Triumph Repair: Triumph Spitfire pushrods, valve stem caps, screw adjustment

Question
I had my stock 1979 Spitfire head shaved 4 millimeters to raise the compression. I will need to have the pushrods custom made to the correct length. Is the new pushrod length shortening a direct 1:1 ratio (4 mills shorter) or is there a different ratio?

Answer
Hi Peter,

The rocker ratio is not the problem. The first problem is the contact patch on the valve stem which is the main problem and the loss of screw adjustment on the adjusting screw. If you can shorten the push rods by the same amount that you take off of the head, you are back to standard on the rocker arm geometry. I would not recommend cutting and welding a push rod. The top of the push rod is a socket and would be very difficult to take any off. You may get away with grinding the bottom mushroom down some. Some choose to make shims to go under the rocker shaft stands but that usually requires the addition of valve stem caps. You may also be able to under cut the rocker arm at the adjusting screw so as to run the adjusting screw up a little.

Probably the best would be a combination of each so all together you can get the adjusting screw in a position to set correct valve lash clearance and have a good contact travel patch of the rocker on the stem with valve stem caps from some other engine.

That sounds like a lot to take off so you will need to use modeling clay on top of the piston to check valve clearance at TDC of the exhaust stroke (over lap). Most high performance engine builders that I have talked to use .040" as a minimum clearance. I have seen some as close as .020" but the RPM must be kept down a little when you use that close clearance.

Are you sure you are not raising the compression ratio too high thus eliminating the use of pump gasoline? You need to measure the CC of the head and use math to figure out what ratio you will end up with unless you know someone who has removed that much.

When you change compression ratio you need to change a lot more. You need to change fueling and for sure ignition timing and timing advance curve. If you turn it to a higher RPM you need to raise valve spring tension, especially the exhaust springs.

When you produce more power, you produce more heat so that needs to be addressed too.

If you are just doing this to gain more power and no plans to use high RPM then you probably can just adjust the carburetor (or replace it) but be sure to set the ignition timing to match the fuel and compression you end up with. This is most important.

Howard