Compression PSI

[Barrman]

Got the engine all buttoned back up, got oil psi while spinning on the starter, got spark at the plugs and fuel in the carb. We get a puff,puff, puffffff every now and then. But, what we don't have is a running engine. Since we only messed with the head and head gasket, I started thinking about rings and valves.

We pulled all the plugs, put a compression tester in one and let the engine spin with the carb wide open. 25,30, 20, 25,20,20 were the results of all six. That sure does seem "throw the engine out" bad to me. But, I don't know flat heads as well as I should. For an OHV engine it is awful. What does a 230 flat head normal post for compression psi readings?

Thanks.

Oh, we haven't done a leak down test yet and I know a little oil in the spark plug hole will really boost those numbers. I am waiting to see what the compression psi numbers should be. If these numbers aren't that far off, then we will go ahead and see which leaks more between the valves and rings. Otherwise I will call the owner when he gets back from vacation and start talking about a repower.

 

[mangus580]

Are you sure all the valves are closing properly?

 

[clinto]

I'm w/Mangus, that's awful uniformly low in all 6.........

 

[mangus580]

Flatheads are notorious for stuck valves too...

 

[CGarbee]

There is a really good thread concerning compression and compression testing on the g741 site. Link to it is provided below in this post... Keep in mind that if you just pour the oil in through the plug hole it will go all over the valves and it will hydro seal them along with the rings thus preventing you from telling which is bad ( valves or rings ). Use a straw to angle the oil in the plug hole so it pours on top of the piston and not the valves! compression readings should be within 10% of each other. TM calls for much higher compression than you are getting, 100 PSI is very much starting to be questionable, 90 PSI, most won't even run or are running very poorly. 120PSI & upwards, with no more than 5PSI difference between cylinders would be good readings.

See this thread for a discussion on taking compression readings, results, and possible causes...
http://www.g741.org/cgi-bin/dcforum/dcboard.cgi?az=show_thread&om=901&forum=DCForumID1

Good luck.

 

[Barrman]

All the valves appeared to be working properly when the head was off and we were turning it over to get oil psi.

The engine seems to turn over extremely slow compared to most every engine I know. Even my Reo Gasser with one dead battery spins faster than this thing with the plugs pulled, 2 fresh batteries installed and a charger on it set to 24HI.

Woops. I saw the owner was here instead of on vacation.

 

[CGarbee]

The M37's have always seemed to be slow turning over... I don't have the cranking speed specification in front of me, but you are right about everything else cranking faster. If everything is in spec (or a tad off to be honest), they still crank and run, you just wonder if they are...

Let us know what you find out when you do the wet retest...

 

[Barrman]

Sure will. We are having severe t-storms right now and my students won't be coming to class today. That means Thursday afternoon before the assigned student get to mess with it now.

 

[Barrman]

Thanks also for the M37 discussion link. Reads like somebody had the same readings I got and asked the same question. Thanks again.

 

[amanco]

CGarbee, that is funny what you said about the cranking speed. My brother was just amazed each time I started my truck. He couldn't believe it starts, said "sounds like it ain't going to start and then away it goes" And that was with a dead generator. Cranks a bit faster now that the charging system is working properly.
Sounds like maybe some stuck rings on Barmann's truck. The oil in the cylinders may do quite a bit of good if he can get enough compression to get it started.

 

[Barrman]

We did the oil over the piston testing yesterday. 147 was the lowest and 198 was the highest with the rest being somewhere in the middle. We put the plugs back in and almost got it to start before all the oil went away and all compression was lost again. We also figured out the fuel pump is shot.

Thanks again for the help gentlemen.

 

[CGarbee]

Well, given those kinds of numbers compared to what you had dry, I'd say that most likely the problem with is worn rings, badly scored or out of round cylinders, or an all of the above combination. I had a engine that was run for a while without an air cleaner (before I purchased it) that had pretty bad rings as a result...
Nice thing about the engine in a M37 is that you can pull the head, drop the pan, and re-ring the engine (and replace the crank bearings and such) with the engine still in the truck for not a whole lot of money...

Of course, if you've got some badly scored bores or egg shaped ones where you have to overbore the block, you most likely will have to haul the block to a decent shop to get the work done in which case you might as well bite the bullet and do a full rebuild or repower...

If I haven't mentioned it before:
I'd suggest the engine and clutch TM for working on the T245...
TM9-1840A Ordance Maintenance Engine Dodge Model T-245 Clutch Borg / Beck Model 11828 The M37 Army technical manual TM 9-1840a contains a description of and procedures for removal, disassembly, inspection, repair, rebuild and assembly of the engine and clutch used in the 3/4-ton 4 x 4 cargo truck M37, command truck M42, ambulance truck M43, and telephone installation, light maintenance, and cable splicing truck V41. This M37 manual also covers parts special tools and equipment for field and depot maintenance trouble shooting M37 engine and clutch. Engine rebuild disassembly of stripped engine into rebuild of cylinders, crankshaft, and flywheel connecting rods, pistons, piston rings, piston pins, valves, valve tappets, and oil and water pumps throttle linkage, oil filter, and oil filter assembly assembly of engine.

It's part of the Military-Media CD or can be had as a reprint from Portrayal Press and others...

Oh yea, almost forgot: If you can get it to run (assuming that it hasn't in a while...), try running it for a while to see if things improve... If you've just got things sticking... Turning it over with some Marvel shot down the cylinders will help, running it will help more... (or won't hurt it much more given that you already are looking at going in...).

Good luck.

 

[Barrman]

Thanks for the info. I need to talk to the owner before we do anymore.