leak down and compression testers (2024)

i asked this once before and never settled on something.

looking to pick up a pair of these.

got mybised torque wrench from snap on but no such luck with leak down or compression testers.

want a solid reliable tool but don't need bling.

recommendations? start trolling the pawn shops in texas? other?

things to watch out for when buying used?

THANKS

Most any of these will be reliable as an anvil. Compression testers can leak down and maybe give false readings.
Leak down testers already leak, that is what they do.
On the cheaper things, gauge accuracy and repeatability can be an issue but 10% of full scale is pretty normal for consumer grade gauges and going to lab grade gauges at 1 or 2% really isn't needed.

would one of these work? do i need to get anything in particular by way of accessories?!

prices looks /stupid/ low for some reason...

http://m.sears.com/search=leak down tester

Quick search shows a HF unit for $40 , but an OTC is only $20 more . I have a Mac at I don't remember it being too expensive .

I wouldn't go for anything with an exposed plastic hose and I'd probably avoid those cheapo things.
The one at $77 seems nice, just be sure it has a means of changing adaptors for the different plug sizes.
I would give Summit Racing a look, they usually have some quality stuff.

that Sears leakdown tester looks ok to me. for that price I'd buy one just because. I have an Imperial/Eastman in my kit for over 40 years because a "leakdown" is the only one we use in aviation (except piston helicopters). it's a different kind of test... a bit tricky because you have to hold the crank at TDC to get a reading. it also requires some skill to do it correctly, and interpret the results. its a good diagnostic tool though because you can hear where the leakage is going. you have to have a compressor to use the tool

the direct reading tester is the automotive standard. easy to use. the only down side on motorcycles is some (big singles) have an automatic compression release. that opens the exhaust valve at low RPM & kicks out when the engine is running. readings will be low unless the release is disabled

And you learn that the difference between TDC and close to TDC determines whether you have a close call with the prop or not.

Come to Houston and borrow mine.

I got the HF one recently. 20% discount. Seems OK.

But you need 100 psi and be able to hold the crank still to get accurate readings.

For cars and motorcycles I suggest only getting a compression tester. The reason being that for either of those vehicles, if your compression is low for whatever reason you need to do the rings and valves at the same time anyway, so no real need to pinpoint where the exact point the problem happens at.

For most bikes I'd agree but for airheads (which the op has) the valve and seat issues that crop up don't mean you need to re ring so both tests are really necessary to isolate the issue

And in that situation how would a leak down tester give you a different diagnosis than than a wet compression test, other than letting you hear which valve leaked worse?

With a leakdown tester you can listen for air coming out the dipstick tube if it's getting past the rings, as well as air coming out the exhaust or intake to isolate which valve is leaking.

Punkinhead, this is proving my point. You do a wet compression test and that isolates the valves from the rings so you don't need to listen for anything. Far as isolating which valve is leaking, why? Are you only going to do one valve and leave the other as-is? I suspect not.

Don't get me wrong, leak down testers have their place just not for motorcycles and other small engines for the most part. Show me an engine that is large and difficult to crank over enough times or fast enough to get a valid compression reading, or has cylinders arranged so that it you cannot reliably perform a wet cylinder test then you start getting into situations where a leakdown test becomes the method of choice.

Don't get me wrong, leakdown testers have their place as I have one too (mines a Mac) but I'm also a commercial rated pilot/CFI/airplane builder and there is no other way to evaluate hand propped or the big engines as an example, but for easily cranked motorcycles they are as the late, great Sydney Wang says: "....like television set on honeymoon: unnecessary."

"Show me an engine that is large and difficult to crank over enough times or fast enough to get a valid compression reading, or.."

in other words every bike with automatic decompression for starting

actually mr. monkey, your point is well taken. the straight 'ol compression tester is easy to use and produces reliable results with minimum fuss. fwiw, I have lapped exhaust valves without pulling cylinders

I agree.... buy the screw in type

I'll go one step further and say no compression tester at all is really necessary for the shadetree mechanic. I wouldn't own a leakdown tester if I wasn't required to do use them for annuals on my planes and if I didn't own that I wouldn't waste money on a traditional compression tester. Whether leakdown or traditional compression tester, it's a crude test at best and different mechanics interpret the results wildly differently. Might as well stick your finger over each of the sparkplug holes and crank the engine and feel the difference in pressure from cylinder to cylinder. I'm trying to think of a case in 30+ years of motorcycle ownership where that test wouldn't have told me all I need to know. If a cylinder feels weak I'm going to pull the head first and look at the valves. If it turns out not to be valves I haven't lost any time because the head would have had to come off to pull the jug and look at the rings anyway.

But my theory of tool ownership is that if I'm going to spend money on a tool I'll bite the bullet and get the best or the one that gives me the most information, so if I'm going to own one I'd rather it were a leakdown tester than a regular compression tester. Better yet would be to own neither if circ*mstances allowed.

The only place I have found a leak down tester to be really useful was on my drag bike engine. High compression NOX motor makes for a lot of heat into the exhaust valves. The engine ran 3x/week with usually 4-6 runs/night. A fresh engine would only leak down about 2-3% and after 50 or so runs, it would be leaking more than 5-8%. Pop cylinder head, freshen valve sealing via cleaning and lapping and a new set of valve springs after 100 runs. All good. This became routine and after, I just left the leak down tester on the shelf.
You can just hook up a compressor to the spark plug hole and do the same listening, I just used my nitrogen tank.

so - anyone know if MAC or Motion Pro are better products or if there is an alternate? getting ready to plunk down on one of each and then going to try to teach myself how to do the tests...

sort of ruled out craftsman and HF at this point.

edit: ach, maybe i'll stop by the local auto parts store and rent one...