Piston Ring Mystery

[david.g.chapman]

I have now found the cause of my earlier compression problems (see my last post on compression).

Both top compression rings on cylinders 1 and 2 have broken neatly in half. giving me a loss of 20 psi or so. This is after fitting these rings less than 15000 miles ago!

What is even more worrying is that the rings had worn down from 1.59mm thick on the outside edge to 1.37mm over most of the surface of the rings- probably the reason for them breaking.

I measured the ring to groove clearance when I fitted the rings as 6 thou. The instructions that came with the rings gave 6 thou as the limit, so I fitted them with confidence. Was this the mistake? How can rings wear like this so quickly?

Dave Chapman.

[alaric]

That's a shame. Presumably the bores have suffered also? If you didn't rebore the block then is it possible that there's a ridge around the top of the bores that perfectly matched the old rings, and the new rings have been forced into it, eventually breaking. Again if no rebore was done, is it possible that the bores are slightly oval. Otherwise I can only suggest checking the oil supply.

Good luck.

Sean.

[TeeJay]

Dave,

Not sure what pistons you have fitted, but both the "A" (large cut outs) and "C" type have a Piston ring to grove as:-
Compression = 0.0016" to 0.0036"
Oil Control = 0.0018" to 0.0038"

Also I assume you checked the Piston ring gap to bore?
Compression = 0.009" to 0.014"
Oil Control = 0.010" to 0.020"

Trust this helps

[david.g.chapman]

There is no wear ridge that I can feel, and the piston ring gaps were OK at the time of fitting. I know that the normal groove to ring clearance is 3.8 thou max, but the instructions that came with the rings allowed me 6 thou.

I have spoken to an engine reconditioner, and they confirm that 6 thou is "right on the limit". They also suggested that detonation could be to blame.

Dave Chapman.

[david.g.chapman]

Some more information...

I have measured the bore 20mm down from the top by first of all measuring a piston at its widest point (90 degrees to the gudgeon pin across the skirt). This was 82.95mm (+20 thou pistons used).

I then inserted the piston upside down into its bore, and tried to slip various feeler gauges between the bore and the piston at the piston's widest point, and 20mm down the bore. I got up to 6 thou for a tight fit.

Encouragingly, I got the same result at 90 degrees to the first measurement, along the long axis of the cylinder block.

This gives the bore measurement as 82.95 + 0.15 (6 thou), or 83.10mm.
A new piston would be 82.50mm + 0.508 (20 thou), or 83.008.

83.10-83.008 is 0.092mm or 3.6 thou. Not bad! I got the same result or slightly betterfor cylinder 2 ( a 5 thou feeler was the maximum).

Please feel free to burst my bubble if I have this wrong.

So I am back to the ring to groove clearance. I now believe that if this is excessive, a runaway effect takes place where excessive ring movement leads to more wear to give more movement until the ring breaks. This must take place with groove clearances more than 4 thou or so. So my original 6 thou was well over the top.

I am not sure about the detonation theory.

It looks like time for new pistons. Can anyone recommend anything to give a bit more pep to a stromberg engine with CPL2 cams and a low compression head (9.5 to 1)? Normal compression is 180 psi.

Dave Chapman.

[rgh0]

Steve has done more on Stromberg head twinks than anyone I know see his web site

http://steveww.org/

There are certainly better cams than CPL2 for the Stromberg head - think short duration ( less than 290 seat to seat) and high lift ( above 0.41). If replacing the pistons a bit more compression to around 10.5 will help also

For a road 1600 around 140 to 150 hp is certainly achievable without resorting to the cost of a McCoy conversion to Webers plus you keep the good fuel economy the Strombergs give

cheers
Rohan