Ring seal question no compression

C

c4mo

New member
I put together an engine with some parts I had laying around and am at a loss as to why it has no compression. It's a cool bore standard bore block, raptor 3 std piston, Burris low tension ring set. Piston to wall clearance .007. The rings gaps are .005 top, .005 2nd, and .006 bottom oil ring. I put them in with dot facing up. I lightly honed the cylinder with 400 sand paper. It does have scratches in it. I realize that the proper thing to do would be to have it Honed, but I would figure it would have more compression than this. The valves were lapped in. Lash is .006 intake, .009 exhaust. Timing stock flywheel with key. Any ideas or do the low tension rings have to have a good hone to seal at all? I can spin it with my hand with the plug in. Thanks
 
Considering these were all used parts, except the rings. Could It be the lash is to large? What would be the max? I guess the only way to check the for headgasket and valve seal is a leak down test. At what pressure is this normally done at on these?
 
Put a little bit of oil in the spark plug hole and see if the compression improves. If it's the rings causing the problem, the compression should improve some. If it's the valves that are leaking, it won't improve at all. As good as a leak down test is, if there is no compression at all, it's going to be hard to pump the cylinder up to any usable pressure.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
 
The engine seals at three points.
The rings.
The valves.
The gaskets.
Unless you have a hole punched in it somewhere you should be able to eliminate these one at a time.
Or...
Knock apart a spark plug and silver solder a hose coupling to it.
Install in the spark plug hole.
Put some air to it and listen for leaks.
 
Oil added it made no difference. And yes Ill apply air to it and listen to hear where its a escaping. At this point I feel its probably a gasket. I would be surprised if its the valves, as they looked good (seats) after lapping them.
 
90% chance its the valves. Were they from a different engine also. Did you check them after lapping, too see if they leaked?
There is only 1 gasket it could be and that is the head gasket.
 
Yes they were from a different engine. The margin looked good after lapping them. But I do agree it probably the problem. Although if my valve guide clearance was excessive, which it was more than it should be. It could cause it not be seating flat when under load. I do have the old valves and will try them.
 
The problem ended up being the exhaust valve sticking open. The cause was the lash was too tight, so after adjusting it (filling) it seems to be running good. Compression is 105 psi, with me rope cranking it. Currently having a off idle bog, so I plan to check the pop off pressure on the carb and see what happens.
 
"I would be surprised if its the valves, as they looked good (seats) after lapping them."

If there was a high spot or burr on the seat sticking out and then you spun the valve. Couldn't the high spot put a nice line around the valve, when it was just the high spot touching? Or could a slightly bent valve not seat ok, yet because of the thickness of the compound make a nice line?

am I thinking right about it?
 
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