Piston rings

baz · Mar 13, 2016

Triton Thrasher said:
Danno said:

I'm having trouble visualizing how the expander, which seems to have no outward tension of it's own in a cylinder, can add tension to the scrapers

Click to expand...

It seems the expander is longer than the circumference of the bore, so is compressed against its spring tension to go into place.

my bsa b44 was not as bad as this one shown in the photo i trimmed/stoned maybe 20 thou off of the ring expander,the top compression ring gap the second ring gap and oil control rail gaps were all spot on but there was no way it would fit into the bore without the expander being trimmed ,whether it was right or wrong to do this i dont really care! it worked for me perfectly and i would not hesitate to do it again cheers baz

pierodn · Mar 14, 2016

B+Bogus said:
Piero,

The Hepolite rings fit the GPM pistons.

I have domed GPM pistons, but mine are aiso marked 'Dunstall' in the casting so I presume they're higher compression than standard. They also have a taller crown.

Have you compared yours to a standard piston?

Hi Andy,
thank you.
The pistons are not marked Dunstall.
Ciao
Piero

splatt · Mar 14, 2016

If the two ends of that ring were pulled in to the bore the ends put together square and pushed back out against the bore all of the stepped cuts in the ring would compress and the ring would probably stay put under tension
Like I said before the ring groove depth is important and would suggest that is the reason most don't fit, I also thought some of the GPM pistons had shallow oil ring grooves

Danno · Mar 14, 2016

Biscuit said:
Danno said:

Until someone can explain either, I don't think they are in any way compressed. The oil rings (scrapers), like the upper two rings, must be compressed to fit into the bore and must be able to move about in their grooves. I think the expander/spacer only acts as a bottom land for the top scraper and a top land for the bottom scraper as well as a pathway for the scraped oil.

Click to expand...

Danno, once the top and then the bottom scrapers are installed onto the expander that has already been placed into the piston groove, the whole three piece system is locked together. The scraper rails are not supposed move around independently to each other or the expander. However the whole assembly will be free to move about in the piston groove just like a one piece oil ring would. When installed on the piston correctly, it essentially becomes a one piece assembly and as such gets compressed like a single ring would by your ring compressor (or fingernails).

That at least sounds plausible. I'd still like to see if a properly-sized expander/spacer would butt up tight before touching the wall of the groove all the way around and see the results of the pull test with and without it.

Danno · Mar 14, 2016

Biscuit said:
Danno said:

Until someone can explain either, I don't think they are in any way compressed. The oil rings (scrapers), like the upper two rings, must be compressed to fit into the bore and must be able to move about in their grooves. I think the expander/spacer only acts as a bottom land for the top scraper and a top land for the bottom scraper as well as a pathway for the scraped oil.

Click to expand...

Danno, once the top and then the bottom scrapers are installed onto the expander that has already been placed into the piston groove, the whole three piece system is locked together. The scraper rails are not supposed move around independently to each other or the expander. However the whole assembly will be free to move about in the piston groove just like a one piece oil ring would. When installed on the piston correctly, it essentially becomes a one piece assembly and as such gets compressed like a single ring would by your ring compressor (or fingernails).

That at least sounds plausible. I'd still like to see if a properly-sized expander/spacer would butt up tight before touching the wall of the groove all the way around and see the results of the pull test with and without it.

Piston rings

baz

pierodn

splatt

Danno

Danno

Similar threads