1915 Compression Test

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Model T Ford Forum: Forum 2015: 1915 Compression Test
Top of pagePrevious messageNext messageBottom of page Link to this message  By John Aldrich Orting Wa on Thursday, June 25, 2015 - 05:34 pm:

If I am hand cranking a stock 1915 engine to do a compression test, how much compression might I expect to find on the gauge?

Or rather what SHOULD I find on the pressure gauge?


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Aldrich Orting Wa on Thursday, June 25, 2015 - 06:05 pm:

Just ran the test and on an un-calibrated gauge they all said about 30 pounds. I know they should be 50-55 but that is done with a starter.

3 of the 4 cylinders bled off very slowly but they did bleed off.

This was done on the engine that has not run in several days. After re-installing the spark plugs, she started on magneto and on the 3rd pull with no choking.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Thursday, June 25, 2015 - 06:29 pm:

30's not the worst and you're not sure about the gauge plus it started well. 50-55 would be very good. Hand cranking's not a problem. Just keep going on the cylinder until the pressure doesn't get any higher. Usually between 5 to 7 full cranks. Choke and throttle fully open too. I like to do the (relatively) low compression T when cold because that is the condition the engine will be in when first starting. If hot you might get a slightly higher reading and easier starting.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Thursday, June 25, 2015 - 07:45 pm:

John - The little I know about it, that sure doesn't sound like a compression problem to me! I don't think you'll ever get as high a reading cranking by hand as you would by cranking with the electric starter. And it's a fact that as long as you have adequate compression ( and you do) the important thing is that there is not too much variation in compression between cylinders. You did say they were all about 30 lbs, and I'd be willing to bet that if that engine was turned over with an electric starter, which would enable 6 or 7 compression strokes one right after another ( minimal leak down time) your compression would be up in the 50's. I know for a fact that a very loose and worn "T" engine with compression in the low 20's can often be a very sweet running engine. Are you hand cranking with all spark plugs out? That's the best (easiest) way to do it, especially when hand cranking.

(.....easy on the back there ol' boy,.....!)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Thursday, June 25, 2015 - 07:49 pm:

Actually, if I'd read more carefully, I guess you did say,...."after re-installing the spark plugs",.....sorry 'bout that John,.....harold


Top of pagePrevious messageNext messageBottom of page Link to this message  By Steve Tomaso - Milton,WA on Thursday, June 25, 2015 - 07:56 pm:

You should be able to "spin" the crank with all 4 plugs out and one wheel jacked up (just in case the clutch adjustment isn't skookum) and get all the spins you need to register on the gauge - no need for electric start - it won't make the compression increase - but it would sure be helpful if you have a reliable gauge.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Thursday, June 25, 2015 - 10:48 pm:

"Leak down" was mentioned. The only way a compression gauge would leak down is if the one way valve, kind of like a tire valve, was shot and bleeding off causing the gauge to go down. Shouldn't happen. The gauge is supposed to hold it's reading until it's manually released. If you're referring to cylinders leaking down, (piston rings/valves), that's the reason you take a compression test first dry then with a shot or 2 of oil in the cylinder to determine engine component condition. You guys know the drill. A shot of oil and the comp goes up: rings/cylinder walls. Doesn't go up? Usually leaky valves.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Thursday, June 25, 2015 - 11:15 pm:

Also as Steve mentioned cranking speed, hand vs. electric, is not a factor.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Aldrich Orting Wa on Friday, June 26, 2015 - 01:10 pm:

I was able to spin crank with all 4 plugs removed and the throttle wide open. I cranked until I heard one particular "thud" 10 times on each cylinder.

The gauge is an el-cheapo gauge which I of course dropped onto an concrete deck before I ever used it.

SO.... I guess I could do a compression check on my Touring which I KNOW has great compression. That way I would know a little more about the gauge telling me more about the engine in the 15.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Friday, June 26, 2015 - 01:58 pm:

If nothing else, you're getting a lot of exercise! :-O


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Aldrich Orting Wa on Friday, June 26, 2015 - 03:54 pm:

Charlie.

Just re-read your input. Will go back and re-do the check with a squirt of oil in each cylinder but not in this heat!


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Aldrich Orting Wa on Friday, June 26, 2015 - 03:57 pm:

Steve,

Have a good neutral in this car so wheels up isn't an issue. The Touring on the other hand! ;)


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