Leak down test
Leak down test
Good morning, doing a leak down test on a 2003 Dyna with 1700 miles. Front cylinder is 87 @ 90 psi but when I go to do the rear cylinder, the pressure pushes the piston down and I can't get a reading. Tried several times. I'm putting the pistons at TDC on compression stroke. Funny, I use a piece of a paper towel over the spark plug hole to make sure I'm on the compression stroke, front is good and the paper blows off. The rear just keeps sucking and doesn't blow the paper off.
Compression test: Front 145/Rear 147
What do you guys think?
Thanks in advance
Compression test: Front 145/Rear 147
What do you guys think?
Thanks in advance
Stellar HDF Member
Joined: Aug 2020
Posts: 3,159
Likes: 2,202
From: College Station, TX
The compression numbers look good.
When I perform a leak down test, it is usually because there is a problem and a compression test doesn't reveal anything useful. I also remove the pushrods to ensure the valves remain closed during the test. But I haven't had the need to do a leak down test in a good while.
As far as the piston being at the bottom of the cylinder, keep in mind then the leak down test is only providing a reading at the place the piston is in ther cylinder. There could be other problems above or below that aren't being tested. A bore scope may be helpful if there is a problem you are trying to diagnose that the compression test doesn't show.
Paul
When I perform a leak down test, it is usually because there is a problem and a compression test doesn't reveal anything useful. I also remove the pushrods to ensure the valves remain closed during the test. But I haven't had the need to do a leak down test in a good while.
As far as the piston being at the bottom of the cylinder, keep in mind then the leak down test is only providing a reading at the place the piston is in ther cylinder. There could be other problems above or below that aren't being tested. A bore scope may be helpful if there is a problem you are trying to diagnose that the compression test doesn't show.
Paul
Ok, so I shouldn't be concerned with the piston being pushed down when air is supplied? I guess I'm stumped because I did the test on the front with no issues and have done this test a number of times and never had this issue.
Seasoned HDF Member
Joined: Mar 2010
Posts: 38,295
Likes: 6,322
From: Honah Lee
I only use a leak down if I see an issue with a old fashion compression test with warm engine and throttle body or carb WOT.
I use the leak down to look for valves or rings. Screw that dump oil in cylinder..
Try approaching TDC in low gear turning rear wheel. You may need to approach it one of two ways to get play out of drive train.
Then holding wheel, hit the air slowly.
I have had same issue keeping valves shut with air while removing valve springs to replace valve seals.
PS. Are you saying you can't find compression stroke on rear no matter how far you turn it? It's apx two turns past front.
Of course turning wheel doesn't show that. Harley use to have a plug in primary in the good old old days before us.
I use my thumb on a TC. Then a screwdriver to get at top. Don't suck something in there.
I use the leak down to look for valves or rings. Screw that dump oil in cylinder..
Try approaching TDC in low gear turning rear wheel. You may need to approach it one of two ways to get play out of drive train.
Then holding wheel, hit the air slowly.
I have had same issue keeping valves shut with air while removing valve springs to replace valve seals.
PS. Are you saying you can't find compression stroke on rear no matter how far you turn it? It's apx two turns past front.
Of course turning wheel doesn't show that. Harley use to have a plug in primary in the good old old days before us.
I use my thumb on a TC. Then a screwdriver to get at top. Don't suck something in there.
Last edited by Jackie Paper; Apr 9, 2025 at 08:04 AM.
I've tried and tried and just tried again. Can't get the cylinder to pressure up. As soon as I apply the air, the piston pushes down and the rear wheel spins. I even put the front spark plug in and put the wheel down on the table under full weight, still spins.
If you guys say the compression numbers are good then I'll roll with that.
Thanks for the responses, I've just never had that happen during a leakdown test.
Ride safe!
If you guys say the compression numbers are good then I'll roll with that.
Thanks for the responses, I've just never had that happen during a leakdown test.
Ride safe!
Seasoned HDF Member
Joined: Mar 2010
Posts: 38,295
Likes: 6,322
From: Honah Lee
I am a firm believer in a leak down is finding an issue that exist and you hear escaping air.
Is there even a standard on the make and calibration for a leak down gage set?
You have so little leak down, crank needs locked down to hold it.
Is there even a standard on the make and calibration for a leak down gage set?
You have so little leak down, crank needs locked down to hold it.
Road Captain
Joined: Mar 2020
Posts: 650
Likes: 351
From: Sierra foothills
Set the piston where you want on compression stroke then use a primary locking tool/flat bar. Check several places. Top, middle, bottom
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Seasoned HDF Member
Joined: Mar 2010
Posts: 38,295
Likes: 6,322
From: Honah Lee
However, when I calibrate the leak down gage set, I don't remember seeing 90 on it. More like 15 like this attachment being calibrated.
What kind of leak down set are you using?
However, I don't have access to it since we are still far south.
Last edited by Jackie Paper; Apr 9, 2025 at 02:05 PM.