Tried a valve seat test on the M50, need advice.....

[Dave M]

Not sure what to think of this, so I'll tell the story.

Background: Swapping the M50 block for a rebuilt one. I'm preparing to see a cracked/broken ring in cylinder #5 due to it dramatically 'failing' a dry/wet test. As the head was rebuilt <35,00 miles ago, and I saw few signs of leakage on exhaust valve stems other than in #5 (others are a nice toasty brown), I was wondering what, if anything, i should do with the head.

I asked yesterday, and someone (thanks) reminded me of the 'liquid' valve seat leakage test. So, I manually set each cylinder as close to TDC as I could and filled the 'bowl' with varsol (spark plug in of course). Now, assuming I'm doing this procedure correctly, here is the interesting part. Of the two cylinders that showed no leakage at all, one of them was dirty #5. Two others showed slow leakage (20 sec for a drop to reach exhaust and/or intake manifold) and I could see a ray of light through the remaining two when looking through the exhaust manifold with a flashlight shining on the 'bowl' (substantial leakage).

So, unless this is within spec, I'm head back to the shop. I had brought it to a supposedly reputable 'head shop' this morning and the gentleman pretty well said, "Ahh, it doesn't look so bad, pretty clean. Its OK if the valves leak a bit, and the exhaust stems/guides are usually looser than the intake. I wouldn't worry about it"

So, if I've done the test properly, and the results are telling, I guess I should find someone a bit more serious about diagnosis. I don't mind spending the $$, just want to make sure i should,

Thanks for listening,

Dave M

[632 Regal]

are you sure the valves are completely closed when you do this test? Is it hard to remove the cam to be sure the valves are seating?

After you redo the test (if needed) I would definetly take it to a different shop, these heads should last quite a while and 35k isnt that.

Some shops still knurl the guides for a tighter clearance, which doesnt last long at all and in my oppinion is totally hack.

[Dave M]

are you sure the valves are completely closed when you do this test? Is it hard to remove the cam to be sure the valves are seating?

I hope you're on to something. I assumed (hopefully incorrectly) that with cams installed and at TDC, that the valves should be seated. It may very well be a stupid question, but I'll risk asking it.....Is this test better performed with cams out? If so, yes, it would be hard to remove them (no tools), but if it warrants it, i may have someone do it for me.

I was also going to mention my compression values (test performed twice before dissasembly) as they don't jive with valves that don't seat. All cylinders but #5 were between 110 and 120 on the first stroke and between 175 and 180 on the fifth (stable). #5 was in the toilet except when wet, it was the same as others. And as I mentioned, the exhaust valve stems are toasty brown in all but cylinder #5.

Thanks Jeff,

I love this $hit,

Dave M

[Kalevera]

Dave, I agree with the statement that the exhausts are loose-er than the intakes. When I saw the first thread on this, I thought, "man, it takes like 5 minutes to check the axial play in a valve guide," but it does require a dial gauge (as shown in the bentley diagram). I can outline the procedure, if needed.

All of our serious machine work goes to Leo Goff @ Memphis Motorwerks; he's one of, if not the best out there. Returned yesterday was a 2.5l bored to 3.0l; beautiful work. This engine is going into a 20k mile E36 factory mtech (rare). Although we do head rebuilds in house, I'm considering sending an M30 head down to him as part of an FI project for a 5 angle valve job and all the other goodies. So, you might consider calling him up.


best, whit

[Bellicose Right Winger]

So compression in cyl 1,2,3,4 and 6 w/rebuilt head was 175-180 after 5 compression strokes and valves in cyl 5 passed leak test. Clearly cyl 5's low compression wasn't due to bad valves. This is great news. I'd put the head on the new(er) block and move on.

Paul Shovestul

I hope you're on to something. I assumed (hopefully incorrectly) that with cams installed and at TDC, that the valves should be seated. It may very well be a stupid question, but I'll risk asking it.....Is this test better performed with cams out? If so, yes, it would be hard to remove them (no tools), but if it warrants it, i may have someone do it for me.

I was also going to mention my compression values (test performed twice before dissasembly) as they don't jive with valves that don't seat. All cylinders but #5 were between 110 and 120 on the first stroke and between 175 and 180 on the fifth (stable). #5 was in the toilet except when wet, it was the same as others. And as I mentioned, the exhaust valve stems are toasty brown in all but cylinder #5.

Thanks Jeff,

I love this $hit,

Dave M

[632 Regal]

Dave, once you turn the cams to TDC, it could be TDC or between cycles leaving the valves slightly open due to overlap.

Bring it to TDC and then "wiggle" the rockers to make sure they arent holding anything tight and try the test again, if they are tight then rotate the cams to the other TDC and they should be loose (lol im losing myself). This should at least narrow down the playing field.

[Dave M]

Bring it to TDC and then "wiggle" the rockers to make sure they arent holding anything tight and try the test again

Jeff, I know where you're coming from, but the M50 head is hydraulic and I don'y know how to make sure the valves are fully seated at TDC. Am I out to lunch?

As Paul stated above, most signs point to drop the head on, and, of course i'd love to believe that and 'move on'. The guy I took it to did come well recommended and, from his apparent age, has seen a few cylinder heads in his day. Can he look at it and say it appears OK? Who knows.

Lowell, as good as your guy might be, I'm in Thunder Bay, Canada. Shipping would cost more than the head job But thanks.

What to do?

Dave M

[632 Regal]

sorry bout the hydraulic thing but the rockers shouldnt be tight...anyways cyl 5 didnt leak and its probably good to go.

Pull the piston on #5 and see what is wrong, then you can move on without worrying about the head.

[Dave M]

Thanks for the peace of mind (for now). I'll try and pull #5 piston ASAP and let the board know either way.

Later,

Dave M

[Bill R.]

results either way, by simply rotating the cams enough to ensure that each chamber has the valves closed when you do the liquid test. And no is the correct answer to leakage on the valves.. none.. Thats not your problem since 5 does seal but i still wouldn't bother putting a head back on if i already have it apart without regrinding or recutting the seats (depending on the equipment your machinist has) and doing a clean up cut on the valves. When the head is cleaned up and the valve stems clean and dry is the only way to accurately check the clearance on them .5 mm , exhaust and intake is the same on that head although a number of heads do specify greater clearance on the exhaust guides and also the exhaust guides wear quicker than the intakes... all that aside you still have a ring or cylinder problem with number 5# as you've already surmised.

I hope you're on to something. I assumed (hopefully incorrectly) that with cams installed and at TDC, that the valves should be seated. It may very well be a stupid question, but I'll risk asking it.....Is this test better performed with cams out? If so, yes, it would be hard to remove them (no tools), but if it warrants it, i may have someone do it for me.

I was also going to mention my compression values (test performed twice before dissasembly) as they don't jive with valves that don't seat. All cylinders but #5 were between 110 and 120 on the first stroke and between 175 and 180 on the fifth (stable). #5 was in the toilet except when wet, it was the same as others. And as I mentioned, the exhaust valve stems are toasty brown in all but cylinder #5.

Thanks Jeff,

I love this $hit,

Dave M