Piston Ring End Gap

[litefoot]

Is the maximum tolerable ring end gap the same on all "modern" rings fitted in pinned ring grooves? Or does that vary from piston to piston? Is the maximum acceptable gap somewhere around .020"?

 

[Lakeside53]

well... depends on the bore size... 20 sucks on an ms180, but is "reasonable" on an 088..

 

[litefoot]

Hi Lakester,
So where do I find those magic numbers. I don't see anything in the service manuals. Or do I need to look harder?

 

[Crofter]

I have seen figures for rule of thumb of .003" clearance per inch of diameter for automotive 4 stroke water cooled, .005 per inch diameter for general 2 stroke and up to.008 for race engines

 

[Lakeside53]

Hi Lakester,
So where do I find those magic numbers. I don't see anything in the service manuals. Or do I need to look harder?

It's not there. OEM rings are pre-gapped. New, I see about 5-8 thou on an 044... (measured in the bottom "unworn" area of the bore). At 20 they are pretty well work out.

Your best measure is compression.

 

[Saw Bones]

I have seen figures for rule of thumb of .003" clearance per inch of diameter for automotive 4 stroke water cooled, .005 per inch diameter for general 2 stroke and up to.008 for race engines

+ 1

 

[litefoot]

So Frank, I used your numbers on a 48 mm bore (1.89"):

1.89 X .005 = .00945

That seems too tight. Am I interpreting your post all wrong?

 

[Lakeside53]

That's not too tight... It's about right, even a little on the wider side than what I see.

 

[timberwolf]

I'v run as tight as .005 to .006 on 52mm bore racer with single ring.

Put your tighter ring on top for best sealing unless it was really tight, then I would put it on the lower groove to wear in for a bit without getting expanded by as much heat from blow-by gases. Though it is very easy to increase ring end gap with fine sand paper if your worried about it being too tight.

Top ring gets more wear, on used saws often see top ring with 20-30 thou gap and lower closer to 10 thou.

 

[litefoot]

OK, excellent! Thanks Frank, Lake and Wolf. Good stuff. I appreciate the info. I want to purchase after-market rings for an old 655 Solo. It's a 48 mm bore, but I'm measuring 1.4 mm ring thickness. Bailey's has 48 X 1.5 rings. Could I assume that these old rings started their life as 1.5 mm and wore to 1.4 mm or would you not see that much wear.

 

[timberwolf]

Hold on there. Thickness and end gap are two different things. You can not use a ring that is not exactly the right thickness.

Up and down play is unacceptable, cant recall exact spec, but up and down you don't want more than like .001.

Rings don't wear significantly in thickness.

 

[litefoot]

Hold on there. Thickness and end gap are two different things. You can not use a ring that is not exactly the right thickness.

Up and down play is unacceptable, cant recall exact spec, but up and down you don't want more than like .001.

Rings don't wear significantly in thickness.

Understood. I was changing the subject from end gap to thickness. But that's what I needed to know regarding thickness. I'm measuring around 1.4mm using an inexpensive dial caliper. Maybe I need to borrow something with better resolution and accuracy. All the Baileys Woodland Pro rings are either 1.2 or 1.5 mm.

 

[litefoot]

OK, I ran down the Snap-On guy, used his micrometer and then ordered one for myself (I need one for welding work anyway). The rings are 1.5 mm. I'm ordering from Baileys (PRS 4815).

 

[Crofter]

Ring material can be important. I think chrome faced rings are a no no in a chromed bore. Maybe Lakeside will comment on this as I was convinced until a few days ago Stihl cylinders were a form of Nikasil which I think can take any ring material. Apparently they are chrome plated.

 

[scotclayshooter]

OK, I ran down the Snap-On guy, used his micrometer and then ordered one for myself (I need one for welding work anyway). The rings are 1.5 mm. I'm ordering from Baileys (PRS 4815).

I check my measuring devices on bullet tips .243 or 6.17mm and its close enough for me!

 

[litefoot]

The rings will be cast iron and the cylinder is Mahle. Not sure if it's chrome or Nikasil finish.

 

[Crofter]

Cast iron rings are good on either!