Four Strokes and Leak Down/Compression Tests - How They Work

[aswracing]

Excellent post.

Also excellent follow-up on the bike wanting to move during the leak down test. We normally have the bike in top gear and get somebody to stand on the rear brake while someone else operates the tester.

A thing or two I'd add ... during the compression test, you need to hold the throttle wide open and if using a CV, hold the slide up. If the cylinder can't freely pull in air, the compression reading will be low. Also, it matters not where the motor is positioned when doing the compression test. Soon as you hit that starter button it ain't gonna be sitting there anymore anyway.

The compression tester has a one-way valve in it's hose, allowing it to see the pressure of the compression stroke but not relieve it on the other strokes.

Always look for a compression tester that has it's check valve as near to the chamber as possible. Otherwise, you're adding the volume of the hose up to the check valve to the chamber volume, and that makes the compression read low.

Temp plays a role in the results of both of these tests. Best to do it with a warm motor.

If squirting some oil into the spark lpug hole improves compression, your rings are leaking.

Also, there are two top dead centers: the one in between the compression & power strokes (known as compression TDC), and the other one in between the exhaust and intake strokes (known as overlap TDC).

During overlap TDC, both valves are slightly open. So you can't do your leak down test in this position. Well, you can, but you'll get a LOT of leakage.

Really you can do the leak down test in any position where both valves are closed. The period of time when both valves are closed is from about halfway up on the compression stroke to about halfway down on the power stroke. Compression TDC is about halfway in between those two places so it makes a good reference point. But it'll work anywhere between those two events.

Some leakage is always normal. A fresh motor will generally show 4 to 6% with conventional rings, 1 to 2% with gapless. 8 to 10% is not unusual for a motor with some miles on it.

Great job!

[xl1200r]

And then the big guns come out...

Thanks Aaron, some good points in there about checking the rings and the position of the valve, etc. This where TONS of experience comes in handy.

[mark883]

I'm gonna stir things up a bit and say that valve guides CAN have an effect on engine sealing. But, this is an indirect effect.

If the valve guide is very worn, the valve will be sloppy in its travel. It will, if lifted off the seat, have side to side 'slop' or wiggle. This slop will allow it to come down 'unsquare' on the valve seat, causing uneven wear on the valve face and seat, eventually creating a leakage of compression thru the valve. It will also wear the valve stem. This is not a major loss of compression, but there are leakdown issues associated with it. In my experience building racing engines out of 5hp Briggs flatheads (I'm sick, I know it) with 1 cylinder.... engine seal is everything. Rings, Cylinder bore perfection, and valve seal are the biggies. And a sloppy valve guide will quickly ruin your intricate valve job and lapping. This is most prevalent on the exhaust side of the briggs due to the exhaust heat and general lack of lubrication.

So yes, a bad guide will cause loss of compression, but then the guides are not a part of the engine sealing itself.

GOTWA- I've been reading about your engine miseries in another thread. If your dealer is going to do this right, he will replace the valve guides, but he will also measure the valve stems to make sure they aren't worn. And he will definately do a valve job, because if you don't do a valve job when you've got the valves out, you're pissing away time and money, and just asking for a bad engine seal. Also, if he's pulling the jug off to replace the rings, he better measure than to make sure its not out of round- if it is, a (oversize) rebore is in order. If not, they better put a crosshatch back on the cylinder walls (light honing, not an overbore) or most likely, your rings will never seal up.

In a v8, you can get away with a marginal cylinder, in a 2 cyl HD engine, one marginal cylinder gives you an oversized Buell Blast.

[xl1200r]

Originally Posted by mark883

I'm gonna stir things up a bit and say that valve guides CAN have an effect on engine sealing. But, this is an indirect effect.

If the valve guide is very worn, the valve will be sloppy in its travel. It will, if lifted off the seat, have side to side 'slop' or wiggle. This slop will allow it to come down 'unsquare' on the valve seat, causing uneven wear on the valve face and seat, eventually creating a leakage of compression thru the valve. It will also wear the valve stem. This is not a major loss of compression, but there are leakdown issues associated with it. In my experience building racing engines out of 5hp Briggs flatheads (I'm sick, I know it) with 1 cylinder.... engine seal is everything. Rings, Cylinder bore perfection, and valve seal are the biggies. And a sloppy valve guide will quickly ruin your intricate valve job and lapping. This is most prevalent on the exhaust side of the briggs due to the exhaust heat and general lack of lubrication.

So yes, a bad guide will cause loss of compression, but then the guides are not a part of the engine sealing itself.

GOTWA- I've been reading about your engine miseries in another thread. If your dealer is going to do this right, he will replace the valve guides, but he will also measure the valve stems to make sure they aren't worn. And he will definately do a valve job, because if you don't do a valve job when you've got the valves out, you're pissing away time and money, and just asking for a bad engine seal. Also, if he's pulling the jug off to replace the rings, he better measure than to make sure its not out of round- if it is, a (oversize) rebore is in order. If not, they better put a crosshatch back on the cylinder walls (light honing, not an overbore) or most likely, your rings will never seal up.

In a v8, you can get away with a marginal cylinder, in a 2 cyl HD engine, one marginal cylinder gives you an oversized Buell Blast.

Excellent point, but what was mentioned was that valve seals, not valve guides, don't contribute to chamber sealing.

Guess you're not stirring the pot as much as you'd like...

[mark883]

Ooops.. I was thinking guides.... Duh. I've got briggs on the brain- their valve guides are their valves seals.... Or is it, they just dont seal the valve (let 'em leak!)

[xl1200r]

Originally Posted by mark883

Ooops.. I was thinking guides.... Duh. I've got briggs on the brain- their valve guides are their valves seals.... Or is it, they just dont seal the valve (let 'em leak!)

Well, I don't think a flat head really needs valve seals seeing as how the oil woudln't run down into the combustion chamber - it would have to run up the valve.

Actually, now that I think about it, that engine might be mounted sideways, so I'm guessin' they just say "the hell with it".

[GOTWA]

Thanks Aaron! And thanks for adding in those, as always, awesome points. Especially, on TDC that is cool to learn and something I did not know.

Mark, the issues you raise with cylinder integrity or exactly what my concerns are. I'll leave that in the Oil Use thread but you are exactly right and that is exactly what I'm worried about.

Thanks again for all the inputs. Just meant to ramble and got some great interaction and learned along the way to boot. Reckon it's like the sign on the door says, "Learn. Share. Ride."

[milmat1]

SIMPLIFIED VERSION:

"Compression test" is a test of the cylinders pumping ability !!!

(Thats why it's very important to rotate the engine through several strokes
and note the increase in pressure with each compression stroke. Compare this to the other cylinder(s). that is more important than the actual numbers!)

"Leak down test" is a test of the cylinders ability to hold pressure. (Seal)

( Apply a know pressure to the cylinder and see how quikly it "leaks Down".)


Generaly if a compression test shows anything suspect, The next step is a leak down test. anything over a few % points, And the heads come off to find out WHY !!!

[milmat1]

However:

Worn Valve guides will totally screw up the valve train geometry !

Causing erratic valve action that Will lead to missfires !!!




Yes its the weekend and i'm back here stirring up $h1t !!