Technical 218 Plymouth acceptable compression?

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 old motors manual for a 1953 sez 105 My older manual for 51 didn't not give a number.

 

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/

 

and also check the reputation of whoever makes the gasket...some new gaskets aren't so good.

 

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.

 

The more I dig into this car it’s apparent that what the previous owner said isn’t true.. I couldn’t get it running this weekend. Last it ran is when he dropped it off. I started it multiple times that day but it coughed the last time I tried and I was sure it was just flooded.

So with my diagnostics, I changed the plugs as they were obvious. Plug wires which were almost certainly original were crumbling and the plug connections loose. I bought parts store carbon core wires listed to fit it. But it’s a universal set…. Then I could get it to fire and cough but it wouldn’t catch. Checked the points and the previous owner left everything loose and the connection were coming apart. He rounded the screw that holds the points and condensor wires together so I just couldn’t go any further as I couldn’t tighten it. An inline spark checker didn’t show any life on #1 but the points do have power and are switching. The coil is new that previous owner had to install, but it could be the wrong one and burned up. I didn’t have an extra plug wire in good enough condition to check spark out of the coil.

next up is a new coil. Fix and gap the points. New condensor as he mangled the electrical connector on it. New rotor for good measure and then I should have life in it.


The real problem is the guy didn’t want to deliver to my house as it was too far, but my mom’s house is an hour and a half closer to him. So it’s currently in her garage. I was trying to load a trailer. My driveway is steep so it has to go under it’s own power

 

do a compression test before you waste too much time on other stuff....

 

I wanna get it to where it can get itself on a trailer and into my garage. I probably should test it to see how many cylinders are down though.

 

the reason I asked is because I have a slightly off topic car with a six, it ran pretty good for a while, then got worse, it will barely make it up my sloped driveway into my garage now. I checked compression, it's pretty bad...40-70-80-90-120-120 and that is not something you can fix with a tune up. You might want to just do the test and see what you're dealing with, before spending too much time trying stuff that won't help.

 

I would put it in my garage non running. Just the problem is my driveway is pretty steep. I’m wondering if I’ve got more than one dead cylinder and it’s just not lighting up enough to kick it off

 

You might also want to do a cranking vacuum test with the plugs installed and the throttle completely closed. You should be able to pull a few inches. If you don't have a vacuum gauge, consider getting or borrowing one.

 

It should run well enough to at least pull itself on and off a trailer no matter how bad it is, if it ran before. You will need to sort out the ignition and maybe the carb.

"Perfect" compression, around 100 - 110. If you have over 80 and all cyls within 10 pounds you have a decent motor.

Later, if you are concerned about the water distribution tube check engine temp with an infrared thermometer. Back of the engine should be the same temp as the front, if the front is cool and the back is hot all the coolant is streaming up the front of the engine and the tube is rusted out.

If the head and block surface are "iffy" a composition gasket will seal better. With freshly machined surfaces you can use the copper job.

 

I brought my compression tester with me. I didn’t test it as, the charger that I have which does 6v is a 4A automatic charger. It works but it won’t recover the charge very quickly.

 

The more I dig into this car it’s apparent that what the previous owner said isn’t true.. I couldn’t get it running

Some people shouldn’t be allowed tools or to work on cars or anything for that matter.
Can you load the car on trailer and unload in your garage?
You can probably use the starter to get it loaded on the trailer.
Dan

 

I’m going today to see what I can do. I have a new coil, a new condenser. Compression tester, good battery charger and a gallon of PB blaster that should make the valves un stick. I also of course got a set of ignition wrenches too.

the problem is my driveway is steep and because of where my house is, the car really needs unloaded on the street. Getting a trailer straight up my driveway will be hard as it’s a very busy road. My neighbors driveway is somewhat adjacent and I could probably get the use of his driveway to pull into and straighten it up. But it’s still a busy road.Tying a tire to the bumper and giving it a push may be the only way if it’s pooched that far.


And as far as the guy not being truthful. He claimed it was one owner and it was the guy he knew who asked him to buy it from him. The title goes back to 1985 in another guys name and the ownership card from the selling dealer when it was new is still in the glove box with another name as well. Actually quite a bit of documentation in the glove box. Pretty cool stuff and the car is still worth it so I don’t have any regrets. I could put 3-4k in the car and still be okay as far as my investment.

 

I'm in the trouble shoot before parts camp. Local dealership spent $1500 on parts to figure out the no crank was a neutral start switch problem.

 

Not looking good but I’ll see

 

Definitely a possibility. I’m going to try to get it running so that I can get it to
My house. I have a head gasket kit ready to go so that’s not an issue. Unfortunately my wife just called me to say that we lost some power at my house ( not all of it) so it’s game over for today. Gotta figure that out

 

False alarm so I’ll keep on chugging


Cause of no spark, and no vroom vroom seems to be that broken wire is the wire that goes from the coil to the inside of the distributor through the pass through. Working on seeing if if I have something to macguyver it together with.