2 Cycle Sleeve Machining

[markba633csi]

I would copy the port dimensions from the cylinder as accurately as possible. Then, if you can have the sleeve replaced you could
cut the ports yourself if the shop can't or won't cut them. Measure the piston carefully too.
I'm guessing it's piston-ported and not rotary or reed valve?
-M
ps looks like a Vespa- if BMW made one

 

[Scra99tch]

I would copy the port dimensions from the cylinder as accurately as possible. Then, if you can have the sleeve replaced you could
cut the ports yourself if the shop can't or won't cut them. Measure the piston carefully too.
I'm guessing it's piston-ported and not rotary or reed valve?
-M
ps looks like a Vespa- if BMW made one

This was my original idea.

Wrap a piece of poper end to end inside the cylinder and either spray paint the locations through the intake and exhaust ports or charcoal pencil. Un-ravel the paper and take some measurements.

I could buy the raw stock and turn it down on a lathe cut the ports rough and hand file them in if anything. I'll snap a picture of what I am looking at. Then assemble it and take a skim cut on the bore to size it correctly to the new piston.

Yes I would say its piston ported.

Its the bevels on the sidewall that have me a bit worried and they themselves look hand filed. They don't match anything in the cylinder itself so I have some leeway I would think as to size, shape and location.

 

[markba633csi]

You mean the edges of the port openings where the rings slide past? Yes, they could have been done by hand. Typically
you want just enough bevel to let the rings go by without snagging a port. The rings actually bulge out into the port slightly as they
go by and then get stuffed back, more or less "gently" by the bevels
Some large ports are bridged in the middle to reduce the bulging effect

 

[Scra99tch]

You mean the edges of the port openings where the rings slide past? Yes, they could have been done by hand. Typically
you want just enough bevel to let the rings go by without snagging a port. The rings actually bulge out into the port slightly as they
go by and then get stuffed back, more or less "gently" by the bevels
Some large ports are bridged in the middle to minimize the bulging effect

Okay these seem a bit more excessive than just a chamfer for rings, as they bridge the whole wall of the sleeve looks to be about a 30 degree angle. Like they wanted to direct the exhaust towards the manifold.

 

[cjtoombs]

Not seeing any pictures, my course of action would be to copy the locations of the ports as you have described, bore the cylinder oversize ~1/8", get some material to make the sleeve and rough turn the outside and inside to near the size, maybe over and under by .030, cut the ports in the sleeve, turn the OD down to a press fit in the cylinder (maybe make a mandrel for this). Press it into the cylinder and bore it to around .003 to .005 undersized and then have an automotive or industrial machine shop with a Sunnen hone finish the bore. That should give it the proper finish for the piston rings as well as getting the piston clearance just right. Not sure what your equipment situation is. As an alternative to farming out the honing, you could make a honing mandrel for the lathe and do it yourself on there.

 

[Scra99tch]

Nice Zorak Pic!

 

[matthewsx]

Before computers there was quite a bit of black magic to porting. I wouldn’t pay much attention to anything besides the port heights and making sure there’s enough of a chamfer for the rings.

Also, there’s no guarantee that someone else didn’t work on it before you got it.

John

 

[Scra99tch]

Here’s a couple of pics.

 

[FOMOGO]

A lot of the magic in two strokes is in the exhaust pipe tuning. Can make a huge difference in performance. you can probably buy an existing liner to modify for less than the raw material to make your own. Mike

 

[FOMOGO]

That appears to be a pretty thick liner. Sure you can't just bore it? How about a pic of the bore. Mike