Testing Cylinder Seal

#1

I’ve been wanting to test the cylinders in one of my engines for a while now. It’s engine #14 and it’s my only engine with oversize pistons. I installed it in new #6 in the Spring but then ended up in clutch hell. After missing an event because of clutch problems I was bored and did some testing on this engine that had only 1 season on it. The result was bottom end leakage on #6. That really surprised me so I pulled the engine out of new #6, lapped the valves on the 2 season head, put it on a Metric Mechanic bottom end with standard size pistons and that’s what is in the car now.

Some measuring of the problem block and the cylinders seemed nicely cylindrical.

Since then this block has been sitting around with the cylinder leakage problem never really understood. I’ve wanted to do some retesting of the block but with it’s head off I didn’t have a good way to do that. Also, I got to thinking, in a perfect world I wouldn’t use a head to seal the cylinders because a head is just another potential source of leaks.

So I got to thinking about how I might make a plate that could cap a cylinder and let me do a leakdown test. Foraging around thru my materials stash came up with some 1/2" AL plate and this gave me a reason to set up my Harbor Freight drill press that has been sitting in it’s box for a month.

It’s not yet a complete success. I got it working a couple times, and a couple times not. I have to play a bit more with gasket materials and it got late enough that I couldn’t use the air compressor last night without risking the ire of glock chick.

Speaking of the damned air compressor, it seems like I have to fix the MFing thing each weekend.

The plate prevents me from going all the way to TDC because the piston will hit it, but I can get pretty close. Maybe 5mm. TDC is the most important place to test the seal tho because that’s where peak pressure is.

I’ve done a lot of measuring of these cylinders, but it’s a little tricky getting “absolute” numbers instead of “relative” numbers. I’m not quite sure yet why the cylinders are leaking, but I’ll figure it out. Since these are my only oversize pistons, one way or another I’m going to get them working for me like they should.

Note that engine #14 is actually old #8. My engine accounting system increments for headwork or engines that come out of storage to go back in.

#2

Part of your sealing problem may be flex of the plate. A thicker plate should help and turning a recess into the plate would allow the pistons to reach TDC. While the plate was on the lathe an o-ring groove could be added. I think I’d use 1/2" AL plate.

#3

I mispoke, it was a 1/2" plate. I’ll fix the post above. I agree that 1/4" would have been asking for trouble.

Re. oring. Too hard. I’ve a sheet a rubber ready to cut and try tonight.

#4

Lacing a CNC mill, the easiest way would be to bolt the plate a lathe faceplate and face cut the piston recess. Then switch to parting tool and plunge cut an o-ring groove. Piece o’ cake really.

A sheet of rubber should work. You could contact glue it plate the make things easier.

#5

[quote=“jlevie” post=68616]Lacing a CNC mill, the easiest way would be to bolt the plate a lathe faceplate and face cut the piston recess. Then switch to parting tool and plunge cut an o-ring groove. Piece o’ cake really.

[/quote]
You overestimate me sir. It would take me all evening to do that.

#6

Scott, what are you using as a gasket/seal for the plate? Do you think you 1/2" is thick enough? I’m going to build one of these contraptions some day… Maybe I’ll just take a bad head and have the valves welded shut.

#7

Has it occurred to you that you should be testing leakdown at operating temp?

#8

Oh my bad Turbo! Thanks for setting us rookies straight!

#9

I tried a number of things. What ended up working for me is a piece of thin dense foam rubber. Each time I do the test I run my hand around the perimeter to feel for air that might be sneaking out. So far so good.

#10

That’s helpful! I plan to test the two old spares I have before tearing into them when I return from my Mid-East vacation next year…