Piston ring thickness- std compression 912

[majcd213]

Hi All- I'm struggling with a rebuild/recon on an early LC 912 excel. In another (more successful) life I would finish surfing, down a pina colada, and kiss my beautiful wife before strolling through the palms to my sumptuously equipped workshop where I would change as many components as I liked! Sadly in this one (although I still have a beautiful wife) I have to cope with rain soaked Surrey, compete with the rabbit and lawnmower for workshop space and have to reuse anything that's not worn out. If it ain't broke don't fix it! So now I have a question about the piston rings.

My Manual speaks of a ring-to-groove gap of around 3 thou maximum for this motor, and it gives one value for compression rings and one for the oil control ring. I assume this means that the ring-to-groove-gap should be the same on both top rings? Trouble is that mine isn't, I'm getting what seems like a huge gap of 6-7 thou on the top ring and then a more tolerable 3-4 on the second. Having removed both rings this difference seems to be accounted for entirely by ring thickness as adding the relevant ring thickness to its clearance generates a similar value of 63/64 thou for the width of both grooves. Its the same on all 4 pistons so I'm wondering if a different ring set has been fitted. Not necessarily in error, this may be a perfectly satisfactory setup, but sadly I don't know as I got this motor partially disassembled and I've never seen it running (hence my wish to check everything). I believe the pistons are original Lotus items and they are marked "STD" and type "B". Similarly I think the liners are the original cast-iron versions. The rings may well not be original as the motor has been rebuilt at least once in the past. They have no markings on them at all as far as I can see, the top ring is chamfered internally and the second has a step as expected.

Has anyone experience of this setup? Is running such a ring-to-grove gap OK?
Or for that matter have all 4 top rings just worn down in thickness? Although this does seem unlikely.
Thanks all

Mike

[bbjonn]

Hi,
I've just finished a complete rebuild of my engine - the wear you describe was comparable to the wear present in the top ring gap of my engine. After taking advice from Mike at Lotusbits I replaced the pistons and liners - the wear was in the piston rather than the ring - just changing the rings wouldn't have been any benefit! Keeping the budget as tight as possible was important to me too - but I took the view that if I was going to do the job I'd do it right and not have to do it again! As a rough guide including machiniing and sundries I spent about £1400 on the rebuild and a couple of months of nights! If I can help you out let me know as it's still pretty fresh in my mind!

Jon

[majcd213]

My new rings are now here. The top ring is 1.55 rather than 1.49 of the existing rings and restores the gap to the expected value. I am pretty sure that this motor has simply had the wrong rings fitted at some point- probably the HC motor ones although the pistons weren't changed. I think an attempt at upgrade to HC is common in these LC motors although I'm after standard. The mains were wrong too in that the plain bearing in the centre was replaced with a groove-and-hole type. I am hoping that the head wasn't overly skimmed but I cant measure that accurately until I get the springs out and can get my micrometer on it vertically.