Hello All,

Kevin from Los Angeles checking in. It's been a while, but life, kids, school, work and all the other stuff gets in the way sometimes.

Got a full day to work on the bike today. First in about 3 months. The old BSA M20 is coming along. Got the magneto off and sent to what I'm told is the best magneto guy on this side of the pond. Got the exhaust pipe off with judicious use of propane, got the cylinder head off, and barrell. Bottom end of the bike looks great.

The timing chest was full of crap. Looks like someone took the timing cover off and left it outside in the blowing wind for a couple of days. There were leaves, dirt, fibers and other shit inside. Luckily, the bottom end didn't look bad when I pulled the barrel. The cylinder is going to need a honing, the clock and ammeter need to come off next, I need to get in the gearbox, and the valves need a grind.

One question stands out. There is what looks like a top groove for a scraper ring at the top of the piston. There's no ring there. Should there be one? I guess I need to get some pictures.

Whew! Time for a Pusser's Rum!

Hi Kevin,

I also had one of those pistons and installed it without the top ring.
(It had a thin ring there, but I took it out)

I remember that Henk told me that he used those pistons with and with no top ring,
And it did not make any noticeable difference.

I think (But cannot commit to that, Ian W. would surly know..) that this top ring type pistons
Used for stationary engines, but the piston is just as good for the M20s.

Noam.

email (option): noam10@gmail.com

Hi Kevin,

This is the normal set up. The piston has three rings already and doesn't need a top ring in that groove. This is a carbon groove. The excess build up of carbon will fill the groove.

Regards,
Leon

email (option): leonhop3@planet.nl

G'day Kevin'
The top groove is a heat dam so no need for a ring. Have a look at item B) in this link.
http://www.hastingsmfg.com/ServiceTips/piston.htm

Thanks for the replies. Good to know this piston doesn't need a top ring. BTW, this bike has a REME tag on the tappet cover, dated 11.2.52, that informs me that it has a 1 mm oversized bore. There is absolutely no hone cross hatch on the bore. I was planning to get a bottle brush in there to get some cross hatch. It should have a cross hatch for oil retention, right?

Hi Kevin,

you can expect the hone cross before the "Brake in"
period, but not after many miles.

The hone, to my understanding, is there more for the
purpose of "filing" the ring and barrel to better fit eachother,than for oiling, and is one of the reasons why a newly run motor runs hotter.

Noam.

email (option): noam10@gmail.com

If it's got a REME plate and no wear ridge on the bore then I think you can fairly happily rely on the fact that they'd done a few of 'em and probably knew what they were doing. Not only that, they weren't paying for the parts and anything that needed it was replaced at the Queen's expense (the old King died on 6th February 1952).

I always thought that honing was essential to carry oil in the bore and provide a lubrication "film". Can anyone clarify this?

email (option): dannydefazio@sumpmagazine.com

"Back in the day" we were taught that the bore had to have the surface glaze broken to allow new rings to seat. The new rings and the new bore surface could then wear together during break-in for a proper seal. New rings on a glazed cylinder bore would not seat properly, causing oil consumption and compression loss

Today, modern machines start out with such a precision fit from the factory that a break-in period is hardly specified, just an early first oil change....

email (option): teladelujo@msn.com

Now I'm more confused. I thought glaze happened after a long period of use where the oil had formed a smooth film that wouldn't carry any more oil. Clearly I need to get back to basics...

email (option): dannydefazio@sumpmagazine.com

Perhaps the use of the term "glaze" is confusing...I am referring to a cylinder bore polished by use, not some sort of baked-on coating on the cylinder walls, which does happen, but not so much in gasoline engines....

New rings will not seat easily (may never seat) on a wear-polished bore. We always would use a fine grit hone to crosshatch a cylinder to create a fresh wear surface for new rings.

Here is a link to diesel marine engines and the subject of bore honing, glazing and polishing...

http://coxengineering.web.officelive.com/bore.aspx

JDE

email (option): teladelujo@msn.com

hi when a cylinder is bored with a tool,effectively the surface
is a screw thread,although it may look smooth it certainly is not.
the reborer will then hone the cylinder to the finished size
to smooth out the peaks and troughs from the boring operation.
he will also remove a little more material to give a running
tollerance,say .004''-.006''so when the piston starts to heat up
it doesnt bind or nip in the cylinder (aluminium expands faster
than cast iron).
the cross hatching produced from the honing operation is caused by the fact that the honing tool is revolving relatively slowly
but is reciprocated up and down the cylinder rather quickly
so it produces a series of very fine scratches.
these will disappear very quickly once an engine is started.
again the peaks and troughs from the honing operation will be smoothed out by the piston rings traversing the cylinder at twice the rpm the engine is running.hope this helps
cheers rick

email (option): richardholt@rocketmail.com

Much appreciated on the replies. Just to be clear... I'm using the same piston with rings. I won't be replacing the rings or even removing them so should I worry about it and run a hone through that bore or not worry? There are some very light score marks, like about 0.0003" or so.

the only reason there is cross hatching is caused by the action of the honing tool it is nothing to do with the cross hatching being the best method a quick rub down with wet and dry paper would do the same as honing the only problem with using wet and dry is getting an even honing to much rubbing in one area would cause its own problems

If you install new rings, the normal practice is to lightly hone the bore surface to create a fresh wear face to allow the rings to seat. If you decide not to replace the rings, leave the bore surface finish as it is, and reassemble....

JDE

email (option): teladelujo@msn.com

Also when fitting a new ring set be sure to carefully remove any wear ridge at the top and bottom of their range of movement and any carbon build up at the top. New unworn rings with 'sharp' corners can collide with the existing wear ridges causing breakage...Ian

email (option): ian@wright52.plus.com