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Triumph TR3 - Compression Test Results

Finally got around to checking compression on the TR3A. Here's how it fared:
#1 - 130
#2 - 120
#3 - 130
#4 - 125

Love to know how this compares with other TRs. High? Low? Average? Any concerns with the 10 psi difference? I have 87mm pistons and liners.

Many thanks,

Bill Stagg
1961 TR3A
Bill Stagg

Bill - I always check my valve clearances before doing a compression check. I do it when the engine is cold.

Nov-2000 Miles=131573
RE-GAP TO 0.010" INTAKE & 0.012" exhaust
8 ADJ 1, 6 ADJ 3, 4 ADJ 5, 7 ADJ 2
1 ADJ 8, 3 ADJ 6, 5 ADJ 4, 2 ADJ 7
Nov-2000 - Miles = 131575 COMPRESSION 150, 150, 150, 120 (Cold)
Aug-2001 - Miles = 134188
This was 54,000 miles after putting in new pistons, rings & liners.

I have the original displacement of 1991 cc.
The bore is like the original at 83 mm.

Did you cock open the butterfly valves ?
Did you have all the spark plugs out ?
10% variation in compression pressures from one cylinder to another between lowest and highest would be about 13 psi. You are well within that.

Don Elliott
Don Elliott


I set the valves the day before my test. Good suggestion. I also checked them "warm" rather than cold. I believe the manual suggests leaving three of the plugs in while the test is performed on a hot engine, but I can't see the difference it would make with plugs in or out. Can you?

TeriAnn Wakeman had a good suggestion of benchmarking the compression 1,000 miles after a valve job, then taking readings over time to monitor the wear of the engine. The benchmark should give a good relative starting point regardless of engine displacement or mods.

To which butterfly are you referring? Carb? I've not seen reference to this step. What's up?

Many thanks, Don!

Bill Stagg

I use my vice grips attached securely to the front swing lever on the throttle arms up front just behind the rhs horn. The butterflies are the two brass circular plates inside each carb throttle venturi - these open when you accelerate. During a compression check, I always use the vise grips to open these butterfly valves to allow maximun air to get into the cylinders. I have heard that this might give higher compression values because the air can get into the cylinders more easily.

Don Elliott

All this talk and I just had to do it.
Factory manual suggests perfoming when cold, three plugs in, and butterflies open a tick. (what's a tick?)

I opened them wide and got the following on 4 strokes:

#1 - 90,120,140,150
#2 - 90,130,150,160
#3 - same as 2
#4 - same as 1

Garage was about 65 degrees when performed. Yesterday I performed the same test at about 80 degrees and got 170 on the 4rth stroke.

I have a rebuilt 3A engine w/ approx 20K miles, 87mm pistons, and stock H6 carbs.

Should ideal pressure be reached prior to 4 strokes?

Thanks for any thoughts,

Mike Gambordella
1956 TR3
Mike Gambordella

Mike - You are not looking for ideal pressures. You want to know maximum pressure for each cylinder - even if it takes 12 revolutions or more of the engine.

All 4 max pressures should lie within 15% of each other. With the numbers you quoted above, it sounds to me that you have a well built engine with lots of "snap".

By the way a "tick" is smaller than a "snap". Look in between your bedsheets tonight. You might just find a tick. To see a "snap" look on your box of Rice Krispies in the morning.

I set my butter flys wide open - the ones in my SU carbs, not the one on my trousers. No, I swear officer, I haven't had a drop.

Mike, how big or small is a drop ?

Don Elliott
Don Elliott

This thread was discussed between 11/08/2004 and 16/08/2004

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