Compression test results

JG1966 · 2025-10-29T15:01:14-0500

I just did a compression test on my 340 for the first time since getting it stroked to a 416 about three years ago. Engine runs great, but I was bored and figured I'd check it out before winter. I also plan on doing a leakdown test in the spring (need an air compressor)
Test was performed dry with engine warm. Here are the results
1 -- 180
2 -- 179
3 -- 168
4 -- 180
5 -- 175
6 -- 179
7 -- 177
8 -- 177
The No. 3 cylinder is the only outlier. It's 6.7% below the top number (I tested it twice). Is that acceptable or am I going to spend all winter being miserable? Thanks

Bird 426 · 2025-10-29T15:17:58-0500

Go out and drive it and recheck. It will probably change.

Doorkicker · 2025-10-29T15:21:24-0500

JG1966 said:
I just did a compression test on my 340 for the first time since getting it stroked to a 416 about three years ago. Engine runs great, but I was bored and figured I'd check it out before winter. I also plan on doing a leakdown test in the spring (need an air compressor)
Test was performed dry with engine warm. Here are the results
1 -- 180
2 -- 179
3 -- 168
4 -- 180
5 -- 175
6 -- 179
7 -- 177
8 -- 177
The No. 3 cylinder is the only outlier. It's 6.7% below the top number (I tested it twice). Is that acceptable or am I going to spend all winter being miserable? Thanks

Stupid question, no insult intended... but... did you take all the sparkplugs out before testing, or removed one at a time?

JG1966 · 2025-10-29T15:23:20-0500

Bird 426 said:
Go out and drive it and recheck. It will probably change.

Thanks Bird. The engine was warm but it wasn't operating temp. Removing the No. 5 and No. 7 plugs is tricky when the headers are scorching hot. I'll take her for a spin and get her nice and hot and then check No. 3 again.

JG1966 · 2025-10-29T15:23:57-0500

Doorkicker said:
Stupid question, no insult intended... but... did you take all the sparkplugs out before testing, or removed one at a time?

All the plugs were out and the throttle was open

icetech · 2025-10-29T15:40:38-0500

Just curious, did you check compression when the motor was first built? and i was always told 10% variation is fine from high to low.. but i'm sure someone else will chime in with more

JG1966 · 2025-10-29T15:44:41-0500

icetech said:
Just curious, did you check compression when the motor was first built? and i was always told 10% variation is fine from high to low.. but i'm sure someone else will chime in with more

No. I never did. It has always run great since getting it built. I only did it today because I was restless and bored and wanted to do something on what might be our last warm day for a while.

Sam69sat · 2025-10-29T16:38:05-0500

I wouldn't give it another thought. Even if #3 was 180 you'd never notice.

Strato58 · 2025-10-29T17:04:44-0500

If it runs good that means it is good. Don't be miserable over this because chasing numbers don't mean much. IMHO.

R413 · 2025-10-29T17:17:29-0500

Does it have a solid lifter camshaft? How is the valve adjustment?
Next time go for a walk or something! LOL

JG1966 · 2025-10-29T17:20:46-0500

R413 said:
Does it have a solid lifter camshaft? How is the valve adjustment?
Next time go for a walk or something! LOL

It's hydraulic. I haven't adjusted the valves in a while, but I never really need to.

icetech · 2025-10-30T05:23:10-0500

JG1966 said:
It's hydraulic. I haven't adjusted the valves in a while, but I never really need to.

If you are worried i would grab a $25 bore scope and just take a quick look at the cyl to make sure it's not getting eaten up or anything.. just for peace of mind.. I love my bore scope.. one of my favorite tools now

Rebelrouser · 2025-10-30T06:34:06-0500

JG1966 said:
I just did a compression test on my 340 for the first time since getting it stroked to a 416 about three years ago. Engine runs great, but I was bored and figured I'd check it out before winter. I also plan on doing a leakdown test in the spring (need an air compressor)
Test was performed dry with engine warm. Here are the results
1 -- 180
2 -- 179
3 -- 168
4 -- 180
5 -- 175
6 -- 179
7 -- 177
8 -- 177
The No. 3 cylinder is the only outlier. It's 6.7% below the top number (I tested it twice). Is that acceptable or am I going to spend all winter being miserable? Thanks

The compression check is fine. You are correct that a leakdown test is much more accurate. And as suggested on a performance engine I always do a leak test and compression test when the engine is fresh as a way to track what is going on with it down the road.

Runcharger · 2025-10-30T09:37:34-0500

I have 5 classic motorcycles. I always do a valve set on them and ensure the carbs are as good as they can be, they run good so I don't do compression checks on them for this very reason. If they run good, be happy, sometimes ignorance is bliss they say.

JARRETT LYNCH · 2025-10-30T09:48:00-0500

Doorkicker said:
Stupid question, no insult intended... but... did you take all the sparkplugs out before testing, or removed one at a time?

Why would that matter other than maybe when plugs out you get a higher crank rpm?

Doorkicker · 2025-10-30T14:02:25-0500

JARRETT LYNCH said:
Why would that matter other than maybe when plugs out you get a higher crank rpm?

Not a great deal, just to keep the airflow consistent between each cylinder test. The OP was concerned about an 8lbs swing, so you want to be sure that the test is as accurate as possible.

Compression test results

JG1966

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Bird 426

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Doorkicker

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JG1966

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JG1966

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icetech

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R413

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