bad valve stem seals shouldn't cause blowby since they're sealing the vacuum in the ports, except maybe if there is back pressure on the exhaust side. Either way if you want to diagnose the problem more thouroughly you should warm up the car, take out all the plugs, stick the throttle wide open and do a compression check. If they're all within 20 psi of each other and around the 140 to 160 range then you don't have a problem, if they seem a little low, pour a tablespoon of motor oil down the plug hole and try again. If there's a noticable improvement then your rings are bad, if not then the valves are leaking. But leaking valves shouldn't cause blowby, just poor performance.
Bad rings let the compression in the cylinders "blow bye" into the crankcase and pressure builds up and escapes up and out through the lifter bores and on out through the rocker arm or valve covers. Also oil can blow out the oil dip stick tube. You should also find a substantial increase in your oil pressure depending on how far gone the piston rings are.
blow by Check the pcv valve ,hose , openings, ect. Some blow by is entirely normal, I would become concerned only if there was "Puffing" out of the oil filler hole. First do a good compression test. the numbers should be around 125 psi or better. 90 psi would be a weak cyl. any lower than that needs repair. blow by comes from worn or stuck rings or a cracked or burned piston. If a piston is cracked or has a hole burned in it, it will "puff" at the oil filler hole , be noisey and have a dead miss on the cyl. the spark plug will be oily. Hard to miss. look at the spark plugs and see if they are oily or black. Does the eng smoke any? The compression test will tell you the most.
thanks for the help guys any one have a 64-66 401 or425 you never no! nailhead for sale in the milwaukee area? :TU:
Gary, both my old Nailheads got rebuilt b/c of blowby........and both had cracked rings in a couple cylinders. My take on this is that when the cylinders wear, they get larger. That excess clearance causes the pistons to "rock" back and forth, putting stress on the rings until they finally break. If you have a deep ridge at the top of your cylinders, that's the first clue. If so, my bet is you also have broken rings, better fire up the impact. :bglasses:
Joe-Agree on what you say, but I've opened and worked on several nailheads and haven't seen a ring ridge on any yet-now SBC is a different matter!
Maybe it was the chrome rings, but my last one had over 15 thousandths skirt clearance after 140,000 miles.........quite a ridge.
Joe- .015 skirt clearance is about what I found on cyls #1 and 2 (front on each bank) in a freshly rebuilt-then overheated Calif Wildcat I picked up a few years ago. (Poor guy was on his way to Hot August Nights in Reno-never did make it). Those scuffed pistons were slapping so bad, I thought it was rod knock til I got into it. Two new sealed power pistons and it's running nice now. It was interesting to me that the other 6 cylinders were still about .002 clearance and the x hatching on cyls 1+2 were as strong as ever with no cyl damage at all.
Yep, my old motor sounded pretty bad.......I thought it was the thrust bearing......just before I took it out of the car on my last run I took her up to about 60 in low, whatever the tach was reading 6,500 and the valves were floating :Brow: .....I was surprised nothing let loose.......and it didn't, but she sure smoked like hell! :grin:
Puffing...sounds like mine. So my 66 has low compression in one cylinder (145 vs 190 in the rest) A new set of heads, valves, etc., an oily plug, noise and puffing out of the oil filler hole on the valve cover. Having it all checked out for me tomorrow. Will let you know if cracked piston or hole in piston or ring or whatever is the cause.
Update Well the rapping noise, the low compression and the "puffing" out the valve covers did, in fact, mean a broken piston. I am trying to think of this as a long awaited rationalization to pull out the engine and make it like new inside and out. I'll post a picture of the result and a list of installed goodies when complete this spring.
A long time coming, I hope it is all worth it So my 401 is now out of the machine shop. Forged pistons from Ross, chrome moly rings. New cam, lifters, push rods, fuel pump, oil pump, water pump, etc. Piston and rod assemblies re-balanced. Engine builder remarked that crank was like new, as were the rods. New bearings and it's like 1966 all over again. Engine and accessories will now be cleaned and painted, and then back in my GS It was a long wait but I expect to pick up some time in the old 1/4 mile once it is broken in. I was running high 13's with two cracked pistons, a broken ring and one very worn lobe on my cam. Plus, 150,000 miles on everthing else. Gotta love those nailheads.
