rebore tolerance

two-stroke-brit · Post by **two-stroke-brit** » Sun Jan 10, 2010 3:14 pm

i am going to try and rebore the buffalo jugs at work if i can get the boss to ok it .

i was hopeing to just hone them and go with new pistons and rings but after the hone showed up steps and scores thats not going to happen.

how much clearance from the next oversize piston size do i want .

thanks mark.

tz375 · Post by **tz375** » Sun Jan 10, 2010 4:41 pm

Do you have access to a proper boring fixture, or are you contemplating doing it on a milling m/c?

If you're thinking a mill will work, I would advise against it. I have seen barrels machined on a lathe and on a mill and they are never as straight as with a boring bar and thick torque plate.

two-stroke-brit · Post by **two-stroke-brit** » Sun Jan 10, 2010 6:19 pm

pardon my ignorance what is a torque plate and i was intending to bore it on a mill.

i was going to lift it from the table on 1/2 inch parralells and use a boreing head. either on a bridgeport or a cnc if available,
thanks mark

Frank · Post by **Frank** » Sun Jan 10, 2010 7:51 pm

I can tell you how my local guy does it (he uses a small cylinder boring machine but I can't see why a mill with boring bar wouldn't work.) He makes sure he has the piston before starting then bores until the piston is a tight slip fit. He then hones and ends up with .0015-.002" clearance.

The required clearance and method-of-measurement is generally given by the manufacturer. You have to be careful where you measure. I like the method described above (have had him do several for me like this) and it seems to work okay.

A torque plate replicates how the cylinder bolts load/distorts the cylinder. Theoretically it allows you to machine the cylinder in the loaded condition even though it may be oblong when unloaded. I'm not sure how big a difference it makes with air-cooled engines as they're inherently sloppy anyway.

durgam · Post by **durgam** » Sun Jan 10, 2010 8:05 pm

"loading" makes really good sense. A good way to replicate what you want would be to clamp the head down to a plate using the actual mounting holes. this will "pull" the material in the proper directions. It may only be a couple ten-thousandths of an inch, but it will be better than even a light squeeze in a kurt vice. My 2 cents.

durgam · Post by **durgam** » Sun Jan 10, 2010 8:10 pm

But to answer your initial question, I believe the bore tolerance is .01mm, total tolerance, or .0039 in. Thats on my 550, your water cooler, im sure is a bit tighter. probly -nothing to +.002. than hone,and fit your ring gap and you should be good.

two-stroke-brit · Post by **two-stroke-brit** » Sun Jan 10, 2010 10:01 pm

i am confused you say 0.01mm or 0.0039 but 0.01mm is 0.00039 ie 4 tenths ,

i get it about loading the jugs to bore them in the state they are in on the bike.
i think i will use the puller plate and just bore thro into the plate enough to clear the rad on the b/bar.

mark.

tz375 · Post by **tz375** » Sun Jan 10, 2010 11:31 pm

A mill is indeed a fine tool and for a street motor it could well be good enough. A typical Boring machine has a really short tool ( like someone... oops I didn't mean that..) and a large diameter tool holder. That makes it incredibly stiff.

A boring tool in a mill is typically less stiff and may wander or chatter.

The torque plate is to simulate the stresses and out of round forces that it experiences in the motor. Admittedly a GT750 block is heavy and although it seems stiff, the upper parts of the bores are not very well supported.

Again, for a street motor, maybe it's not much of an issue. And yes, the hone should be used to take it out to finished clearances of .0018" - .0022" (1.8 to 2.2 thou)

WISECO pistons expand more and need a lot more clearance, so if you use them, follow their instructions.

And don't forget to chamfer all the ports.

Suzukidave · Post by **Suzukidave** » Mon Jan 11, 2010 5:01 am

This page and the next several talk about clearance http://www.3cyl.com/mraxl/gt/manuals/gt750man/sr26.htm

H2RICK · Post by **H2RICK** » Fri Jan 15, 2010 12:12 am

i was intending to bore it on a mill.

Call me old-fashioned if you will but you're playing with fire trying to bore stroker cylinders that way. "Smarter" guys have already tried this method in an effort to save a few bucks. Very few, if any, have succeeded.
You will be FARRRRR better off and get a more accurate job with a real boring machine, IMO. Short term (financial) pain but long term (mechanical) happiness.....as long as the guy running the boring machine knows his job.
There is a good reason why they make boring machines.....and that is because they do a proper job.
My .02 worth.....

two-stroke-brit · Post by **two-stroke-brit** » Fri Jan 15, 2010 1:16 pm

many thanks guys,
i am going to try and find a recomended shop in my area (houston).
i went to one place and saw his setup ,basically a pillar drill with auto feed and a cutting tool that he would "tap" out "abit" and try it until the piston fell in.
any recomendations are welcome.
thanks mark

bikegeezer · Post by **bikegeezer** » Sun Jan 17, 2010 10:08 am

Box up the cylinder, and send it to Bill Bune. http://www.billbune.com/

Stu

Suzukidave · Post by **Suzukidave** » Mon Jan 18, 2010 5:50 pm

bikegeezer wrote:Box up the cylinder, and send it to Bill Bune. http://www.billbune.com/

Stu

and the pistons so he can match them to the bores

rebore tolerance

rebore tolerance

more please

queery on tol

thanks