Author Topic: Is widening the piston oil ring gap worth it to fit 3-piece oil rings?  (Read 426 times)

Online mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 615
  • Karma: 12
One for the engineers… I can’t find any 1/8” 3-piece oil rings, so to fit a 3-piece oil ring in the a10 (+.080") I’d need to widen the rig gap to 4mm height. Does this sound feasible or is it likely to cause some trouble?

I could probably do it myself tho assume I’d need to be accurate, so maybe get a proper engineer to do it.

Thanks for any comments,
Mike
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline Swarfcut

  • Wise & Enlightened
  • *
  • Join Date: Oct 2018
  • Posts: 1383
  • Karma: 23
  Mike. Perfectly feasible with the right equipment and attention to detail.  Way back when,  widening the ring grooves was service offered by the  good old Cord Piston Ring Co. Bear in mind this was at a time when make do and mend was the norm rather than our modern throwaway attitude to precision engineering.

  The idea was to widen the groove to accommodate a new set of standard width rings, plus a spacer ring and thus compensate for the wear in the casting. In this way pistons with worn grooves could be reclaimed for further use at minimal cost. The spacer went above the ring, as this is where the piston is loaded against the ring as the engine fires.

 As to whether the material is removed from the top or bottom on the groove, I don't know, but suppose in well worn cases it may have to come from both sides.

 Some years ago I went on a Bike Club tour of the PowerTorque factory in Coventry.  Their speciality was converting standard mass produced engines to run in unusual and arduous applications.  The machining carried out to improve the lubrication and longevity was truly amazing, and pistons grooves were routinely remachined to take different sizes of non standard width rings in the interest of reliable and long term operation.  Many engines were used in so called Combined Heat and Power units, basically a big engine and generator set, running 247/365.

 Swarfy.

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8677
  • Karma: 112
  • Lithgow NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
G'day Mike.
I have never done it but it is quite feasible. If they are new pistons only one side of the groove would need machining. If used pistons I'd take it off top and bottom as the groove may have worn.
Clearance wise I'd be looking at 0.002" - 0.0025". Three piece rings may be hard to measure the stack height to end up with the right clearance. Also double check the width to allow enough clearance on the depth of the groove, at least 0.010".

I love the three piece oil rings but they can be a pita to install. I've done many but one time the rail cocked over un-noticed. Destroyed the piston and bore in just a few seconds!
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online RDfella

  • Wise & Enlightened
  • *
  • Join Date: Aug 2017
  • Posts: 1192
  • Karma: 10
Have done a few over the years, but not quite as easy as imagined. First of all a piston is not only oval but barrel-shaped as well, so truing up in a lathe is a bit of a fiddle. Also, unless you've got a largish lathe, there's a fair bit of piston sticking out of the chuck - bit dodgy should the tool decide to 'dig in' a bit and rip the piston out of the chuck (no, never happened to me yet, but that worries me whenever I do that job).
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 615
  • Karma: 12
Thanks guys. it sounds feasible so I'll probably do it, or get someone else to do so.
the barrels are now off the a10 and the +.080" JPs look ok to reuse with new rings, preferably ones that keep the oil in this time  *eek*
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline Beeza

  • A's Best Friend
  • ***
  • Join Date: Oct 2014
  • Posts: 143
  • Karma: 3
Hi Mike, I have 5 A10s all with 3 piece oil rings, I do use a guy who is competent in this job as it is not a straight forward machining job. See if you can find someone who knows what they are doing, as far as pistons are concerned.

Online mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 615
  • Karma: 12
thanks Beeza, I'll take your advice
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Online mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 615
  • Karma: 12
ok well, suitable rings organized and then i had a proper look at the current JPs and not so good:
i'd only looked at the right, this is the left.
looks like I now have bigger problems to solve  *sad2*
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline Swarfcut

  • Wise & Enlightened
  • *
  • Join Date: Oct 2018
  • Posts: 1383
  • Karma: 23
 Back in the day with a symmetrical (non split skirt piston) we'd shine off the high spots with Brasso and refit with the damage to the (front non thrust) side of the motor. Better than walking........and almost as cheap.

 Swarfy.

Online groily

  • Wise & Enlightened
  • *
  • Join Date: Jul 2007
  • Posts: 1174
  • Karma: 19
    • www.brightsparkmagnetos.com
You don't say how many miles the pistons had done (if you know, that is). If 'not many' I wonder if the person who bored the cylinders to +80 had the pistons to hand when doing it?
JPs need a generous skirt clearance. It looks to me that it hasn't been provided, unless some other horror show has caused the damage.

When I had some barrels resleeved for another make, and used JPs supplied by the same people, the skirt clearance provided was nigh on 6 thou, on a 72.5mm bore (which is near as dammit where you are at +80 on an 'A'). They have done 15000 miles without any problems and with what I regard as 'normal' modest oil consumption for an engine of the era.
On my own A, newly fitted with a pair of IMDs during lockdown, I have a similar minimum clearance of 5thou+. Too early to say how good they'll be, but not a whiff of smoke from the very first start-up, and spins very nicely.
Personally, I prefer to go a bit 'large' as a matter of course  . . . makes no real odds to the lifetime of engines that aren't being used as daily essential transport.

Looking also at Chaterlea's wise comments in the other live thread on here at the moment,'Big Jim's decoke  . . .' it just goes to show how important it is that the bottom half be put together properly too. If 5000 miles is all it took to cause the top end trouble there, the job certainly wasn't a good one. Sadly, jobs that 'aren't a good one' are not rare!
Bill

Online mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 615
  • Karma: 12
Swarfy - i thought about it but got talked out of it by our own RR (who coincidentally has a fairly firm dislike of JPs). probably the right call.

Groily - I put those in 6000 miles ago when i did a major rebuild (new everything). they've always burned a bit of oil hence going after the rings. the guy who bored it had the pistons in hand and the JP spec that says 5 thou, and I believe he is capable and informed about old bikes. I can't recall what he bored it to, clearance wise. today with feeler gauges an inch down at the front it measured about .0045-.005 on the right and .0065 on the left. I can be a spirited rider but have never thrashed the A10, tho it has been wide open few times under load. i don't recall anything that felt like that looks. I did have a look at that other thread - i don't see any suggestion of misalignment tho will look again.

in the meantime I've just put my money down for some imd pistons and liners that will hopefully be here in a week or 2. good bye to the JPs! the imds are 7.25:1 which is a bit low for the super rocket but at least they come with a 3-peice oil ring. no doubt they'll change the balance factor so I'll weigh them when they arrive and decide what to do.

the one plus side of this job so far is getting to see the thunder rods again - they do look great sticking their head out of the cases.

thanks for your comments everyone.
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline ironhead

  • A's Best Friend
  • ***
  • Join Date: Feb 2019
  • Posts: 214
  • Karma: 0
 I can't recall what he bored it to, clearance wise. today with feeler gauges an inch down at the front it measured about .0045-.005 on the right and .0065 on the left.


I think you have found the problem right there.  Whenever i have to use JPs I always give an extra thou clearance. If you were to hone the right cyl to the left dimention  ie 0.006" & fit new JP 0.080 you would be able to use up the remaining life of those barrels. JIMHO.
SA