Leak down test

I put 100 to 125 usally when you get to 100 you will know all you need to know. hope this helps. I have bought heads from some big time head people an all of them leaked around the valves. even a high dollar billet head leaked. had to lap the valves in. a lot of work an time but you can get them fixed. I took a flat piece of alum an made a fixure so you could bolt a head down with a gasket an test them. I wondered how many people just bolt the head down an never check them. joe
 

gregp07

Member
I made a plate and threaded it with the head bolt pattern then I made a gasket out of gasket material, I torque the head down per spec and check the head assembly only. When I build our motors I cut the seats, lap the valves and then do a leak down on the head only, I make sure it is sealed with zero leakage before I finish my build.
 

astrowrench

Member
Remember when doing a 45*valve&46*seat, valves do not seat entirely until initial run in to seal@ approx.45.5* you can do a lot of lapping. This is from NEWAY possibly you have heard of them??


zombieheadservice
 
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alvin l nunley

Site Supporter
Crazy Al here; just can’t help myself, I need to speak.
Leak down tests are good, and with some of the suggestions I’m seeing, really good, but what good is a leak down test going to do you at the track? Having a fixture to bolt the head to is hardly a viable, at the track, method of finding out why the sudden drop in RPM’s, or lap times.
When we were at the track, admittedly with a 2 cycle, and our times were down, we immediately did a compression check. It’s quick and easy and tells you in just a short time if a lack of compression is the problem. If the compression is what you would expect it to be, it tells you very quickly that something else is the problem.
People pump their heads up to 100psi to see if the valves leak. They then want to see no more than a 5lbe drop in pressure in the first five minutes, or so I understand. What I wonder is; how does that relate to the time the valves have to leak when the engine is turning 3000-6000 RPM??
I also wonder; if you’re doing a compression check, how does using the pull start relate to those RPMs. People say you can’t do a compression check with the clone because of the compression release, and that may, in some ways, be true, but I would want to know what the compression is with the engine spinning close to what you see on the track.
I have also seen rings not seating with my 2 cycles. A quick compression check shows this with little effort.
I wonder what the problem is? It’s such an easy thing to do and it tells you so much.
You take a fresh engine, check the compression, do that after every race and you’ll see for sure if the compression is getting better, or worse. Sure it could be the valves, or the rings, so if the compression is down, when you have the time at home, you check the leak down to determine where the compression loss is coming from, rings or valves.
I don’t know for sure, but it seems a leak down test is time consuming, and you may not have the time for it during a race, while a compression check is an easy and quick thing to do.
Now I know people are going to jump all over this, but it’s just my way of looking at things.

Comments, compliments, criticisms and questions always welcome.
 
I don't disagree with Al if your at the track. But at the shop/garage a leakdown can diagnose a problem more specifically than a compression test. Ideally you want to do both...BTW , I get a bunch of diagnostic info at about 10psi just listening to the air and turning the motor by hand through its cycle. A stethoscope is very handy. I also agree with astrowrench. You should not see any light showing through the ports but that may not be enough at 100psi. And the 45* valve and 46* Neway cut does take a wee bit of breakin, However if you lap, lap, and lap again and really look at the result under magnification you can get valves leakfree...When lapping , the sound will tell ya when your done. The gritiness stops and it becomes very smooth. Clean everything and then lap again with WD40 or any cutting oil without the compound. Things should be really , really smooooth at this point...
 

astrowrench

Member
Sneaks; I appreciate your agreeing with me; Using a 45*-46* fit causes an interference fit which is positive 100% if done correctly,and I agree the sound of lapping does change,you learn that after decades of doing it,That's why i don't like using a drill to spin the valve in one direction only.TRUE lapping is Omani-directional;Using an interference fit, the valves seat themselves rather quickly considering they are hitting the seat hundreds of times a second;The only downside is having to adjust the valves once or twice,but you get to look for Gold?? floating around in the oil.Which could be an indication to look elsewhere.
Dave zombieheadservice
 

alvin l nunley

Site Supporter
I don't disagree with Al if your at the track. But at the shop/garage a leakdown can diagnose a problem more specifically than a compression test.
Sneaks, there’s no “But”, that’s exactly what “I” said.
I understand it’s not cool to agree with me, but really, “BUT”? I would have expected more from you!
Comments, compliments, criticisms and questions always welcome.
 
I tried that shine a light in the ports an if u don't see any light at the valves they are ok. then I put head on the alum plate with a gasket an torqued it down an put the air to it an it leaked really bad at 30 lbs . even a couple of the high dollar billet heads for animals I bought leaked bad. 3 hours later they didn't leak after I lapped them in.
 

Kart43

Member
Neway used to discourage lapping they preferred that you allowed the valve and seat to run right off their cutters and they would peen themselves to a perfect seal. I must admit after cutting I always lapped, I just couldn't bring myself to putting the head on an engine knowing there was leakage.
 

jjchat

Member
I find that new valves will leak down about 8-10 percent but after a day of running/breaking in, they will seal completely.

However, I have had a few heads that were good but after a season or two they would no longer seal. When re-cutting the seat, the angle has changed; not sure whether it was the valve guide or the seat.
 

astrowrench

Member
I find that new valves will leak down about 8-10 percent but after a day of running/breaking in, they will seal completely.

However, I have had a few heads that were good but after a season or two they would no longer seal. When re-cutting the seat, the angle has changed; not sure whether it was the valve guide or the seat.
Usually due to the constant re-formulation of gas and the removal of lead causing manufacturers to keep reformulating the valve and valve seat materials In other words they keep screwin with our gas so they can make more money by making it cheaper-- no matter what anybody tells you!!!
 

astrowrench

Member
Neway used to discourage lapping they preferred that you allowed the valve and seat to run right off their cutters and they would peen themselves to a perfect seal. I must admit after cutting I always lapped, I just couldn't bring myself to putting the head on an engine knowing there was leakage.
They still Do My quote was from Neways newest info. As far as the interference fit and run in goes
 

Chipg56

Member
I made a plate and threaded it with the head bolt pattern then I made a gasket out of gasket material, I torque the head down per spec and check the head assembly only. When I build our motors I cut the seats, lap the valves and then do a leak down on the head only, I make sure it is sealed with zero leakage before I finish my build.

I like to bolt them to the flow bench before installation, vacuum shows imperfections where pressure will not because the pressure exerted on the valve head tends to seal it. When on the motor I look at pressure of 50, 75 and then 100.
 
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