Compression figures after cylinder hone

[david.g.chapman]

I recently suffered a broken top piston ring on number 1 cylinder - the compression figures were:

1 140
2 170
3 175
4 170.

I have now fitted new piston rings (nothing else was required) and honed the cylinders for the new rings. As I don't have a slow speed drill for the hone, I had to finish off by hand with 400 grit wet and dry paper, to get the correct 120 degree angle for the finish.
Compression figures on re-assembly were:

1 140
2 140
3 140
4 140

I have now done 250 miles, and the compressions have all risen to 150 psi.

How many miles does it take to regain full compression? 500? Or have I done something that might delay this?

Dave Chapman.

[type26owner]

Not long if you provide enough cylinder pressure. Start giving it wide open throttle frequently. Short bursts though so nothing overheats. IIRC, especially true if you've got the chrome Hepolite rings. Call those folks and speak to their engineer for confirmation to what I'm telling you. Chrome rings are prone to graze the cylinders if you baby them.

Recommend you have the cylinders plateau honed next time on a honing machine. That cross-hatch angle is crucial to get the rings to not spin according to the engineers I've spoken to.

[david.g.chapman]

Thanks for that, Keith

I was supplied with Deves rings ( the ones with a backing spring strip on lower rings 2 and 3). Interestingly, the old top rings on all cylinders had worn 10 thou thinner than the minimum allowed. The aluminium piston grooves had hardly worn (about 1 thou from new). This could be why one of them broke, possibly because of over rotation as you suggest.

Weirdly, the old lower rings had much less wear in their grooves. Perhaps because they were protected from carbon particles by the top ring. Once the rings start to thin, then maybe they rotate more. Hmmm.

I will try bursts at full throttle and check the commpression again at 500 miles.

One plus point. The engine has not dropped any oil since the "rebuild". It's all dry underneath! Including the bottom of the bell housing! Can it last!

Dave Chapman.

[type26owner]

Dave,
IIRC, I had an hour long conversation with the engineer at J&E pistons about a year ago. The fact the ring wears thin quickly is a clue the ring has been rotating around too much. The forces on the top ring are extreme because they are designed to allow the combustion pressure to get behind the ring and force it out against the cylinder wall. This is exactly where the synthetic oil kicks the dinosaur stuff's butt. Of course, don't add the synthetic until after the rings have bedded otherwise they may not.

You probably have a six-bolt crank with a rear main lipseal right? That's considered cheating from a four-bolt (ropeseal) guy! I managed after considerable effort to retrofit a lipseal also to join you in the leak-free zone!

[david.g.chapman]

Keith,

You are right, I have the later design. The engine is I21300 from 1970.

I thought I might be in luck when I had the sump off, as the flywheel face right next to the seal was dry. Further out it was a different story.

Thanks again,

Dave.

[david.g.chapman]

I have now done 500 miles, and have measured the compressions again. Disappointingly, they are the same as at 250 miles - all 145-150 psi hot.

I have a confession to make, though. I had re-used the old oil when I replaced the rings, as it had only recently been changed before. Unfortunately I had added Wynns supercharge to it, which may have affected the rings bedding in.

Hopefully it has just delayed the running-in process (I have just changed the oil for pure 15/50 mineral) , but I may be unlucky, as the cutting action of the honing may have been used up by now. Alternatively the bores may have glazed.
It that is the case, is there anything I can do apart from lifting the head and re-honing the bores?

I even though of emery paper on a L-shaped arm though the spark plug hole - I am that sort of person!

Dave Chapman

[types26/36]

David,
Here's a method I have used to bed rings in fast and it may help if your bores are glazed,
Get the engine to normal operating temp.
Find a quiet stretch of road.
Drive the car at as low a speed as possible in top gear (1000rpm if it will take it)
Accelerate hard up to about 3000rpm, repeat this 4 or 5 times then drive the car normally for a couple of days, check compressions after a week or so.
If you cant drive the car in top gear as I stated use 3rd gear but I think you get the idea.
Hope this helps.
Brian