Compression in an old flattie

6

66miles

Senior Member
Hi guys. Just tried searching through past threads for this one cos I know it must have been covered many times. Trouble is I'm not the quickest at this sort of thing so I'll start a new thread (at least it'll be short n sweet).
What kind of pressures would one expect in an older flattie v a rebuilt motor. I'm about to compression test this old thing and was thinking somewhere around 100PSI +/- 10% ?
 
remember to hold the throttle completely open, remove all plugs, do two cold and then one at operating temp. depending on flathead 125lbs-150lbs is not out of the question. on cold one squirt a little light oil in the cylinder after the first time and note the changes
 
Compression has come back at 65 70 65 75 75 75 80 75. After a 5-mile run and retested while still hot the pressures had come up slightly. She's a merc, so has the T head with the slightly lower compression than the Ford. I would expect slightly higher pressures though just the same. Given that the pressures have improved after a short high-speed run, is it possible that there could be an issue with sticky valves, or even gummed-up rings, that are freeing up as the motor runs?
 
65 seems quite low and 150 seems high for a stock flathead. 110 to 120 should be about right and within 10% of each other. But these things will run with low compression , sticking valves , low oil pressure , cracks and broken parts. Yes they are built like tanks.
 
I have seen a lot of these engines with compressions in the 75 lb range that all ran good with very little oil use. I would soak the cylinders with MMO for a few day and turn the engine a little to get the high pistons down and repeat the soak. The MMO will
free up the rings and the compression should come up a little. The MMO will also free the valve which if slightly sticking will lower compression. As long as it runs smooth and don't smoke drive it. G.M.
 
Thanks GM. This car was always intended to be a driver not a show-car. To my mind they're not a car unless they drive, otherwise they're a bag of bolts! Thanks for your thoughts on the MMO to help with sticky valves and rings. Think I'll pop the heads off soon for a look at those valves too. Might be a good opportunity to clean out the lifter gallery also. Apart from that the only other thing she definately needs is a rebuilt carb. PS. Soon as I get a fine day and get my camera sorted, I will post some pics!
 
do what GM suggested, that was my next move--did you pump a little light oil in the cylinders while doing the compression check and if yes did the numbers go up? Be sure and put a little MMO in the gas which will also help with sticky valves.
 
Yep. Poured a little diesel in plug holes, left for half hour then tested again. Pressure has come up 5-10 pounds on all pots. The lowest one did the best. Came up from 65 to 75.
 
not quite what I meant, I wanted you to put a little motor oil in the spark plug holes. Putting diesel into the spark plug hole and the compression going up probably means you do have stuck rings and that helped to remove some of the crud on the rings
 
Thanks Alan. I'm about to get really bold now with the diesel. Diesel is cheap (no MMO in stores round here). I've made a plug fitting with rubber hose to wind into the plug hole and fill each pot while it is at bottom. Then I'm gonna slowly hand-crank her and force that stuff right down into the ring grooves as well as up into the valve guides. If she keeps cleaning up that way then I gonna change the oil and give her a soaking with MMO.
 
Mate, I've held off till now....but reading this thread I'm just sitting here shaking my head! .....sorry!
I will try to explain;
The reason to do a compression test is to get an indication of compression pressures. In a perfect scenario, all cylinders would be of identical reading; in actuality, you can expect to see a slight variation from one cyl to the next.
Right; you do your test with engine at operating pressure, all plugs removed, throttle wide open. Record the readings; any variation over 10 psi indicates a problem in the low cyl. This could be worn rings/cylinders OR bad valve. Other things could also play a role; leaky head gaskets etc, but would be also indicated by a miss, so we'll disregard that.
Squirting a bit of OIL, [not diesel] into the weak reading cylinder provides a temporary piston seal, so then if the repeated compression test comes up, this indicates that the rings/cylinders are crook, if the reading is the same, this tells you the issue is with the rings.
Obviously, high mileage, worn out engines will not readily respond, but they also give those other symptoms mentioned; burning oil etc.
Now, I could well be wrong, have been numerous other times in my life [ask my wife], but I personally think that pouring diesel into your cylinders and expecting it to magically cure the low compression issue is a waste of time. In fact, it could possibly remove carbon build ups etc, that are in fact assisting some sortta piston seal, with the result the engine actually loses compression!
Good luck buddy, flatheads are remarkable in their ability to run when absolutely buggered, which is a big part of their appeal. That guy Kiwitony [a good mate of mine] is close to you and pretty clued up, having owned his 37 since new [almost], at least it goes better than new! Meet up with him, he is a stand up guy and I am sure will be only to happy to assist.
Also, feel welcome to drop me a pm any time, I'll help out if possible, but live a coupla hours away [Air NZ]
 
Thanks Brian. Could be sticky valves too I suppose given how she's a little rough under load (with the odd backfire too)! Yea I've had a quick chat with Tony on the barn and look forward to catching up with him in the not-too-distant future. He's not too far from where I live and sounds like he would know a heap of stuff and would be an interesting guy to talk to. Thanks to yourself also. Don't worry mate. I promise to not break her! (amazing the ideas I come up with after a whiskey or 2).
 
You won't break her- heck, I've been trying to break mine since, like, forever, seems like, as they get looser, they rev out better! Just don't develop as much power any more!
 
Good on ya mate! I'll keep ya posted if I have a sudden unexpected breakthrough. Don't hold ya breath though. Miles.
 
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