Low Compression

[Hamish Coutts]

Doing a bit of head scratching today. Here's why -

Was regularly fouling No4 plug. Tried changing to hotter plugs but no improvement. So did a compression test (actually 2 or 3) the results were:
Nos 1 to 3 cyls - 190 lbs/sq in.
No4 - 100 lbs/sq in.

Checked cam clearances - within tolerance
Took off head to see what the gasket was like - it looked OK. Also the oil was still black and there was nothing unusual with the coolant.
Cleaned up the head and did a petrol test on the valves. There was some weepage past the valves but nothing much. All valves weeped the same amount.
The valve seats look OK.

Aha, must be a broken compression ring methinks
So engine came out this morning and I pulled no4 piston. Piston looks OK and Rings were all fine :huh:
Cylinders look OK. No scratches.
The only thing I can see is that the gap in the compression rings is about 18 thou when I put them in the cylinder. (a bit wider that the book says, 9 to 14 th) Is this extra 4 thou enough to cause the problem?

Any other ideas as to what could be the cause? The engine was rebuilt about 12 thousand miles ago, before I got the car.

Regards,

Hamish.

[Hamish Coutts]

Forgot to add that the engine seems to have a bit of piston slap. This is more noisy when cold but is still there a wee bit when warm.

Hamish.

[rgh0]

Hamish

You have the engine problem I hate the most. A real problem in fouling plugs and low compression but no identified cause when you have stripped down the motor so you dont know what to fix !

A few things to check

1. Bore ovality and piston to bore clearance
2. Were the rings free in the piston grooves and ring grooves within spec width
3. Any difference in ring appearance between no 4 and other cylinders
4. Piston diameter profile along its length and any difference between No4 and others due to wear.
5. Gudgeon pin and big end bearing clearances and any differences between No 4 and other pistons

The plug fouling in No4 and low compression in no 4 suggest a ring problem letting oil in and compresson pressure out past the piston. The fact that its both at the same time suggest a fundamental wear problem affecting the bore / cylinder / ring assembly. In the end the difference is there somewhere you just need to find it.

Rohan

[type26owner]

IIRC, my guess if the rings are Hepolite ones is that the top ring was installed upside down. The chamfer on the inside edge of the ring is there to allow the combustion pressure to push the ring out against the cylinder wall. Is there a ridge at the top of cylinder?

Funny how those rings wear out. They wear thin and leave the aluminum in the piston intact. You'd think it would be the other way round. The ring wear rate is directly tied to the surface finish of the cylinder wall. The roughness and the angle of the cross-hatching causes the rings to rotate or not on the piston. Get the wall finish wrong and the rings will spin themselves to death rather quickly.

If you're using dino oil then the four oil drain holes in the piston underneath the oil control ring are probably plugged up. Once the drain holes are plugged the rate of oil making it past the rings increases. Some ChemTool in the oil will help dissolve the gunky ash and carbon deposits and should be used every 6000 miles or so. Just add it right before an oil change. Put some through your fuel system too while you're at it.

Check for a broken valve spring. Just push on the bucket with the cams in place and you can tell if a spring has failed.

[steveww]

I know it is a bit late now that you have teken everything apart.....

You could have done a leak down test. A bit like a compression test except that you use a compressor for the pressure then measure how fast the pressure goes. As the pressure leaks away you can listen for where it is going.

Perhaps a useful hint for next time

[builder618]

You should ALWAYS put oil in the cyl. and retest compression. If compression goes up, you have ring problems. Easy way to define valve or ring.

[twincamman]

but a VERY SMALL AMOUNT OF OIL re a drop Ive seen the compression gauge explode and the operators put on his can with too much oil in the bore --- ed

[Hamish Coutts]

Thanks everyone for the advice - as ever, it's great that people want to help.

In case you think me ungreatful and are ignoring you over the next couple of weeks, I'm not. I'm off on hioliday so won't be around. My son is baby sitting the Lotus.

I'll get stuck in about the engine when I get back. Maybe by then I will have worked out what to do with it.

Thanks again.

Regards,

Hamish.

PS - Geez Ed thats quite a type 26 you've got. It looks fantastic. Very, very neat and tidy. Thanks for all the photos. By the way, what's the blue mini in the background? I cut my teeth on minis in the 60s - used to rally & autocross them. Ah, they were the days.

[twincamman]

the mini is my sons---when he was 16 I asked what kind of a car he wanted ---I figured a 32 chevy with a 327 that would stop the cows milking the chickens from laying and make the barn fall down ---to my dismay he asked for a mini---found one dragged it home fixed it up and he drove it all through school-found it sideways in parking spots etc etc - now he works in Toronto and drives my Buick :angry: ed

[Frank Howard]

If you haven't taken the cams off the head, there is one more thing you may want to check. Valve clearance. Too thick a shim and the valve won't close. This happened to me when I reassembled a twin cam and thought I was putting the shims back in the same places they came from. :huh:

Frank Howard
'71 S4 SE
Minnesota