acceptable compression

[Don Jordan]

I'm not sure if I want to know what is good compression or just enough to get by. It sure seems low.

1. 45

2. 50

3. 50

4. 50+

5. 50

6. 65

I have a compression tester (Hastings) that is older than dirt. It read 20 lbs less than my neighbor's - which I borrowed. I guess it's time for a new one.

How many times should you turn the engine over? 6 or 7?

And while I'm on the subject what are recommended plugs? Right now it's got Autolite 303...

Thanks

[greg g]

I would put my faith in the neighbor's gauge. Basically you crank with till the needle doesn't move any further. Were all teh plugs out? Did you try it with some oil in the cylinders via the plug hole. Even with 20 added to those 70's are OK for a dry test. I would be a bit worried about number 1. But get and accurate gauge and do another test with a squirt of oil down wach hole before cranking. I believe new the reading was in the 110 neighborhood.

[randroid]

Don,

What Greg said. Usually about five turns for each cylinder is good but it's more important to have the same number of turns than a specific count. Do one test dry and one wet, and if there's a significant difference your readings it's most likely your rings are wearing. Otherwise it indicates valves. These puppies aren't that hard to rebuild, the worst case you'll need to pull the engine to have machine work done but that's cheaper than a bad paint job.

You already know your engine has at least one weakness so if you have the time and the space, pull the crank and look at the bearings, then check the cylinders for wear. Pulling the head is no biggie so you might just rebuild it. It takes time but unless you have a serious problem you'll start with a new engine for not a great outlay of cash. If you have serious problems you'll at least know about them and deal with them as you will.

That's how I spent my money so I guess I can put my mouth there, but until such time as I begin sending checks to you or anybody else I don't feel it's any of my business what you do with your drive.

Best to you, and keep us posted.

-Randy

[Don Coatney]

[Don Jordan]

When I got my 48 it had been sitting in the desert for 25 years and the engine was frozen - it wouldn't even budge. So I just took the engine in and had it completely rebuilt. You are right - it was worth it. Very reliable.

The 47 ran/runs. Some little annoying idiosyncrasies. So I thought I start first with the compression. Next step oil. I guess since this is my project car I will take the engine apart and have it redone a piece at a time. That should be cheaper shouldn't it?

Here's a silly question: Does 47 P15 have babbitt bearings or inserts? I know my Chevy didn't get inserts until 54.

Just think - if I didn't have this to do I'd probably have to paint the house.

Thanks for the tech page - I'll print it out and add it to my collection.

[Young Ed]

Don you will soon learn the superiority of the mopar flathead. Your p15 has insert bearings and full pressure oiling. Don't let the non-overhead valves fool you

[Plymouthy Adams]

If you drive this car, basically I see that if you were to add the 20 that the neighbors gauge read..you are still even and as such suggest that the engine is just tired rings...you would be a good candidate for an overhaul. The car will probably be a bit hard to start but will run smooth and may have a tad of smoke and not what you call zippy on the highway.

If this engine has been sitting..do a wet test (oil added to top of piston) with all plugs removed and crank about 5 turns per..it takes at least 2-3 to get a good reading

[FMSPEED49]

Do a search on "snake oil science" it may be somthing you want to try or not.

[TodFitch]

Here's a silly question: Does 47 P15 have babbitt bearings or inserts? I know my Chevy didn't get inserts until 54.

Plymouth got inserts in 1933. Along with aluminum pistons and a four bearing balanced crank with full pressure lubrication to all bearings and hardened exhaust valve seats. All in 1933 (or earlier for some features as all Plymouths have had full pressure lubrication since 1928). It may look like a lump of iron but the Plymouth engine was a long way ahead of Chevrolet in terms of engineering.

[TodFitch]

Do a search on "snake oil science" it may be somthing you want to try or not.

That search should be on this forum. And it will be a relatively recent (last month or two) thread. I imagine a search on Google will turn up a lot of other stuff unrelated to getting higher compression on an old engine.