Chris.

Just bought a 1970 Duster 340. Driven it a few times but figured I'd check everything over so I know she's good to go.

Checked compression today
Cylinders 2-8 all had between 170-185psi
Cylinder 1 has a reading of 16-18

Decided to do a wet compression test and the reading went up to 45psi

Any idea why my piston rings would fail?


The rebuild only has 9500 miles (10 years ago.) It was built to run on a strip but the previous owner only ran it a few times when he first rebuilt the engine.
I don't have all the information on the rebuild but what I do know is this:
-12:1 Keith Black Pistons
-Hi Lift Solid Lifter Crane Cam
-4 barrel holley 750 double pumper carb
-Griffin Rad

There was some sludge when I changed the oil yesterday. Not much but it was there. It had 20w50 Synthetic in it.
The oil level was low the last time I drove it. I had to add 2 quarts. Didn't realize it was missing that much oil. The oil pan holds about 8.5 quarts though so I never thought much of it.
Also one of the first times I drove the car it was backfiring about every 2-20 seconds. Got worse near the end of the drive and backfired every second or two. I drove for about 10 minutes until I was home, checked the plugs, wires, and octane level. Everything was good and it never backfired again.

Whether it was my fault or whether I bought it with a bad cylinder is my problem and I know that.
Any tips on rebuilding, a quick/easy fix (unlikely), or any help would be appreciated.

Thanks

Coronet 500

First thing with a flat tappet cam the synthetic worries me, is it a roller?

I would do a compressed air test in that cylinder through the spark plug.

Air sound at valve cover or dipstick is rings, at the carb is intake valve and at the exhaust is exhaust valve.

Mr.4spd

That sucks, but I'm not sure it's ever had 12.5:1 compression, 185psi? Not unless the cam has a butt load of overlap. Even if it does though, I think a teardown is in order. The oil level being 2 quarts low isn't the issue, you still had at least 6 quarts, right? Maybe a broken ring set from improper end gaps?

Chris.

I have no idea if it's a roller.

I'll try the compressed air idea. My compressor is loud though so I might not be able to hear the hiss over the loudness of the compressor

Chris.

I'm going to try and not sound stupid but I'm not sure I understand everything from your reply.
-Yes I still had 6 quarts in the engine
-Why do you not think it ever had 12.5:1 compression? Is 180psi too low for that type of piston?
-You think when they did the rebuild they improperly set the end gaps on cylinder #1? Do you think it finally lost compression or is it possible the old fellow before me drove the car for 10 years on 7 cylinders and had no idea?

Thanks

Coronet 500

I forgot to say have both valves closed with piston at TDC is best. Be careful the piston can move fast to BDC when pressure is applied. I would not jump to tear down until you have had a full diagnosis and are sure.

Chris.

Sounds good. Thanks! Will keep you posted

Mr.4spd

I would assume 220s psi unless the cam overlap is crazy, not an exact science though, just a ballpark figure. If the ring end gap is too tight the rings can butt (from heat and expantion) and break or cause the ring lands to break.
And it's entirely possible to drive an engine that's down one hole and not know it if the engine is healthy otherwise (it's happened to me). Or maybe he wounded it with nitrous and that's why he sold it?

pro-tech

If it is a solid lifter cam it means it could be the valves are adjusted to tight causing the valves to hang open and not seat properly. All the above is good advise and a very good possibility that it might be one of those problems also.. But the valves are an easy check...Bill

moe7404

two big Qs. 1. have you over reved the engine? there goes the piston and rings. 2. backfire intake? case, cracked cap, cyl 5 and 7 crossed. Mopar single fire V8 dist cap wiring
My dad made this saying about small and big block dist cap wireing. To reduce wiring confuseion. “the small block has the dist in the wrong place, in the back of the engine. BUT the shaft rotates the right way clockwise. The big block has the dist in the right place, in the front of the engine BUT the shaft rotates the wrong direction counterclockwise. Both wire #5 and #7 on both ALL ways have #5 before #7 on the cap AND block. And most / some stock V8s have the fireing order cast in to the intake manifold. And more info: if the cap has a vent hole next to the center tower. Plug it with JB-weld. just mr idea.