically looking for:

  • stem diameter, intake & exhaust
  • margin thickness, intake & exhaust

Also, how do you know if the springs are tired? Their “free” height is within Haynes manual specs, but the engine has 300 000 miles on it…

The standard method of checking if an old valve spring is ‘tired’ is to mount it and a new (or known good) spring of similar or equal free length end-to-end in opposition in a vice with a small piece of sheet metal between the two. Close the vice jaws a few millimeters to apply pressure to the springs and then measure the compressed length of both springs. The weaker spring will be somewhat shorter under compression if it is ‘tired’.

I can’t help with the dimensions of the valves themselves - I only know that if at 0.25" lift from the seat a valve rocks more than about 3 thou side to side then the valve guide should be replaced.

BTW, what do you mean by “margin thickness”? Not an expression with which I am familiar.

Re “margin thickness”, not sure how to describe it; maybe like: the thickness of the part of the valve from the face to the point where the valve-seat surface begins. Look up “valve margin thickness” on Google under Images. First dozen or so have it all marked. The reason I question 1/32 as stated in Haynes, is because my exhaust valves have that margin consistently above 1/32, and intake valves are all consistently below that dimension; I don’t mind buying a set of intake valves, just wonder if necessary, since the guides are worn, and with new valves I’d need 0 guides, and all I can find out there are +1 and +2…

Great suggestion on comparing an old and new spring!

On the question of replacing valve guides - the + 1 and + 2 refer to the outside diameter of the guides. When replacing the guides you must fit the next oversize and ream the head accordingly giving a 2 thou interference fit.

OK I see what you mean by valve ‘margin thickness’. I would think that is important on the exhaust valves to avoid the edges being eroded/burnt away but I would not worry overmuch about it on inlet valves. In my motorcycle tuning days I would radius the inlet valve seats so that the ‘land’ was a tiny ‘knife-edge’ strip around the valve circumference to enhance inlet swirl but I always ensured that the exhaust valves had a good area of ‘land’ to dissipate heat from the valve head through the valve seat.

Eye-opener on the guide oversize dimension; in my mind the only business side was the in-side :slight_smile: I wondered if I was looking at the wrong guides since their size was too big, not realizing they talk about the OD!!!