TOPIC:

Thanks for the advice guys! But could someone describe the leak down test for me?

kzwolfsr wrote:

Thanks for the advice guys! But could someone describe the leak down test for me?

Here's some info from the archives ---

KZrider Archives » Engine » leakdown test question


leakdown test question

I was looking at ..this page about the leakdown tester on Murphyau's site, and I came up with some questions I was wondering if anyone's got some ideas..

1. you connect the device to the air compressor first, right?

2. I think I can do this with stuff I have already. My compression gauge might have had a leakdown adapter option, at one point, it has this hose with the quick-disconnect at one end and the spark plug thread at the other. I have an 80-gallon air compressor with a regulator on it. What if I set that regulator for 100 psi and use its gauge to measure? it will be at the end of 50' of 3/8 hose, but the air pressure should be equal at both ends, right?

I'm noticed before that the regulator's gauge dips when you use air tools, I guess I would just be hooking up the cylinder as some kind of air tool and measuring its consumption by recording the pressure drop. Then I could pull the cutoff valve lever at the tank, and come back the next morning to see how much air leaks out overnight?

wiredgeorge



The point of testing compression is to determine how well the rings and valves hold this compression and a regular compression gauge that costs less than $20 will tell you specifically that number. You need to warm your engine and then pull ALL the plugs and crank the throttle wide open. Put the gauge in a plug hole and crank a bit. It will pop right up to whatever value compression. OK... what does this prove? That the compression is within service limits and none should vary more than say 10 lbs from any other cylinder (I don't kow what kind of vehicle you are looking at checking so cant' talk specific numbers). If the bike has the right compression for each cylinder, then the valves are seating properly and the rings working as they should... if it DOES NOT...

Then leakdown! What does the leakdown do? You shoot some air into the combustion chamber... since there are really only two places where it can escape:
a. past rings
b. past valves
You need to LISTEN for the air escaping. The point where the air escapes will indicate which area of the engine (head or cylinder) needs to be addressed to fix the low compression problem. Air coming out your crankcase breather indicates rings and from the exhausts indicates exhaust valve seal is poor or from intakes (remove carbs?) indicates intake valve seal. Since some of the valves will be open and other closed, you NEED to understand that you will get air coming out of either the intake or exhaust ports regardless of where your crankshaft is oriented...

My personal opinion is that while the leakdown is a wonderful diagnostic tool, if you have poor compression on the initiall straight up compression check, then you will be removing the head anyway so you might as well look at the piston/bore clearance and rings too... and head too since it is off so why bother with the leakdown... just my 2 cents.



wiredgeorge



warning, tongue-in-cheek comments ahead

I get it, it is a method for mechanics to waste the customers paid time; and a divining rod to guesstimate for said customer how much the repairs are gonna cost!

Yeah I just was wondering in general, I think you figured out from my other thread what engine I had in mind

So if I'm already gonna be tearing it down and measuring, playing with the leakdown test could just be wasting my time.


I get it, it is a method for mechanics to waste the customers paid time; and a divining rod to guesstimate for said customer how much the repairs are gonna cost!

Yeah I just was wondering in general, I think you figured out from my other thread what engine I had in mind

So if I'm already gonna be tearing it down and measuring, playing with the leakdown test could just be wasting my time.

---

The scenario goes like this, Compression gage only tells you that the cylinder is good or low. The leakdown helps a mechanic determine whether it is Rings, Pistons, Valves, Gaskets.

Quoting someone a repair for a burned valve is going to be a lot different than replacing a piston or a set of rings.

Mechanics in my shop when I was an SM were required to a comp test before a tuneup. All the tune up in the world will not fix a burned valve. If the cylinder was low then the leak-down test was done to find out how deep we had to go estimate wise.

Don't waste your time with odd automotive-type compression testers that can be bought at Auto Zone, Pep Boys,... Go down to Sears, and get what I got. A good compression tester with a correct spark plug adapter. I bought this kit after I found out that no one will sell just the adapter needed for my bike, all by itself.

Note. Compression test needs to be done with the engine warmed up. And like WG said above, all plugs removed, and throttle wide open. 4-5 compression pumps, and that should be enough for an accurate reading. I can't remember, but some people say the first compression pump tells something about the engine, but what it says escapes me at the moment.

Craftsman Compression Test Kit

Includes a full assortment of adapters for most automotive or small engine applications. Shock-resistant gauge that reads 0-300 psi/0-20 Kpa x100. Rubber boot on gauge. Blow-molded case for storage.
Sears item #00947089000
Mfr. model #47089

38,000 miles on my bike, and I had readings of 140, 135, 135, and 135. Those are strong readings. 119-185 psi is what's stated in the factory manual.


that's a nice kit, Corey. I bought two filled rolling chests when I bought this house; they had a compression tester with spark plug adapter.

I hear ya Lt.Dan, I was just trying to joke around with it.. after reading WG's post, I used the hose I described to hook to my air compressor and paid more attention to where the air was coming out than what the gauge was saying. Very interesting.


leakdown test or differential pressure test same thing.. aircraft mechanics use this all the time to determine condition of engines.. it is NOT a waste of time and is a lot more accurate than than cranking over and reading,,, engine must be warmed up ,, pull all plugs hook up the tester put piston on tdc than apply 80 psi you do need to hold the crank somehow which isn't all that easy on a bike.. but after you read your pressure and it is less than 60 generally something needs some work just another thought for you guys i've used d/p test for years its more work but i think it is worth the time it gives a better idea whats going on inside the engine




wiredgeorge

KZ King








Posts 5506
Registered 30-7-2003
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posted on 30-3-2004 at 16:39

Didn't mean to imply that leakdown tests are a waste of time. The equipment costs considerably more than a standard compression tester. The one from Sears is nice but a regular "hold the rubber end in the hole" tester that costs $20 works as well at giving straight cylinder compression readings. Once you know that you have a problem, and you are riding the bike, is there really value in knowing specifically WHERE the problem is? In a situation where you are in a repair shop, there sure is... You can quote X for a burnt valve as has been pointed out or Y for new rings... two markedly different estimates. The problem with the leakdown is that it WILL NOT really help tell you if your cylinder/bore clearance is too great and you will need new pistons or if your valve guides are also bad on the top end. You will still need to get into the engine, even in the best equipped shop, to assess the final dollar amount the repairs will cost. Is a leakdown tester necessary for a repair shop? Sure is... but for home use, where you will be tearing the engine apart in the event of any compression issue? Not as necessary. If I take the head off, I WILL ALWAYS disassemble the head, lap the valves/seats and put in new stem oil seals even if there is nothing wrong... I like to feel the play of the valves in the guides... I am kind of anal that way... In a repair shop, well, that costs the customer money that the customer might not appreciated paying so the head will come off and go on a bench while the pistons are worked on... yes the leakdown has some uses/benefits but you always have costs associated with any benefit. How you make a decision on this should be based on cost vs benefit... my take...

---

wiredgeorge








Hey there. I have both testers. I can see this from Wired George's point of view, but in an automotive world where the cylinders dont come apart so easily as the head a leak down in invaluable. In a cycle environ, if the compression is o.k. and fails to jump with the application of oil... having both I would hit it with the leakdown to determine a valve or gasket issue. Maybe overkill but where
time = money you might find it worthwhile.




Jethro Tull

Senior Member










Leakdown

The best test is ... both.

The leakdown test allows you to pinpoint problems and is a far more accurate test of the cylinders sealing ability.

A compression test is a quick blast of air for a partial second. A leakdown can catch things that a compression test will not. Such as very small leaks that do not affect the compression much (or how the engine runs) but can indicate the start of a what may now be a minor problem (such as a burning valve or a hairline crack in a piston) that could become a major problem on that 300 mile ride to the big bike rally. How would you feel broken down by the side of the road because the shop you took the bike to did not catch the problem because they did not do a leakdown test?

In particular a leakdown is almost the only way and an invaluable tool when used to find leaks in brass head gaskets (or any other gaskets for that matter). Imagine a older bike without electric start. How are you going to find that (for now) tiny leak in the head gasket while jumping on the kickstarter (or even on a modern bike with an electric starter and clutch making all that noise) with only a quick blast of compression per cylinder for a split second? With the leakdown you can hook up and if you listen carefully find the leak every time. This means that you or the mechanic need only replace a gasket instead of doing an uneccesary overhaul because the true cause of the problem was not found.

Given the cost of shop time a leakdown test can save a customer money by allowing the mechanic to know what is wrong and only quote and perform the services that really need to be done.

A leakdown test is good it can tell you not just that the valves are bad but which valve is bad and by how much such as the intake valve on the #2 cylinder is bypassing 10% more air than it should (air hissing out the carb on #2 will you it is the intake valve and the reading tells you how bad it is).

There are things a leakdown can not tell you. Such as: A cam lobe on the intake cam is flat, a throttle valve is not opening, the cams have been removed ... none of these things would show up on a leakdown but they would show up on a compression test. Because in the cases listed above the cylinder would sael fine but it would never allow any air in to be compressed. This is why the compression test is needed.

The reality is the best test to do is the leakdown and the compression test. The compression test gives a picture of the overall condition of the engine and the leakdown allows the mechanic to pinpoint problem areas. If you are going to only do one of them a compression test is fine. But if you are a shop and you need to tear someones engine down you need to do the leakdown test as well. Then by comparing the two you should be able to tell exactly what is wrong and by how much before you ever tear the engine down at all.

Jethro Tull

[Edited on 31-3-2004 by Jethro Tull]










leakdown tested

Very interesting results. As posted in my other thread, a squirt of oil had raised compression in #2 cylinder.

However, putting the cylinder at TDC and filling with air from the above method revealed quite a bit more air coming out the intake port than out the oil filler hole. Covering the intake port with my hand (carbs removed, obviously) revealed the presence of air coming from the crankcase, as well as much more air leaking around the carb holder than I would have expected. I'm definately gonna use a sealer there.

Removing #2 intake valve revealed a shiny metal pimple on the valve's shoulder, and a divot it was wearing into the seat. Picked up some debris somewhere.

Ignoring the PSI gauge and concentrating more on where the air was coming from paid off nicely. Thanks again, George; I wouldn't have known that otherwise.


I like what you said, too, Jethro Tull. But me not being a shop means I have time to do multiple test, and take the engine apart in between tests, up to the limit of my patience. I think I'll put it all back together just to see what difference it made. See, Cycle_Todd, even when time != money the leakdown is still useful, it tells me where to start looking first.

:cool:

---

'83 1100 LTD [KZ1100L1]


Here is a good thread explaining the leak down test, especially WG's posts. Not sure how to cite the archive articles, so if this link fails, will try again.




KZrider Archives » Engine » leakdown test question

Post edited by: Patton, at: 2006/12/06 21:29

Post edited by: Patton, at: 2006/12/06 21:30

Thank You Patton!

If I were to pull my cylinder head FOR ANY REASON, I would check valves sealing, check the guides and install new valve stem oil seals IF I had not done so within the last 5K miles. I would also install new thread inserts in the cam caps and plug holes while the head was off if any needed this.

OK... what would a leakdown test do? If you had any compression issues, it would tell you that they are in the cylinder head and you wouldn't necessarily have to pull the block off. the cylinder gasket is a pain to clean up and you will need a new gasket. If you were running a shop, knowing you didn't need to look at this area would save the customer money. Now if you are working on your own bike, perhaps removing the block, installing new rings and honing AFTER you checked the cylinders would be wise. Catch everything at once is my thought. You should install a new cam chain and new cam chain rubber bits. You should measure the cylinders as has been pointed out... that is, check for round. You do this by using T bore gauges at three different depths in the cylinder bore and at two different angles (6 measurements total). If you find that at the mid-point measurement, in one direction, you are 62.5mm and in the other direction (90 degrees away from the first) you are 63.5mm, the cylinder is out of round and you are wasting money with new rings and honing. You need a new block/pistons or bore your old and install new pistons. This is why I have about half a dozen spare 903 and 1015 blocks with matched pistons on my spares shelf. I have measured each and they are all within spec. If I find an out of round cylinder, I mark it as such and use another. This is cheaper than buying pistons kits and getting a cylinder bored (I don't have the equipment). You also need to watch the amount of clearance between the piston and bore. You measure the piston and if any of the measurements exceed the service spec, you will not have the rings seat well or scrape as they should.

Last, I THINK Jeff at www.z1enterprises.com can come up with rings, if needed. Best to give him a call as I am sure that the 650 rings are not on his web site.

I got ya WG. Thats what I was planning on doing on Saturday. The inside of my cylinder 2 is always fairly wet from the looks of it and my spark plug is oil foiled, so I have to pull the head anyways. I will check all those clearances and roundness with my cylinder and pistons.

Please forgive a novice question, but what is "honing"? Is is something I can do myself, or do I need to take it to a machine shop?

Skyman wrote:

Please forgive a novice question, but what is "honing"? Is is something I can do myself, or do I need to take it to a machine shop?

Here's a decent article on the basics of honing ---


www.realclassic.co.uk/techfiles/tech05010600.html

Thanks, Patton.

So, obviously, this is not a DIY thing.

Skyman wrote:

... So, obviously, this is not a DIY thing.

May stand to be corrected on this, but believe when just replacing rings (same size and no overbore) to "sand" (the service manual says #400 emery paper will suffice) or "hone" the inside cylinder walls to "break the glaze" and thereby better enable the new rings to "seat". In the old days, we also washed out the the cylinders with soapy water and rinsed well before re-assembly with liberal application of fresh new oil in and about the new rings and groves.
So simple "deglazing" would seem a diy thing; however, where more precise honing cross-hatch patterns are desired, you may elect to have it done rather inexpensively at a machine shop.

On the other hand, a cylinder bore job(for over-size pistons and rings) requires precision special tooling and experience beyond the diy capacity of amatures such as myself.

Wow, thanks again, Patton.

Right now, I have the head off (this is on my KZ1000). I did a leakdown test, and I heard some air escaping through the crankcase and a pressure loss at about 9% on cyls 1, 2, & 4. On #3, I had a loss at 15%, and it seemed like I could hear air escaping through the gasket.

So, from what I've read here, I'm thinking I should remove the block, measure the cyls for ovaling. If the cyls are still good, then replace the rings. I'll also replace the head gaskets. Am I missing anything?

Post edited by: Skyman, at: 2006/12/25 14:22

I would use something like this to hone the cylinders:

cylinder hone at NAPA

It's a cylinder hone from NAPA, about $30. If your rings are not seating well and they are not worn out, a light honing will break the glaze on the cylinders.

You want to be really careful because honing does remove material from your cylinder walls and will increase the clearances if done too much.

The procedure - put the hone in an electric drill

Put some light oil in the cylinder
Spin the hone at a medium rpm and move it in and out of the cylinder a few times. Try to get the angle of the cross-hatch lines (created by the stones on the hone) at an acute angel (less than 90 degrees).

Do this for each cylinder and then rinse the cylinder walls to clean out any grit. Lightly oil and reassemble.

I used to use a "dingleberry" hone back in the day when I was doing this for 2-strokes.

Post edited by: steell, at: 2006/12/25 19:42

floridamba wrote:

... I used to use a "dingleberry" hone back in the day ....