TR2/3/3A 1960 TR3A engine compression question.

[Tinkerman]

I just ran an engine compression check on my non starting post 60,000 engine and got the following numbers:

piston 1 through piston 3: 85

piston 4 75

So we have 85,85,85 and 75, which I feel, is too low. What should the numbers be for a stock TR3A?
This could be the root cause of my starting problem. If that is the case what do I do about it? How can low compression be fixed?

Your thoughts and comments, as always, greatly appreciated.

Cheers, Dick

 

[TR3driver]

Normally, compression readings are only useful in a relative sense.

But those are low enough to probably indicate some sort of systemic problem. I would double-check the valve timing, valve lash, and whether the gauge is accurate.

If that is all OK, start looking for where the air is escaping from the cylinders. You can try the old trick of squirting a small amount of oil into each cylinder just before checking it's compression again. If the reading goes up dramatically, you've probably got bad rings. A small bump says that's probably not the issue (which more or less leaves valves unless you've got something really weird like 4 broken pistons).

But a better test IMO is to pressurize each cylinder and listen for the hiss of escaping air. With the compression gauge I have, it's a fairly simple matter to unscrew the hose from the gauge, and connect an airline fitting with the help of a few adapters. There's a Schrader valve in the tip of the hose (where it screws into the plug hole) that has to be removed.

Of course, by now, it's fairly certain the engine will have to come apart; and the problem(s) will likely be obvious once you can inspect the valve seats, piston rings, etc. But it's good to know what you're looking for, just in case.

The good news is that you can do that with the engine still in the car, without pulling the front bumper, apron, radiator, etc.

 

[Geo Hahn]

No disagreement with anything Randall said - but I would think that an engine with 85/85/85/75 dry compression would start and run if there was nothing else amiss. Might even run rather well.

...my non starting post 60,000 engine...

I do not recall what symptoms you have but it might be worthwhile to determine if anything else is wrong before you start tearing it down.

 

[Andrew Mace]

I agree with everything above. But I'm curious: How quickly does each cylinder come up to that reading? Very gradually, or within 2-3 compression strokes?

 

[CJD]

Since the engine sat so long, I would bet those readings are from gummed up rings. You could try freeing them before breaking into the engine by adding solvent in the plug holes every several hours and turning over the engine...repeating for a few days. May not work, but it's worth a try before having to break down the "new" engine.

I've run into that many times with motor bikes and old tractor engines. The interesting thing is if you could get it to start, the rings usually free themselves pretty quickly.

 

[sp53]

Try a different gauge and check the valve lash. I think it should pop even at 85psi. If you take #1 plug out, put your thumb over the plug hole and crank the engine until it puffs your thumb off-- then lift the dist cap off --where is the rotor pointing? Plus those little screws in the cap do not index exactly.

 

[Got_All_4]

You should do 3 compression strokes. Your first compression stroke should jump up rather quickly then 2 more with lesser increases. I'd do what John suggested. Squirt some penetrating fluid or Marvel Mystery Oil in the sparkplug holes and let that sit over night then crank the engine. may take a couple of times to get them loose but best if you can get it to start and run for a while.

 

[Geo Hahn]

Stating the obvious just in case -- you do need to have the throttle wide open when doing a compression test. Not opening it will yield low numbers as the cylinder will not suck in enough air to give proper compression.