Hone Grit and Re-Ringing Necessity

[gene xs]

New to me '78 with 11,000 miles. I got this bike as a short term project - my wife says that 1 vintage motorcycle (my '74 RD350) is enough (and I agree).

It had light smoking from one cylinder so I tore down the top end yesterday so that I can replace the valve seals. While I was in there, I pulled the jugs to see what was up.

The bore mikes out to 79.5 mm, so it look like the PO went big bore on it. I found very light scratches (they just catch a fingernail) in both bores. Since this is not going to be a long term bike for me, I plan to use a ball hone to remove the scratches.

Question 1: What grit ball hone should I use? They come in 120, 180, and 240 grit.

Question 2: Is it really necessary that I re-ring? This is not a long term bike, so I'm trying to keep the costs down.

Thanks!

 

[angus67]

how did you measure the bore?
Do you have a manual?
Hmmm...is it necessary to re-ring. Ask the guy you sell it to if he would use old rings in a resurfaced bore.

 

[Fleksta]

Seems kind of silly to have it torn down that far and not re-ring. In the grand scheme of things they aren't that expensive and I doubt the old rings would seat properly again.

 

[5twins]

Checking the bore size is only half of the equation. You have to measure the pistons too, then determine the clearance between them and the bore. As this increases, the pistons rock back and forth on the rod as they go up and down. The skirts start contacting the bore. That's what causes the vertical scratches in the bore.

You made no mention of checking the current ring end gaps. That's what I look at most when determining whether or not to replace the rings. I don't find many that can be re-used.

 

[jbenedict]

Reusing rings is a 'have no other choice' thing to do. A ring set at Mike's XS is about $35 and you already have to buy all the gaskets and stuff. A wild guess is, with only 11K, the pistons and bores are going to be OK.

Do it.

 

[gene xs]

I checked the bore with a vernier caliper (not very precise, I know) and it came up at 79.4 mm. Obviously way over the stock 75 mm. The cylinders look stock.

I think my next step is to figure out what kind of pistons are in this beast, then see if I can track down some new rings.

 

[gene xs]

OK, the pistons say Yamaha and 80 inside. So it looks like the PO went to an 80 mm bore.

Any idea what they might be? I'm guessing that they are from another model of bike.