The ‘51 Plymouth I just bought has a dead miss so I need to go through the diagnostics. How many psi compression is acceptable for this engine? I don’t know as I’ve ever checked the compression on an engine with under 7:1 compression before. It’s 6v so I know I need a fully charged battery with plugs out. Hoping I find a bad plug or a wire or something as it doesn’t sound like a lack of compression but I haven’t had a chance to do anything with it. It was runnning on 6 when I first saw it.
If the compression on any cylinder is more than 10-15% lower than the rest, it really needs attention. The variation is more important than the actual number.
Agreed. RPM, having the throttle open and how much oil is on the cylinders and rings will all change the hard number. You mentioned you had the book in your other thread. I'd go with that, but not treat it as exact. I've only had a few, and they typically ran 80s to low hundreds. Don't expect an OHV number of 150. It would be a good idea to have the side cover gaskets at hand in case you go in. Expect to get a couple cheap wrenches and possibly bend/grind to fit, too. I was going to post a picture, but found this. I would treat at least the first go around as NOT speedy but go with a do it until it's right approach. However there may be good info in there for you. https://p15-d24.com/topic/59574-adjusting-valves-mopar-flathead-6-speedy-guide/
I haven’t looked in the motors manual to see if there is a spec for a number.. I was thinking around 100 psi if the cylinder and valves are in optimal shape but that was just a wild guess to myself. I did forget to order side cover gaskets. The valve adjustment process in the manual is similar to that… if I’m low on compression I’m going to guess it’s got a stuck valve or needs adjustment… modern technology with bore scopes should make it easy to figure that out
My '51 Pontiac has similar static compression, 6.3:1 and in reality on the starter it'll make high 80's because it's got a really soft cam with high overlap. Like the others say, in reality so long as I'm within 75-85 range across the board is good. I've got a couple cylinders only making 65 because of two valves that need lapping in again; that makes a noticeable difference to the running of the engine, particularly at low rpm and light load. If you're in that ballpark range and the variance is minimal then it's overall going to be pretty good. Phil
With the possibility that I’ll need to pull the head, should I get a copper or a composite head gasket? Both are listed and the price difference isn’t much. I know copper was probably original
There will probably be fans on both sides. I'd look at installed (compressed) thickness and lean toward the thinner one, and also look at the surface of block and head and see what kind of surface you've got. https://www.cometic.com/tech-support https://dsportmag.com/the-tech/education/quick-tech-the-ins-and-outs-of-copper-head-gaskets/
50 some years ago I bought a '51 "Club Coupe" (Plymouth for 2dr sedan) that had been neglected since birth. It ran "OK", but was kinda weak. Since the first thing you do when getting a new rig is tune it, I was amazed to find a couple 40s, a 65, a 70 & some 80s during the compression test. Went ahead with plugs & wires, filed the points, & changed the oil. I wasn't about to screw around under the fender adjusting what was left of the valves ... Torched the front springs, split the manifold, & just drove hell out of it for a few months ... It started fine when it was warm, but at freezing or below, had to plug in the headbolt heater before it would light off. The flatmotor MoPar is one tough powerplant ...
Fel pro makes the original copper gasket, and Victor Rienz makes the composite gasket. Vintage power wagons lists an NOS gasket that has an extra hump. I don’t know what that means I’ll be happy if I get all 6 cylinders to light up.
I grabbed a Fel Pro head gasket kit from RA with the copper gasket. Being a kit it came with valve cover gaskets too. I had to order tie rods for my work beater, and the gasket kit came out of the same warehouse.
Don't quote me on this but I believe the extra hump refers to an internal vs. external thermostat bypass. On later engines 1950+ (?) the bypass was changed to internal. So no hose small hose between the top of the thermostat housing and water pump. External does however have a extra water passage between the top of the block and head. See the attached picture. The passage is located on the passenger side near the front edge of the block and near the thermo housing. The extra hump is to seal this passage. Older external bypass head gaskets are straight across the front edge. Newer internal bypass gaskets bump out on the front edge at the passage location to seal it. If you incorrectly use the older style straight edge gasket then you'll have a coolant leak out from under the front edge of the head.
I forgot to mention the famous thing to check on every Mopar flathead. Look at the water distribution tube. Check the P15/D24 site for all the info. Some stuff here, too. If it's got ugly coolant or runs hot, this needs to be done.
The coolant tube is in the back of my mind. The water pump is newer and it looks to have nice coolant in it but if it runs hot I know I’ll have to replace it. Andy Bernbaum sells new tubes. I’ll have to check the gasket to make sure it’s right when I get it. I had no clue there were differences.
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