BS50 Sport iron cylinder

[CL-100]

I'm in the process of getting set up to start a full restoration on a BS50 Sport.  The bike was thought to be a runner, although I was never able to get it started.  I spent some time trying to figure out what wasn't working and I found that I was only getting an intermittent and weak spark so I just packed the bike away and bought a new coil and ignition parts that went into the storage bin for that bike.  I recently pulled the cylinder off with an intent to send it out for a fresh bore to accommodate the first-over(0.25) piston I already had.  The machine shop cautioned me that a first-over bore might not clean up the cylinder enough if there was existing damage.  Well, as it turns out, there is some damage (see attached pictures). 
What really caught my attention is that the area between cylinder bore and the transfer port opposite the exhaust seem to be real close.  I put a digital caliper on it and it measures about 1.7mm at the thinnest part of the skirt.  The farther you go down the skirt, toward the top of the cylinder as it's laying upside down, the thicker it gets. 

Also, on the photo with the burnt and scarred piston you can see that both rings look to have a shiny appearance.  It was my understanding that the iron cylinder used a chrome plated top ring and an iron bottom ring.  Both rings look the same to me. 

So my first concern is whether or not the cylinder is suitable for a rebore, assuming I can find a second-over (0.50) piston and rings.  Secondly, is the damage to the piston and bore from someone installing the wrong rings on this piston?

Any thoughts out there from anyone with iron cylinder experience is appreciated.  Thanks.

 

[Chibi62]

I have had a similar cylinder condition successfully bored. From what I'm seeing with yours the level of damage on the bore wall would indicate a decent amount of material needs to be removed. It appears to me your likely to get into trouble with the cylinder port thickness. Not knowing what your machine shop will charge you. I suppose it's worth a shot to have it bored up to the point the damage is removed. Otherwise you might be better off looking for a good used cylinder. It's a coin toss .. Your machinist should be able to definitely say yes or no ..

As for possible mismatched rings. I have unknowing used wrong rings in a few applications. Bikes ran well with no real issues. I'm seeing chrome rings here in a iron cylinder. From the pictures it appears the cylinder was seizing due to lack of lubrication. Check the piston for signs of the wrist pin starting to scuff & seize. That is another sign is was ran dry ..   

Kevin
   

[OldSwartout]

I agree with Kevin, definitely ran without lube for a short time.  Wrong rings won't cause that, only chrome rings on a chrome bore will. Iron rings or chrome are OK in an iron bore.

IHMO, the 0.25 over piston will work. The serious damage is all below the transfer ports, so if it doesn't totally clean up, it won't hurt performance. The rings will seal properly above the ports. There could be some long term effect on piston and ring longevity, but you probably won't put 15-20,000 miles on it, so no worries.

Karl

[CL-100]

Kevin and Karl, thanks for your responses.  Since I'm at the beginning end of this project, I'll spend some time looking for replacement parts first.  I'm hopeful but not optimistic I'll find a 0.50 second-over piston and rings or even a good used cylinder.  At least I have an option of trying the first-over bore.  I'm not likely to put more than 20 miles a year on it.  I've always been partial to the little Sport 50's and 60's so I'm determined to make this work. 

Rowland

[Chibi62]

I also wanted to mention that when you see a recess hole on the top of the piston (see picture) indicates it's intended for use on a iron wall cylinder. A piston with a "swirl mark" on the dome of the piston indicates use in a chrome bore cylinder.

Under most applications. The upper ring is chrome (compression) while the bottom ring is Iron (for oil sweep). In race motors it's not uncommon to remove the lower iron ring, and run the engine with only the upper chrome ring.

Kevin     

[CL-100]

I did not know that the recess hole in the top of the piston indicated that it was for an iron wall cylinder.  I appreciate the lesson as well as the  information on the purpose of the two piston rings. 

Thanks!