Another PETTA AVA Question regarding piston ring gaps.

Hi,
Further to my last post, I have now stripped down the engine, and measured the old piston ring gap. The bore looks OK. Piston ring gap appears to be around 50 thou. As the bore appears to be 81mm(roughly) I think at 4 thou to the inch the gap aught to be around 13 thou? Any advice anyone please?

Thanks
David.

It sounds as if the gap is a bit big to me, I would have said around the 10 - 12 thou mark. It does sound strange for the gap to be that big, was the engine a runner before you stripped it down?

Stu.

Hi Stu,
Thanks for your reply,yes the engine would run but would only start with the oil trick passed to me on this forum.Would this gap explain poor compression do you think?
Regards David.

Yes David it would give you poor compression, and it sounds as if the oil trick was maybe helping to make a seal on the rings to help it start. I think the only way forward now is to replace the rings and gap them to the correct size.

Stu.

Thanks Stu,
I will buy some more rings,let you know how job goes.

Regards David.

Only a very small percentage of compression is lost through the end gap of the rings, despite what most people think. Most compression is lost around the back of the ring, so to speak, through the groove. So it's really more about the fit of the ring in the groove more than the end gap. But any end gap over about 30 thou is to be considered very excessive, & obviously the larger the end gap, the more of a problem it poses.

I found all this out on an American piston ring sellers website. I will see if I can find it, & post a link.

If the rings are sloppy in the grooves it could be ring wear, or groove wear. If it's groove wear then you should really source a new piston. You could use over size rings, but the grooves would need machining out square again, as they tend to wear rounded. You can get groove spacers put into pistons as well, but it's a professional machining job, once again. Just putting over size rings into worn grooves is not really an option.

But this is all conjecture really, I just thought I would pass on the info & say that people tend to put a lot into ring end gaps, when they are rarely the major source of compression loss (it's a very small hole to get all that compression through, especially when you take into consideration the speeds going on, whereas the gap around the back of the ring, although smaller, could extend around the entire circumference of the piston. But, like I said, the larger end gap, the more the percentage of compression loss is affected by this. Kind of makes sense when you think about it).

How is the ring fit in the groove? Does the groove appear worn? Might be worth your while measuring the ring (accurately, preferably with a micrometer or, failing that, a digital vernier might be accurate enough) & checking this with the measurements of a new ring (you can usually find out what the ring dimensions should be), that way you can see how badly worn the ring is, & that could tell you if it's the ring or the piston at fault.

I would hazard a guess that it's just the rings worn on this engine, probably due to the prolonged use of easy start. So I expect a new set of rings is all that's called for, but without seeing it or knowing more, I'm just guessing really.

Here's hoping for you, best of luck .

Thanks for info nutty,turns out that this is an over size set up[4 thou]and mainly because this engine works regularly have decided to splash the cash and fit a new cylinder and piston set .Parts are ordered,hope everything goes back together ok I will post update as the work is done.
David

Wow, someone's treating themselves!

Well, you should be guaranteed a good runner at the end of it.

Best of luck.