Tech Notes
British Car Day
Chapter Bylaws

Cylinder Heads, XPAG & XPEG Style
(as appeared in the Fall, 2011 TSR, reprinted from Aug/Sep 1972 TSR)

By Chip Old

At our July 23 Technical Session two different people asked the question “I’m using a TD head on an TF block. Will this cause any problems?” The answer is no. TB, TC, TD, and TF 1500 heads are not all exactly the same, but they are interchangeable.

The TB-TC-TD head has combustion chambers of 45.5 cc each, giving a 7.25-1 compression ratio. The intake valves in this head are 33 mm diameter, and the exhaust valves are 31 mm. This head is usually fitted with 1¼ inch carburetors. Used on any 1250 cc block (XPAG) regardless of whether the block is from a TB, TC, TD, TD Mk. II, or TF, this head will produce standard TB-TC-TD power: 54 horsepower. If this head is used on a TF 1500 (XPEG) block, you will lose power because this head has smaller valves and carburetors than the standard TF 1500 head.

The TD Mk. II and TF 1250 head has combustion chambers of 43.75 cc each, giving an 8.1 to 1 compression ratio. Valves are larger: 36 mm intake and 34 mm exhaust. This head is fitted with 1½ inch carburetors. Used on any 1250 cc block, this will give you about 57 horsepower. Used on a 1500 cc block, you will get a compression ratio of 9 to 1, and about 75 horsepower. but you will have to polish the combustion chambers to eliminate detonation-causing hot spots.

The TF 1500 head has 45.5 cc combustion chambers, just like the TB-TC-TD head, but because of the larger bore of the 1500 cc XPEG engine you get a compression ratio of 8.3 to 1. This head uses the same large valves and carburetors as the Mk. II and TF 1250, and on a 1500 cc block it will produce 68 horsepower. Used on any 1250 cc block, this head gives you standard TB-TC-TD compression ratio (7.25 to 1), but power will be raised very slightly because of the larger valves and carburetors.

As I said earlier, these heads are all interchangeable, but as you can see from the above you may end up with either more or less power than you originally had. There is only one important thing to watch for when you are swapping heads from one engine to another. The heads used on TB, TC and TD engines up to engine number XPAG/TD2/22734 were cast with oblong water holes (actually sort of banana-shaped). This head is often referred to as the “banana head,” and it is designed to use ½ inch reach spark plugs (Champion L-series). Starting with engine number XPAG/TD2/22735 and continuing with the TF and TF 1500, the heads were cast with round water holes. This type head is often called the “round head” for obvious reasons, and it is designed to use ¾ inch reach spark plugs (Champion N-series). You must use the correct plugs for whichever head you are using, regardless of which type plug your engine originally called for. Half-inch reach plugs will work in a round head, but the engine probably won’t run very well because the plug will be shrouded by the longer plug hole. Three-quarter inch reach plugs will extend out into the combustion chambers of a banana head, and once carbon builds up on the exposed threads you’ll never get the plugs out. These exposed threads are also a potential hot spot which can cause pre-ignition or detonation. What’s even worse, the over-long plug might interfere with the top of the piston in some cases, and I don’t think I need to tell you what that will do to your engine. Since I mentioned some engine numbers earlier, the “TD2” designation in the engine number does not mean it is a TD Mk. II engine; it simply means that the engine has a larger flywheel and an 8 inch clutch instead of the 7¼ inch clutch used in earlier engines. A lot of TD owners seem to be confused about this, judging by the application forms we have been receiving.

TD Clutch Linkage

This one’s a quickie. At the tech session someone asked whether or not an early TD with a cableoperated clutch can be converted to the rod-operated clutch used on later TDs. The answer is yes, and you end up with a smoother and more trouble-free clutch. All you need is the correct rod from a late TD to replace the old cable, and you have to modify some of the brackets slightly. The best idea is to examine a rodclutch TD to see exactly what parts and modifications are necessary. The rod is adjusted in the same manner as the old cable. Someone else at the tech session supplied the answer to this one; I don’t remember who, but thanks anyway.