MTFCA Forum

I just replaced my aluminum Z cylinder head with a aluminum Sherman style (Prus) head from Lang"s due to blowing head gaskets between # 1 and #two cylinders on my 1927 roadster. The Z head must have been machined wrong because the casting was VERY thin compared to the area between #3 and #4. I talked to Steve at Langs and we took some measurements ant the Sherman style (Kevin Prus) was a lot beefier in the problem area. It came today and I followed the instructions TO A TEE and after the initial start up. something is hitting. I DO NOT HAVE THE HIGH COMPRESSION DOMED PISTONS. I've got a call in to Kevin Prus , being a holiday weekend, I haven't heard anything yet. The instructions said to tighten a few head bolts and crank the engine over to make sure nothing was hitting and I did not hear anything. SOOO- I just finished removing the new head and I cant seem to see anything where it was hitting. Does anybody out there have any ideas? Thanks, bobt

One or more pistons may be hitting the head gasket at the edge of the cylinder. You might try this: Turn the engine so that the pistons are all down the bores about half way. Leave the head gasket off and set the new head with spark plugs out and half the head bolts in, but left loose, on the engine. It should sit flat and not rock or wobble. (Put in about half the head bolts, in every other hole, but leave them several turns loose) Then crank the engine with the hand crank SLOWLY, and watch the head for the least sign of movement and listen for any sound of something hitting. It would be best to have another person crank the engine slowly while you watch the head closely for movement. Keep fingertips lightly on the head to feel for any movement.

That's a great idea. I will try it tomorrow. When I installed the new head I put #1 and #4 pistons all the way up to center the gasket, then put the new copper sandwiched gasket with the large water opening towards the rear on the block ,then installed the head.I used to be a mechanic back in the 1960's but age is catching up to me. I do remember having a clearance problem on a Chevrolet six and the fix was using two head gaskets. Thanks, bobt

It sounds like you did a good installation. Your block may have been decked, which would have the effect of making the pistons taller. I have no idea how many different types of pistons have been sold for model Ts, but It's probably a bunch. It's possible valves are hitting the head, too.

I would closely inspect the copper head gasket too. When they came back on line after the recent drought, I ordered three of them to replace my reserve stock. They were all a bit suspect around the large water passages at each end, being miss-crimped, and missing whatever is used in between the cooper sheets.

Allan from down under.

Beside the block having been decked, the cylinders could be bored very large oversize. Try putting some prussian blue on the top edges of the pistons and then put on the head without a gasket. Turn over the engine at least two revolutions and remove the head. If any of the blue has transfered to the head, you will know where and you can cut it down just a little in that area.
Norm

You can put a little bit of playdoh on the pistons, reinstall the head and gasket. Turn it over a full revolution. Where it is hitting the playdoh will be pinched.

OK. I just put the new head on the block with no gasket and when I hand crank the engine over the piston(s) are hitting the edge of the head. I measured the depth of the old head combustion chamber versus the new head and if anything, the new head has more depth clearance.I measured the WIDTH of the combustion chamber at the pistons and the new head is narrower than the old Z head so I'm thinking that's my problem. Lang's sells standard aluminum pistons in sizes-standard,.020, .030, .040, .060, and .080 oversize. Since I bought my 26 roadster used two years ago, I don't know what I have but I'm guessing I have oversize pistons. I'll be waiting to hear from Mr. Prus on a solution. I guessing machine work? Comments , solutions are welcome. Thanks, bobt

Clean the tops of the pistons, they will be marked with size STD .020, .030 etc

Thanks Mark. I just found out my pistons are .080 oversize. The largest they offer. (JUST MY LUCK!) Any recommendations on what size to have the Prus head milled out to? I'm not a hot rodder and drive about 35 mph max. I drive both my tees a lot but just around town. Please remember that I bought this car used. I wish it had standard size pistons in it. Thanks again, bobt

There is probably a rule of thumb specification for what the minimum piston to head clearance ought to be. I would want to have at least that much, and probably a little more. You'd best assume the engine will get over-revved at some point, and allow a safe margin of clearance. Ts have elastic crankshafts and are prone to having excess bearing clearances, which can allow piston over-travel. A piston touching the head during operation probably wouldn't jam the engine, but it might very well cause the piston to drop its skirt or it could easily distort the top ring land enough to pinch the top ring, which would soon lead to problems. .080 over also reduces the sealing surface area available to the head gasket, so I'd want to take every precaution to get a good gasket intallation, both in assembly and by careful torquing and retorquing.

Use of modeling clay might be the best way to determine clearance as you adjust the head or pistons or both. If removing metal from the pistons is indicated, be careful to remove the same amount from each one and in the same pattern. Allow clearance for head gasket crush and expansion. The gasket will be both thinner and slightly larger than when it is new, once it has been fully torqued down.

You should also test the piston to gasket clearance. The gasket should not overhang the edge of the cylinder. I personally do not like to go over .060 on bore and if it is too worn for that, re-sleeve the block and bore to standard. But if the pistons and rings and cylinders are OK, next best thing is to cut the edges of the head chambers just enough to clear.
Norm

I'm running a Prus head with a 80 over bore, it hit on every cylinder without a gasket. The prussian blue showed where to remove material. I took material off the head, it didn't take much to get clearance.

Years ago the aluminum pistons had sharp edge on the top corners, they have since turned a radius on the outside top which reduce chances of hitting the head gasket with an over bore piston. Years ago I chased a noise on a new rebuild, tore it down six different times to find it, finally saw where the piston was hitting the edge of the head gasket. I raised the pistons to the top centered the gasket and torqued it down, problem solved!

Kelly, Thanks for your post. Can I ask what did you use to remove the material from the head where the prussian blue was imprinted? Also, how DEEP did go into the head? Thanks, bobt

I would consider removing metal from the pistons. They are cheaper than heads. Removing metal from the pistons could raise balance issues, but it also is an opportunity to bring all 4 pistons to the same weight and take up the rod bearings and replace rings. If you do take your pistons out, you'd best lightly hone the cylinders and replace the rings, since it would be very difficult to put the old rings back exactly as they were, and they probably won't work right unless you do.

I only had to remove about a 1/32" max and only on spots around the squish area. I used a ball end die grinder. The Prus head looked to have a lot of material in the area I worked. Take only a little at a time and check clearance again. Take your time.

The reason I took material off the head was the pistons were balanced very close so I didn't want to mess that up.