[bob bezin]

yah i did the 535 kit the british exhaust and the 34mm carb  i still like 60mph but i get there a lot faster

[ace.cafe]

Ok Eric, you've convinced us that there's a real problem here!

Now, this is something that Bullet old-timers have seen before, and it's not terribly uncommon under certain circumstances.
Let me do some 'splainin'.

First, this is normally caused by heat.
The heat can be caused by a variety of things, or a combination of things.
I'm thinking that the "rattling" you were hearing actually was detonation or pre-ignition problems that manifested into an overheated piston.
I'm not sure exactly what order that the progression occurred, but it is likely that the compression rings got hot enough to stick, and caused blow-by of hot combustion gasses.
This overheated the next ring and the skirt, which cooked any oil that was on the skirt.
The piston got even hotter and expanded even more, thus tightening the wall clearance tighter.
The tight clearance, hot metal, and lack of lubrication from the oil burning off from blow-by seized the piston in the classic 4-point manner.

I've seen this a number of times, and you're lucky that you didn't also bust the con-rod, because that often happens when a seizure occurs.

So anyway, what you need to do is look all the remaining parts over carefully, and determine the extent of the damage. Hopefully it will be limited to the piston and the bore alone. Then you can just replace the piston and re-bore to suit.

You CAN go to a higher compression piston, as suggested above.
However, this is something that needs some care in doing, because you will need to re-adjust your timing to suit the higher compression ratio or you will seize that new piston too.
AND THIS IS IMPORTANT, the 8.5:1 forged piston has a VERY big expansion characteristic and it MUST be clearanced at .006". This means the measurement  of the new bore diameter MUST be .006" larger than the measurement of the new piston skirt diameter. That's diameter measurement, from front to back on the skirt.
If you do not do this, it will seize, and I know of at least a dozen people who have seized these 8.5:1 forged pistons by trying to clearance them too small. The piston will rattle when cold, and when warming up, but when fully hot it will quiet down and run at proper clearance.
You have to know this before going into that piston. It's a nice piston and made very nicely, and has great rings on it. But it needs ROOM to expand.
And, it will REQUIRE a more retarded ignition advance by about 4 degrees less total advance, or around 30-32 degrees total advance, and no more than that. Perhaps a little less, which you can determine when fine tuning your particular engine with it.

Also, make sure you don't run that piston lean at any throttle position.

I like that piston, and I have 2 of them for my builds. But, you have to know what it behaves like, and large expansion is the hallmark of it. Not trying to scare you off of it, but to let you know what to do if/when you get it.

The normal replacement pistons that are of cast construction do not require as much clearance, and can be clearanced at around .0035" or even a bit less if you break then in careful.
But the stock pistons such as you just removed, have very poor quality ring sets, and there are no good replacement ring sets for them that I know of. There used to be good sets from Cords, but they discontinued them in the stock sizes for the Bullet. maybe if you go .020" oversize and get the Cords rings for that size from CMW, that would be a good option for standard piston replacement.
The stock piston isn't too bad, but not great. However, the stock rings really suck.

[erk187]

I'm not sure that I want to go with upping the compression, as I was okay with the power that it had before and would like to get her to a good starting spot. I am wondering however, and may have a clue as to why it seized in the first place, as I notice in the spots where there is no wear on the cylinder, ( very top, down below ) If I put one of the rings that were melted into the piston into the cylinder, the space on the end of the ring is so tight I cannot put even a piece of paper in-between the two ring-ends. I thought I recall .030 being the factory setting, whereas this is much tighter than that. Question is whether I should plan on boring this cylinder to proper tolerances? I can see that the piston was too tight to begin with and as we know, the aluminum will expand a lot faster than the cast iron..
 In all my years of working on cars, being a race mechanic for a professional motocrosser, racing motocross myself, rebuilding cars, motorcycles, tractors, aircraft.. I can honestly say that I have never ever seen a piston as tight in a cylinder ( without the rings ) as this one is. Keeping in mind that it does have some wear on the skirt and with the carbon removed from the top, you cannot stick a matchbook cover in between the cylinder and the piston. Is it possible that it wasn't bored correctly from the factory? Are they always this tight?

[erk187]

As for the rattling, it was from the piston. The rings were siezed tight into the piston so that the piston was doing the work of the rings, flopping from the top to the bottom. Piston rattling some call it, some call it piston slap, it can be seen via the wear on the piston. I thought from the beginning that maybe it was some big single detonation from whatever and retarded the timing to account for that. As it turned out, when I got home and static-timed it, as it was it was retarded a degree or so and upon checking the jets, the pilot was a 25 and the main was a 125 both richer than stock.

[ace.cafe]

Well, anything's possible.
But, the stock ring gap should be about .015"
Stock piston-to-cylinder clearance should be right around .003" - .0035" more than the skirt.

If you bore .020" oversize, or whatever CMW can give you a piston and a set of those Cords oversize rings to fit it, you'll be golden with that combo, if you don't want to increase the compression.
Those Cords rings are the only decent rings that you could get for that stock piston, and they discontinued the std size, but there are still some ring sets left at CMW for some of the oversize piston sizes.
Get a piston and matching Cords ring set for whatever oversize you can get matched up with(+.020" piston and matching +.020" rings would be great), and bore the cylinder to suit, with a .003" - .0035" diameter clearance more than the skirt measurement, and you're good to go.

As long as nothing else is wrong, that is. We wish you good luck with it.
You're by no means the first to have this happen, and you won't be the last, either.

And remember, you'll have to break that bore in again. If you go right out and try to do 60mph with that new piston and bore, you're gonna wind up in the same place again. 1000 miles before running hard on that new piston and bore.

[erk187]

Boy don't I know it, another 1k miles, shouldn't take too long though. What kind of oil is suggested for break in? I'd rather be wearing my own piston anyhow I guess.. Good that the crank is clean and straight. I like the idea that my power will be a lot better!
 Thanks for the help!

[ace.cafe]

My personal preference for oil during break in would be a dino oil, not a synthetic.
Normally i would suggest an oil rated with an older API rating such as SL instead of the newer SM rating, because they have changed the additive packages in the new SM rating oils to reduce ZDDP and phosphorous, to extend the life of catalytic converters in new vehicles. These ZDDP and phosphorous additives are important for new cams and solid lifters to break in well without excessive wear. It's only an issue with classic type vehicles or vehicles with solid lifters during break in.
However, since you have already broken in the cams and lifters, it would seem to allow wider leeway for an oil selection for just the bore and rings to break in.
I'd pick a good dino oil that you like, and if you can find one that isn't SM rated, so much the better.
As a general rule, I'd try to use an SL rated oil at least once in a while to replenish the ZDDP and phosphorous on the metal surfaces of the cams and lifters, because it won't be provided by the SM rated oils in sufficient quantities. But you probably don't need to use it all the time after break in is done.

You can reduce your break in period by using wider clearances in the piston to cylinder wall clearance, but that would also lead to earlier wear-out too. It's your call on that one.

[mrunderhill1975a]

I must say I find this information very interesting.  However, I am not quite sure where the clearance tolerances are supposed to be measured.  As you rebuilt your motor, would it be possible for you to post some photos of how the various clearances are measured?

[Foggy_Auggie]

The most accurate way is to measure the piston at the bottom of the skirt and at the top of the crown using an outside micrometer.  Measure four circular points 90 degrees apart at both ends.

Note:  Since the piston skirt usually expands more at operating temperature than the piston crown there could be a difference where the skirt will be smaller diameter.  This not a hard fast rule and would depend on manufacturer and production tolerances.  And some piston skirts will measure differences 90 degrees apart to the wrist pin boss.

Use a precision bore micrometer and measure the cylinder at the bottom, middle and top at four circular points 90 degrees apart.

These micrometers can rented from a tool rental.  I wouldn't rely on a feeler gauge stuck between the piston and cylinder bore.

Or - drop the piston and cylinder off at a machine shop with specific clearances and let them do it.  At my age, anymore, that's what I do.

Make sure the connecting rod has no felt up and down movement from the big end.

Make sure the installed wrist pin has no felt side rocking couple along the axis of the pin in relation to the con rod small end.