Piston Ring Fitting

Discussion in 'Help Wanted' started by Skankin_giant, Feb 8, 2012.

  1. Skankin_giant

    Skankin_giant Well-Known Member


    Next fun thing I have to do Is fitting new piston rings, not seen the new rings yet, but the stop in the piston is angled, also the inside of the ring is tapered towards the tips.


    So shaping may be a bit fun!

    Also gapping rings for pistons with stop pegs, anyone got any advice?

    Cheers Steve
  2. petternut

    petternut Administrator

    I use chainsaw files for the peg cut outs.

  3. Skankin_giant

    Skankin_giant Well-Known Member

    Ok thanks Roland, ordered some precision files, as I only have a couple of files, did pick up a normal fine file today, but think it will be a bit over kill, sadly got rid of all my modelling stuff...

    Cheers Steve
  4. Kevbuts

    Kevbuts Member

    I must confess to being a little daunted by the prospect of fitting new piston rings to my recently restored 2hp Petter M which also had pins in the grooves to prevent the rings rotating on the piston. I also found that chainsaw files were ideally sized to file the pin notches in my new rings. Take your time over gapping the rings and filing the pin notches Steve and you should be fine. Also - just a word of advice, if you're fitting and gapping your new rings on a cold day put the new rings in a bowl of hot water before trying to fit then onto the piston. If you don't there is an increased risk that you may break the ring when opening it up! I broke one of my new rings doing this, and learnt the hard way. Using hot water worked well for me, and was advised by Philip Daintree who I bought the rings from.

    Hope that helps,
  5. Numpty1

    Numpty1 Member

    Thanks for that advice Kevin, Often overlooked and as you say, you can be wise after the event!

  6. Skankin_giant

    Skankin_giant Well-Known Member

    Thanks for that information!

    I have had to send the rings back as they were not right, the angle for the pegs were the wrong angle and filing them to the correct one would remove to much material, so hoping to get back a set of butted rings so I can file to suit.

    Attached is a quick drawing of the problem with the rings and the bottom rough drawing (I cant work out how to use the eraser correctly) is how the ring need to be shaped.
  7. petternut

    petternut Administrator

    That must be a very odd shaped pin. Any chance of a pic please.
    I did once know the hot water trick but had forgotten it. Thanks for the reminder.
    I assume everyone knows about using brass slips?

  8. Skankin_giant

    Skankin_giant Well-Known Member

    Will do one when I can, in the mean time here is a photo of the old ring, think that just about shows it?

    Cheers Steve
  9. Skankin_giant

    Skankin_giant Well-Known Member

    Hmm didnt come out to well..



    Will see if I can edit it.

    Cheers Steve
  10. Kevbuts

    Kevbuts Member

    Nope - please enlighten us Roland
  11. Kevbuts

    Kevbuts Member

    Roland - Do you mean brass slips to bridge the ring grooves and assist in sliding the new rings down into their grooves? I recall seeing a detail of this in the Ruston Hornsby AP instruction manual. Just stops the ring dropping into the wrong groove during installation. I have to say, I didn't do this when installing my new Petter M rings, but I can see the benefit of doing this - would make installation easier, and reduce the risk of ring breakage I imagine.
  12. admin

    admin New Member

    Yes, if you are worried put the shims in at 90 degrees around the piston.

    I tend to do it by putting thumbs at the gap and second or third fingers (depending on bore sze) at 90 degrees around the ring. Light pressure on the fingers when opening the ring with the thumbs and then drop the ring on. Just done the 8" rings on the Allen S30 that way.

  13. Skankin_giant

    Skankin_giant Well-Known Member

    Bugger bugger bugger! Cox and Turner have no 78 x 1/8th with a butted joint, or the opposite diagonal joint...

    So got 3 options

    1 - looks else where for 78mm x 1/8th piston rings
    2 - get some made (£££)
    3 - see if I can remove the peg and run with out..... (rotating them wont change the angle). Scrap that not a good idea! might have a look at getting new pegs made...

    Hmmm which way forward.

    Cheers steve
  14. air-cooled

    air-cooled Member


    If this is for the Scott I would imagine the bore size may be common to other Scott engines, have you tried the Scott motorcycle club ?

  15. Skankin_giant

    Skankin_giant Well-Known Member

    Hi Phillip, we discovered its not, on measuring the bore, its 78mm not the believed/printed 73mm been down that route with Cox and Turner.

    Nick checked his SE as well which is also near as dame it 78mm.

    Cheers Steve
  16. Skankin_giant

    Skankin_giant Well-Known Member

    Hmm, can anyone convert this into simpletun for me please?

    Hi Stephen-
    I have good news and bad news. I have in stock the 3.077" x 1/8" compression rings with angle cut ends. The trouble is your anti rotating "pins". You will need to get a drill or slip of metal the same thickness as the "pin". Then you will have to file the ring ends till the slip or drill will go into the end gap of the rings when inserted into the cylinder without the piston. This must fit just perfectly or you will not be getting any compression and the engine will not run. You will have to file away approximately the same thickness as the "pin" Whomever was the designer that came up with the flawed "pin" design deserved to be run out of business as I am sure he quickly was. This is a modern "product defect" that today would attract lawyers. $5.00 per ring plus shipping and handling.
    Dave Reed
    Otto Gas Engine Works

    Is he explaining the method of gapping the rings to the Pin?

    Cheers Steve
  17. petternut

    petternut Administrator

    Subtracting the trade mark rant I believe Dave means that you should gap the rings using a slip the same width, and angle as the "peg" and though he doesn't say so + the expansion allowance. His usual good prices.

    I do agree with him that its a crass design as it creates a gas path. I suggest discussing with Nick his past replacement of the bridge with a peg.

    naturally this opinion is worth what you paid for it :)
  18. Skankin_giant

    Skankin_giant Well-Known Member

    Thanks for that, took me a few reads to get the gist of it, recovering from a nice nappy!

    Think im just about capable to do that.

    Bridge, Peg....was working before... :lol:

    Cheers Steve
  19. ploughman

    ploughman New Member

    Hi Everyone,
    I am not familiar with this piston/ring set up, but, I think it is probable that the peg is fitted into a round hole in the piston, if so it would be quite simple to cut the diamond portion away with a fine cutting disk then drill or punch out the remaining piece. A new brass pin could then be manufactured and fitted.
  20. Skankin_giant

    Skankin_giant Well-Known Member

    Hi Harry,

    The peg protrudes into the piston wall, and is also saddled to the back of the ring grove.

    Cheers Steve

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