The Mysteries of Pushrod Tubes

One of the troubles of building these old British engines is when you start mixing parts from different years. I have a '67, with a '66 cylinder head, '71 pushrod tubes, and '66 tappet blocks. My old set up was '71 head, tappet blocks, and tubes.
When I ordered my new tappet blocks to put into the big bore kit, I ordered '67 style. I'll admit, I wasn't thinking about all of the mismatching. Had I ordered '71 style, I could have saved some work. However, the '66 cylinder head would have still required a little work.
I mentioned this mismatch the other day, but I didn't take many pictures. Since I've got the top end back off of the engine, it's sharing time!

This is one of the tappet blocks from my old engine. Look down on the left side...you'll see some cracks in the bottom of the cylinder. Oops! Anyway, this block is different from what I'm running now. The differences are subtle, but they will change the way the engine goes together.

This is the new tappet block. It's a '67 style from Morgo. The sealing surfaces are different from the '71.

This is a '71 pushrod tube. It WILL NOT fit a pre-'71 engine. Well, not without a little work. The base is to the left. I took about .030" off the end, and ran a boring bar .400" into the base to open the ID by a few thou. It was really just a skim cut to get a good clearance. The end to the right mates with the cylinder head and holds an O ring. I reduced the thickness of the O ring "shelf" to get the proper squish (we'll look at that on the next picture).

When you set the head onto the cylinders, it will sit on top of the pushrod tubes before contacting the head gasket. The read arrow indicates the head gasket. Look in there closely, you'll see some light between the top of the gasket and the bottom of the head. You want that gap to be between .030" and .040". Oh...and you want to measure that with the O ring installed on the tube. I set this up and measured the gap, then I reduced the height of the tube in order to get the right gap. The gap establishes the amount of squish on the O ring.

This is where the head interfaces with the pushrod tube. The O ring will seal against the countersunk surface around the bore.

I'm not an expert engine builder (said it before and I'll say it again). Lots of people will say: "you can't fit those parts together." Indeed you can. Study what makes them different. Do a little research, see what kind of specs you need to hit. If you have the time and/or skills, you can make just about any combination of parts work together.
More on the O ring squish: the ideal amount is .030" to .040". Less than that means that you may not have enough pressure on the O ring to make a good seal. More than that is even worse! Two things can happen if the gap is two big: first, you extrude the O ring and get a bad seal. Second (and worse) you place too much pressure on the head itself and cause it to warp. Once the head is warped, you'll have all sorts of issues that you really just don't want to deal with.
Obviously, if you are worried about keeping an engine in "original" condition, something like this isn't an option. However, if you're goofing around with a combination of years...you weren't worried about keeping it original to begin with.