I would do all the diagnostics you can with the heads on. However if you are pulling the intake and heads - now would be the time to refresh the motor. Going the extra mile to have everything replaced/checked is only a few hundred dollar and piece of mind. That is assuming everthing checks out and if not yes you will spend more, but not as much if it lets go at 6k+rpm.

You have the time now - go through everything real good. Taking a motor out with a buddy is only a 1-2, maybe 3 hour job if your new to it. bag everything and label it.

quote:

Originally posted by ski_dwn_it:
I would do all the diagnostics you can with the heads on. However if you are pulling the intake and heads - now would be the time to refresh the motor. Going the extra mile to have everything replaced/checked is only a few hundred dollar and piece of mind. That is assuming everthing checks out and if not yes you will spend more, but not as much if it lets go at 6k+rpm.

You have the time now - go through everything real good. Taking a motor out with a buddy is only a 1-2, maybe 3 hour job if your new to it. bag everything and label it.

I put it in this March. It was fresh at that point. The previous engine held together for 35+ years. I'm gonna see if the issue is limited to the heads (crossing my fingers). Take care. Tom Worthington

Some progress. Today I pulled the passenger side valvecover to try tapping on the valves. With the leakdown tester installed, I tapped on the intake valve first. Basically no change. But then I tapped on the exhaust valve. It dropped immediately from approximately 50% leakage to approximately 10% leakage (!!!). Thank you Green1! Now. If the valve is actually bent, how can it seal at 10%? Wouldn't it still be pretty bad, even after the tapping (I don't think of 10% as being really bad). And if it is the valve sticking in the guide, why? I did really give the spring a good looking over, and it sure didn't show any sign of breakage.

Yes, the heads are coming off. But the more of this stuff I can determine first, the better. Thanks again for all of your responses already. I'll probably sleep better tonight, convinced that I'm not looking at a shortblock issue. Take care. Tom Worthington

Tom, that is good news, (better the heads than the short block). Were the heads fresh on that engine when you put it in? You would think if the guides were a bit tight, they would have either loosened up by now, (or grabbed a valve sooner) Do you run oil seals on them?
On the advice of a few guys I trust and respect, I started leaving them off a few builds back, to ensure the guides get some lubrication. No negative consequences so far...

I don’t think a valve leaking that bad if bent would seal up tapping it but depending on how hard you were tapping it I guess there’s the potential for the air pressure to force it closed as it flexed from being tapped. Mine wasn’t that bad when I had it happen, thinking it was only in the 20% range but was a long time ago.
Do you run air cleaner? Just curios because of the fact you said cylinders 5 and 6.

These are the heads off the engine that let go. I had them freshened up before putting them on this fresh shortblock. I am running a seal on them now, I didn't, it the past. But in the past, I was getting some smoke I didn't like, so I asked the machinist to put some seals on, this time. He said the guides didn't need any work. I always run an air filter. Take care. Tom Worthington

Iron or aluminum heads ? I had a valve seat come out one time and made a big mess.

Personally I would have to pull heads just to see what was going on.Since you have pretty much narrowed it down to being the heads them selves that are problem.

quote:

Originally posted by NFC:
Iron or aluminum heads ? I had a valve seat come out one time and made a big mess.

OEM small chamber oval port cast iron heads. Take care. Tom Worthington

quote:

Originally posted by wideopen231:
Personally I would have to pull heads just to see what was going on.Since you have pretty much narrowed it down to being the heads them selves that are problem.

I will be pulling the heads, but I'm glad I ran these checks before I did. Take care. Tom Worthington

All it would take is some carbon to cause a poor leak down reading.

Over time seats rust on iron heads and carbon builds up on valves and around seats.

I will add that the machinist should check valve to guide clearance. I had a new set of Indy heads that the clearance was to tight causing the valves to hang in the guides..

Dave

Stop the presses!

I decided I wanted to see if the #5 cylinder would behave the same way as the #6 cylinder did. So I pulled the driver's side valvecover off, and what do I see? The #7 cylinder intake rocker has completely backed off and the pushrod is no longer under the rocker. They are polylock locking nuts, but I must have not tightened it enough. Mind you, the pass I made before the one where things went south was the best mph I'd ever gotten with this combo. Therefore, all this rocker arm backing off took place in very little time. No, I didn't notice a bunch of extra rocker noise, but then, this is a solid lifter flat tappet cam.

If I button it back up and everything is dandy, do I sweat those leakdown numbers? I'm not planning on it, but I suspect many of ya'll will suggest otherwise. I sure am glad I am taking a lot of time with this. Could turn out to be a quick and very inexpensive fix. Take care. Tom Worthington

Happy New Year Tom! What did the compression test reveal? I don’t leak anymore, need to know entire strokes story. Could be garbage in the rings, seats etc.. Hoping it’s simple for you.

BP

quote:

If I button it back up and everything is dandy, do I sweat those leakdown numbers? I'm not planning on it, but I suspect many of ya'll will suggest otherwise

as already suggested may be rings have contaminates.this could be true because of the cylinder that was not fireing properly.run engine and get it hot and try to make sure of clean combustion then recheck numbers

There's a problem with the valve where the leakage dropped from 50% to 10% when you tapped it. The spring pressure is straightening or somehow forcing the valve to seat. To verify remove the spring and leak test it with a weak spring.