Compression Testing....couple questions
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Road Warrior
Joined: Feb 2009
Posts: 1,274
Likes: 130
From: Now Western TN
Compression Testing....couple questions
I would like to take my compression readings on my bike. It has around 34k on it now and even though probably should have done this sooner, I want to take the readings for reference. This way I can check it as I get higher mileage on it.
So 2 questions:
1) I read the manual on how to do it but it doesn't mention anything about pulling the fuse for the Fuel injection pump. Should you do this and if you don't will it just be pumping fuel into the cylinders when you are checking the compression?
2) what size adapter do I need for the compression gauge? I bought a compression gauge at HF that had 4 adapters, but of course none of them are the right size/thread.
Thanks!
So 2 questions:
1) I read the manual on how to do it but it doesn't mention anything about pulling the fuse for the Fuel injection pump. Should you do this and if you don't will it just be pumping fuel into the cylinders when you are checking the compression?
2) what size adapter do I need for the compression gauge? I bought a compression gauge at HF that had 4 adapters, but of course none of them are the right size/thread.
Thanks!
Thread Starter
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Road Warrior
Joined: Feb 2009
Posts: 1,274
Likes: 130
From: Now Western TN
Well I went out and looked at the compression gauge kit I bought and it is the 12mm adapter I was trying to use. It starts about 1-2 threads and then stops.
Any ideas? Guess I could check the adapter threads for a bur
Any ideas? Guess I could check the adapter threads for a bur
Road Master
Joined: Mar 2010
Posts: 934
Likes: 54
From: SE Wisconsin
I just looked at the harbor freight comp testers, not real impressed.
Not good reviews there either, guys say the readings are off.
It looks like they come with the 12 x 1.25mm attachment, or at least supposed to, maybe poor quality on the threads.
It's no wonder the pressure is off, I noticed the hose is 20 inches long.
In case you haven't thought about it, a 20 inch hose at 1/4" diameter essentially adds about 1 full cubic inch of area to the head. Not really representative of whats going on normally. If the check valve was on the other end of the hose it would be fine, but it's not.
If you are just using the readings for reference and use the same gauge every time, I guess you could at least see a trend.
I'd probably return it and look for one that puts the gauge or the check valve very close to the head.
Not good reviews there either, guys say the readings are off.
It looks like they come with the 12 x 1.25mm attachment, or at least supposed to, maybe poor quality on the threads.
It's no wonder the pressure is off, I noticed the hose is 20 inches long.
In case you haven't thought about it, a 20 inch hose at 1/4" diameter essentially adds about 1 full cubic inch of area to the head. Not really representative of whats going on normally. If the check valve was on the other end of the hose it would be fine, but it's not.
If you are just using the readings for reference and use the same gauge every time, I guess you could at least see a trend.
I'd probably return it and look for one that puts the gauge or the check valve very close to the head.
Road Master
Joined: Jun 2011
Posts: 974
Likes: 13
From: Chesapeake Virginia
I own some HF tools and have been happy with them but I am usually cautious about Chinese made items that have threads. Bolts & nuts are usually not so bad but specialty items like the adapters you need are hard to find in good quality for cheap. I don't think they have the full picture about how important the tolerance of thread pitches are and have a hard time holding them. I have seen quite a few threaded fittings that have threads cast in with a parting line. This is a huge no can do but they still do it. You might run by a local machine shop and get them to chase the threads on your adaptor. The fitting more than likely has an "O" ring on it for sealing to the head and it doesn't need to be very tight so chasing should not cause an issue. Don't ever buy a tap and die set from HF or Northern China's ability to harden Steel is like east coast weather. It changes by the minute.
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Road Master
Joined: Jun 2010
Posts: 1,221
Likes: 4
From: Annemasse (border of Geneva-Switzerland) facing Mt-Blanc.
To get a reliable reading you may need to add a few drops of oil or gasoline in the cylinder and spin the crankshaft a few cycles to distribute the 'sealant'.
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Road Master
Joined: Jun 2011
Posts: 974
Likes: 13
From: Chesapeake Virginia
I only add oil to a compression test if I get a low reading. If the low reading is from the valves leaking by it won't change after adding oil and it helps determine root cause of the low reading. Usually follow up with a cyl leak down test.
Road Master
Joined: Jun 2010
Posts: 1,221
Likes: 4
From: Annemasse (border of Geneva-Switzerland) facing Mt-Blanc.
