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High-compression Alloy pistons


MikeWilliamsUK
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My 1938 Zephyr engine was rebuilt in 1982 by Jim Rodia at Ocottillo Automotive, Ocotillo, California. It was a total rebuild job with all new parts, a replacement used crank which was chromed and ground, and the block was line-bored. An 8EL oil pump was fitted. It all seems to have been a good quality job. It was bored plus .060in and fitted with new alloy pistons with domed heads, which Jim "regrooved for late ring set (top groove)".

Twenty five years later the rest of the engine was assembled last year, but I am having a few problems with it.

Does anyone have experience of domed alloy pistons? Do they increase the compression ratio too much?

Was the plus .060in too much, or quite normal?

Anyone remember Jim Rodia and whether he was reliable?

Thanks,

Mike

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Mike, the domed pistons are quite normal, but there are degrees of "dome". The original pistons were domed and you will notice the cylinder heads are domed also. The .060" over bore is normal but at the limit for oversize 2-3/4" pistons. You could actually bore this engine to a standard 2-7/8" ( 1940 to 1948 size) if need be.

The combination of the overbore and slightly higher pistons will raise the compression ratio especially if the cylinder heads were "shaved" or re-surfaced. A volume check (cc)would be the only way to really determine what the true compression ratio is.

What kind of problems are you having?

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Hi Peecher,

Its a problem I've had for some time now. Engine starts on the button and runs beautifully. Temperature starts to build quite easily and once it starts it gets hot quickly and starts a heavy screeching noise - I switch off quick. When first built it did this after about 5 minutes of running in the garage, but now it will run on the road about 15 miles if careful. Obviously I turn off before the noise starts when I can. The car has now done about 100 miles and in addition maybe 20 hours running on tickover in the garage.

I am wondering whether the alloy pistons expand and stick in the bores if the clearances are not sufficient. When you say domed pistone are 'normal' do you mean all Zephyrs have them as standard? I am wondering whether standard pistons would be better than these alloy ones.

I am not aware that the heads have been shaved but I know the block has been faced - how much I don't know.

It may be that more and more miles will gradually wear it in, but I am worried about damaging the engine. What do you think?

Mike

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Mike, The original pistons used in these engines were a cast steel and are not the best replacements. The aluminum alloy pistons are the best and are universally used by most rebuilders. The "quick heating" you describe does sound like you have a lot of friction altho these engine do warm up quickly with the exhaust going thru the block. The fit on the pistons or the rings sounds suspect to me. Generally there should be about .003" clearance between the piston and cylinder wall and the end gap on the piston rings should not be less than .008". The bores may be tapered causing the piston and/or the rings to seize up in the smaller portion? Another cause could be bent or twisted connecting rods. What you have is diffintely not right. Even a newly rebuilt "tight" engine should not react like yours is doing. If this was mine I would remove the engine, disassemble it and find the cause. Good luck, Dee

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  • 2 weeks later...

I was just reviewing your posting on the engine noise. The 1941 LZOC coupe that my father had and is now owned by my brother in laws brother had the engine rebuilt by my father in the late 60's or the early 70's. When the engine was rebuilt my father had the original pistons knurled if I have that spelled currectly. There were indentations or dimples put into the side of the pistons to make them a bit larger. They were the steel pistons and may have been the originals. Anyway, the engine would crank cold and run fine. The problem that occurred is that when the engine heated up it was difficult to turn over with the starter and crank after getting hot. He had to install a 12 volt battery as a back up to get it to crank when the engine was hot. The basic problem to all of this was that the tolerances were too tight with the pistons making for difficulty cranking the engine.

If you are having tolerance problems with the pistons are you having any difficulty cranking the engine after it gets hot?

Mims

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Thanks for that Mims.

I would say 'no' - no problem cranking when hot, but, when it starts to tighten up and make the noise I shut off and have not dared crank it over at that point. When it cools it cranks and starts fine.

Had one recent thought. The temperature stays down when running, but when I stop for fuel and turn off it rises quite a lot. When I restart I'd expect the water pumps to then drag the temperature back down, but they don't and that's when it starts to rise out of control. The pumps were supposed to have been new in 1982, but I wonder whether some of Skip's hign volume pumps would help the problem. Anyone had experience of them?

Second thing is that the radiator is total loss type with no catch tank and after a run it can need over a gallon to bring it back to the level - its all gone down the overflow. Is there a catch tank available which suits these cars and doesn't look too modern?

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Guest imported_V12Bill

Your radiator should not be filled to the top!! There should be enough fluid to cover the core with about an inch of coolant. Too much coolant and it goes out the overflow when it heats up and expands.

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Guest imported_V12Bill

Your radiator should not be filled to the top!! There should be enough fluid to cover the core with about an inch of coolant. Too much coolant and it goes out the overflow when it heats up and expands.

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Mike, Didn't know about this temperature problem. The water pump(s) themselves might be causing your problem, perhaps seizing up from lack of lubricant? that squelling noise could even be the fan belt slipping. If later model water pumps were used on your '38 block there might even be interference with the impellers in the water jacket. Impeller(s) may be loose on the shaft? I think you should take a look at those water pumps before going into the engine at this point.

It's qite natural for the temperature to rise after shut down but it should drop very soon after the water starts to circulate.

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