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How To: Broken Piston, Velocette MSS

October 23, 2011

IMG_9762

While on the Girder Fork Ride two weeks ago, the MSS was down on power during one stretch of road.  It hesitated and died, and the roadside prognosis was a loss of compression, with a prod on the footstarter giving very little resistance.  The kicker will typically support my weight unless I lift the exhaust valve.   (early motors with no valve overlap can be a bit tough to kick through the compression stroke).

 

Back at home, I knew that there were just a few ways to lose compression:  faulty valve lifter, spark plug is loose in the head, failed head gasket, bent valves or broken piston rings.  After a quick look at the plug and the valve lifter, I moved on to the time-honored method of diagnosing a loss of compression:  pouring oil into the combustion chamber.  The spark plug came off, and three quick squirts from the oil can, then plug in and kick over the motor to check for compression.  This test is a good one, since it will identify if the problem is in the piston rings or valves.  The oil will seal around the rings and give compression.  If no compression comes back, then the problem is likely in the valves. 

 

The oil found its way around the rings and I had compression for two or three kicks.  Then it went back to no comp.  So the problem was in the barrel.  Off with the head and barrel, then the piston was on the bench.  This particular piston is a Hepolite, original type as used when the bike was new.  It was fitted 15 years ago, just before I bought the bike, and has now done something like 25,000 miles, including a dozen of the Velo club 1000 mile rallies at or near full throttle to pull the sidecar.

IMG_9867

 

A crack had developed across the crown of the piston, and down through the ring lands and onto the thrust face.  The crack wasn’t the source of the compression loss, but the top rings had been nipped in their grooves and weren’t free to expand into the bore.

 

One tip:  if your piston pin is a tight fit in your piston, warm the piston to expand the hole and give the pin some room.  A rag soaked in boiling water will work, or the gentle application of a propane torch.

IMG_9870 

A call to Ed Gilkison and I had a NOS piston in my workshop.  A genuine Hepolite, with the same part number, including rings.  And to the same +.030” bore size, so I didn’t have to bore the barrel over to fit a .040” piston.  This one was marked Ernie Pico 1973, so it was probably sitting on a shelf at Ernie’s shop in SoCal back when I was still wearing short pants.  more Ed stuff here on DQ’s site.

 

A light hone of the bore with a three-stone expanding hone in a hand held drill, then it was time to check the rings.  Typical ring gap is about .004” to .005” per inch of bore, so I used a fine file to cut down one edge of the ring until it gapped at .015” using feeler gages.  The photo below shows the general method.  Or check in an old book, most of them describe the operation.  I should mention that the bore looked pretty good.  No ridge or deep grooves, and just a bit oversize for the piston diameter.IMG_9866 

The valves were given a quick test by filling the ports with a half pint of solvent.  No leaks into the combustion chamber were seen, so the valves were left alone.  If yours need some attention, here is a How To article on valves.

 

Rutland stove paint works well on old cast iron motors.  But make sure to paint outside, it smells nasty.

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Funny how the motor can come apart in a half-hour, then an hour to fix and fit the new parts.  But then it takes half a day to put the motor back together and seal up all the various leak paths.  IMG_9874

When he laid out the design for the M Series motors at Veloce Ltd, Charles Udall decided to use 19 screws to hold the valve covers together.  But Valentine Page used just 8 screws to do the same job when he designed the enclosed rockers on the 1938 Ariel Red Hunter.  After fettling each of those over the last dozen years, my feeling is that the Ariel is easier to work on and make oil tight, but the Velo runs sweeter. 

 

your mileage may vary.

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17 Comments leave one →
  1. October 23, 2011 7:32 pm

    >015 ring gap not .004??? Thanks, Paul

    • October 23, 2011 9:46 pm

      Hi Paul. .004″ or .005″ per inch of bore. This motor has a bore of 3.2″ so I gapped the new rings at .015″

  2. charlie 101 permalink
    October 23, 2011 7:51 pm

    It will be interesting to hear if it blows blue smoke and if the oil consumption goes up with that shallow honing you’ve done. I was in the same situation with on one of my Indians and honed perhaps lighter than you did and after some 500 miles the rings still wasn’t settled. I had to redo it.

    • October 23, 2011 9:48 pm

      It smoked a little today on a quick 2-3 mile ride, but not much. I’ll keep an eye on it on the next few rides and see how they bed in.

  3. Anonymous permalink
    October 23, 2011 10:43 pm

    Hi Pete, I too have a mighty MSS, love it, but struggle to find the magic potion to seal the rocker box and valve cups for any length of time, especially the exhaust which gets smoking hot. I find if I use good quality oil I will always smell nice!
    Russell
    Sunny Melbourne, Oz.

  4. October 24, 2011 1:28 am

    nice post pete….

  5. Somer permalink
    October 24, 2011 3:41 am

    Best way to seat rings is go for a ride in a mountainous area.On throttle/ off throttle .

  6. October 24, 2011 8:24 am

    If I can’t fix it with a ziptie, maybe I shouldn’t think about owning one of these!

    • October 24, 2011 9:14 am

      Hi Jason. don’t sweat it. A big part of the fun of owning an early bike is the occasional fettling that is required. You are a handy guy, you’ll figure out a Velocette.

  7. Anonymous permalink
    October 24, 2011 6:22 pm

    Pete, can you determine where the piston crack originated? What a strange failure mode.

    Fred Meyer

    • October 24, 2011 8:07 pm

      Hi Fred. I’m not sure. The crack may have started in the crown, or maybe at the ring lands. The lands had flowed a bit over the rings, which might have caused some stiction and then a crack? but I’m not 100% sure what happened first, the crack or the sticking rings. I could see that the crack was present on the inside of the piston, but it was wider on the outside. I’m not too worried about it. A $50 piston that lasts for 15 years of very hard service, and can be replaced in an afternoon, is ok with me.

  8. charlie 101 permalink
    October 24, 2011 9:32 pm

    Well, it’s a very interesting technical question. I’d like to hear what some of the reputational piston manuf. has to say. Probably material fatigue, maybe stemmed from a overheated spot at an old damage. I think I spot an old damage at the top ring land, maybe from soot deposit that would impose a local friction spot. The crack continues past a oil drainage hole as is a weakened spot. Would be interesting to see the underside and opposed side.

  9. Jim Abbott permalink
    October 25, 2011 8:58 pm

    Pete:
    Dab a little Locktite green (very thin and wicks into threads even after the nut/bolt has been tightened). Ed G. has used it on leaks at the bottom of front forks with good results.It just wicks into all the cracks and seals ’em up toot sweet. I’ve also used it although not on a cracked piston, but give it a try. We can all learn something from your experiments.

    Cheers, Jim A., Tucson, AZ

    • October 26, 2011 8:42 am

      Thanks Jim. I typically use the green loctite only for bearing locking. But it is good stuff. On the iron MSS, it isn’t the umpteen screws that leak, it is the huge amount of joints around the valve enclosures. There are two steel spring cups that seal against three different aluminum cases, plus another aluminum case all that stuff seals against.

  10. October 26, 2011 11:45 am

    Instead of heating the piston I freeze my piston pins. Just throw’em in the freezer for an hour or so. Cool Blog. Keep up the good work!

    JT
    Chicago

  11. November 13, 2011 1:23 pm

    Just ordered some of the Rutlan Stove Paint for the old Beesa. Thanks for the tip!

  12. caryandjohn permalink
    November 29, 2011 6:57 pm

    Hi Pete,
    I had fun chasing Blaise on George’s Velo at the All British Ride recently … neat bike, and faster than me … ha! Thanks for the link, I saw a big jump in traffic … Wonderful blog …
    See you in the parking lot,
    — caryandjohn

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