Iron sleeve boring

[Molochnik]

I'm going to try boring an iron-sleeved cylinder on an old 125cc dirt bike - I've never done this before. I intend to put it on my mill and use a boring head and then finish it with a silicon carbide ball-hone.
My primary concern is how far under my target dimension do I bore so I don't go over w/ the ball-hone?
I'd hate to find that I've blown the final dimension after achieving the desired finish.
Is there a rule of thumb like X% under for every X inches of diameter?

Thanks

[Molochnik]

FWIW, the max I.D. of the bore is currently 2.2069, the O.D. of the new piston is 2.2419, and the target bore size is 2.2434 min. to 2.2439 max.

I'm astonished that Yamaha is specifying a clearance as tight as .0013 for an air cooled engine. I'm going rogue and using .0015 for a min.

Thanks again!

[RICHARD ZASTROW]

The Yamaha specs may be compensation for the expansion of the cylinders under operating conditions. They do have experience in this area. LOL

Dick Z

[holbieone]

hone it with x amount of strokes and see what it takes off

[Molochnik]

I would expect the aluminum piston to expand more than the iron sleeve. Even the liquid-cooled bikes use .0015 as a min.

[Molochnik]

hone it with x amount of strokes and see what it takes off

Thanks Hobie, but wouldn't the hone take a different amount off of the worn cylinder as it sits, as opposed to one with the finish from the bore-job? I would think the hone would tear through the machine finish quicker than the much smoother surface the piston has been beating up since 1978 Do you think the difference would negligible?

I would also prefer not to disturb the set-up, i.e., have to re-jig it after a partial bore-job.

I suppose I was hoping someone had done a lot of this (sizing by ball-hone)and could say something like "you're always safe going .010 under" or "figure .004 for every inch of bore" or something like that.

[holbieone]

those dingle berry hones don't take much off

if it was me doing this , I'd bore it to size and then hone it (as long as the cutter marks are not to bad)

those old engines could handle a little piston slap

[Molochnik]

I think the finest feed on my mill is .001 To be honest I haven't messed w/ the feature much, nor do I have much formal instruction. I was just going to use the most appropo cutter speed formula and the slowest feed, and hope for the best. I still have to make a jig for it.

I wanted to send it up to a hot-shot engine place that's local but they said it's too small for them to do

I figure it's a good opportunity to learn the operation myself. Everything else I've worked with has been of the more modern plated variety, so those just get sent out regardless.

Thanks Holbie.

[RICHARD ZASTROW]

Ref. post #'s 3+5: First, your right alum. expands more than iron. That's why pistons are smaller at the top, more expansion from the larger mass of alum.

In crankcase induction two-strokes the incoming charge cools the underside of the piston dome, confusing everyone about heat growth vs cooling shrinkage. On top of that, there's more mass in the wrist pin boss causing more expansion in that area. One of the reason cold pistons aren't round, but are a bit elliptical. (they round up when at operating temp)

Ref. ball hones. Be careful, they will radius the port edges. That's good to avoid "ring snag". If it's excessive, it'll change your port timing and flow direction.

Just trying to help.

Dick Z

[holbieone]

Ref. ball hones. Be careful, they will radius the port edges. That's good to avoid "ring snag". If it's excessive, it'll change your port timing and flow direction.

there should be a chamfer on the port edges ,a dingle berry hone will not hurt the ports

[Molochnik]

Ref. post #'s 3+5: First, your right alum. expands more than iron. That's why pistons are smaller at the top, more expansion from the larger mass of alum.

In crankcase induction two-strokes the incoming charge cools the underside of the piston dome, confusing everyone about heat growth vs cooling shrinkage. On top of that, there's more mass in the wrist pin boss causing more expansion in that area. One of the reason cold pistons aren't round, but are a bit elliptical. (they round up when at operating temp)

Ref. ball hones. Be careful, they will radius the port edges. That's good to avoid "ring snag". If it's excessive, it'll change your port timing and flow direction.

Just trying to help.

Dick Z

At the risk of getting way off topic, could you elaborate on the heat growth vs cooling shrinkage confusion? I'm unclear as to what you're getting at.

Also, I'm curious as to why the cooling effect would be any different on a piston port or cylinder-mounted reed cage arrangement?

I trust the Yamaha engineers :-) I was just surprised to see such a tight spec. I just got done w/ a '59 Triumph that needs .0046 at the skirt, and .0085 at the crown!

[RICHARD ZASTROW]

I was referencing "crankcase induction". Piston ported, reed induction and rotary valve are all crankcase induction. That is, they all pull the incoming charge into the crankcase.

This incoming charge circulates under the piston, providing cooling to the inside of the piston dome. Meanwhile, the fuel is being compressed and ignited on the top of the dome in the combustion chamber. (not in the same exact moment, but near enough at engine operating speeds)

The piston skirts are also being cooled by the incoming charge, so they don't expand as much as the dome.

If the operating conditions are correct, all the expansions will move toward proper clearances. Every engine being mass produced has been very carefully engineered to be as close to optimal as possible.

It's us who mess with these engine and find ourselves slightly beyond the limits. Result is seized engines and/or donuts for pistons. They always run the best just before the blow up.LOL

Dick Z