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Boring cylinders, clearances etc
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Author:  2stroke [ Sat Jan 08, 2022 4:41 pm ]
Post subject:  Boring cylinders, clearances etc
I have some questions about boring a 2 stroke cylinder.

Is it common for a cylinder to wear more down towards the skirt area?

The piston is tapered, the top is smaller to allow for more expansion because it runs hotter than the skirt area.
So lets imagine a new bore and a new piston. The bore, of course should be square, that is the same diameter top to bottom. (?)
Lets say the clearance should be .0055"

Measuring clearance at the top and bottom would give different values because of the piston taper.
WHERE exactly should one measure when looking for the proper clearance?

If one shot for the proper clearance at the top of the bore, then the skirt would be tight if it fit at all. If one targeted the skirt, the top of the piston would have excess clearance, which I know it is supposed to have hence the larger diameter at the crown. As I am typing this, that seems to make sense. (I find that if I try and write out my questions as if to ask others then sometimes it comes together in my mind.)

Measure the skirt of the piston an inch up, and then an inch down into the bore of the cylinder and use those numbers to target the clearance?
I suppose that would give the desired clearance at the skirt and would show some excess at the top.

In the process of boring, I would say it would still be important to measure both areas of the cylinder to check for holding square...I am waiting for my upper area to catch up to the skirt area, there is still a greater measurement in the skirt area but i am assuming that is because the cylinder had worn that way. I am going a very little at a time and tracking the measurements so I can see the trend between the skirt area and the top cylinder measurements.

This is the first time I have attempted to bore a cylinder. It is one of those things that I have been burned on before sending them out and I have been doing some machine work at my new (4 years so far) job. I finished up a 40 hour class on precision measurement and alignment they sent me to a couple months back. My method is fairly crude--I am using a lisle hone. If the result is not perfect, I can live with that. As little at a time that this is going, I feel confident about not overshooting it, thats for sure.
I have access to a nice lathe and a bridgeport at work but for the amount of time I would fret over the process, it would be a little stressful to try and pull that off there. heheh... I cant afford a boring machine or even a used bridgeport. I have seen some vintage jig hole borers go by for sale, I am thinking I could probably modify one of those to work fairly well at some point.
Basically, i am experimenting here, knowing that my methods are not what they "should" be, but that is the story of my life, and I enjoy the process as much as the results or even in spite of them. I have done a whole lot in my life that the "Internet says cant be done". Of course I had my start turning a wrench about 38 years ago so there was no Youtube to run to to figure out why my SL70 wasn't getting 2nd gear.

Thanks for any input!
Author:  Retro 3 [ Sat Jan 08, 2022 5:22 pm ]
Post subject:  Re: Boring cylinders, clearances etc
Too much thinking.
1. Measure piston at the skirt (piston manufacturers usually say where) roughly 1/2” from bottom of skirt.
2. Add piston manufacturers recommendation on clearance.
3. Bore cylinder desired size. (Straight) measure top, center, bottom. All same measurements.
Author:  Jim [ Sat Jan 08, 2022 6:55 pm ]
Post subject:  Re: Boring cylinders, clearances etc
Wossner actually says to just bore to the nominal size because their pistons are very accurately made to provide their specified clearance if you do that. I'm not sure all machinists would like to do it that way though.
Author:  Retro 3 [ Sat Jan 08, 2022 7:06 pm ]
Post subject:  Re: Boring cylinders, clearances etc
Jim wrote:
Wossner actually says to just bore to the nominal size because their pistons are very accurately made to provide their specified clearance if you do that. I'm not sure all machinists would like to do it that way though.



Yes they (Wossner)do.
I would never bore without having measured the piston first.
You know how assumptions usually go.
Author:  Jim [ Sat Jan 08, 2022 9:44 pm ]
Post subject:  Re: Boring cylinders, clearances etc
Retro 3 wrote:
Yes they (Wossner)do.
I would never bore without having measured the piston first.
You know how assumptions usually go.

Yes, I agree. When I got three Wossner pistons for my H2 I measured them. I found that the three of them were the same. and sized as they say, within my measurement ability, which is a tenth or two. I took one piston to the machine shop with my three cylinders. The machinist was a little perturbed, but bored all three to match. I was milling slots in them for John's reed setup and hadn't finished all three yet, so I wanted to work on the others while the cylinders were being bored. Of course this was all being done with the Deals Gap meet only a week or two away.

I also got the usual "Should we give the center cylinder a little more clearance because it runs hotter?", to which I said "No thank you, please make them all the same."
Author:  Retro 3 [ Sat Jan 08, 2022 10:47 pm ]
Post subject:  Re: Boring cylinders, clearances etc
I was basically trying to explain to 2stroke that the piston manufacturers have already done all of the “thinking” that he was doing about the expansion qualities of their pistons.
No need to re-think, especially since he’s utilizing a pretty crude boring method.
Author:  DGA [ Sun Jan 09, 2022 12:09 pm ]
Post subject:  Re: Boring cylinders, clearances etc
When you say you are using a "Lisle" hone, i hope that isn't a large brake hone, the type that has non removable stones on center pivots. Only type of hone should be a Sunnen type. Two stroke cylinders wear the most just above the exhaust port, the lower bores hardly wear, below where the rings never go to. I shudder when you use the clearance number .0055", i hope you will go for the minimum clearance the piston manufacturer suggests, and no more. Wossner probably have less piston taper, the early triples had far too much piston taper top to bottom, so even with the lowest clearance, they would still rattle.
Author:  H2RTuner [ Sun Jan 09, 2022 9:55 pm ]
Post subject:  Re: Boring cylinders, clearances etc
Most "bore Gods" insist on clearancin pistons already giant enough to literally worn out, and refuse to do it right. I have never set up a piston to bore size bigger than .0015 inch, and have never seized a piston in an H, or H-R engine. Even the factory forged H-R pistons collapsed the bottom of their skirts in the first two laps, and cast pistons don't need bigger clearances. When I bear a top end so loose, then hear the owner proudly say it is a brand new bore job, it makes me want to go drill some common sense into the "bore God" that pulled that idiocy in the first place.
Author:  2stroke [ Mon Jan 10, 2022 5:27 pm ]
Post subject:  Re: Boring cylinders, clearances etc
Too much thinking is my calling card. I enjoy it though. Especially as I am learning something.
The thing is I have been burned by "experts" before. I sent a KX400 cylinder to an expert and got back something that had piston slap from the get go. Took a couple years to find that piston. I sent it to Swain and had the "thick" coating put on it, and took it down until it fit. . I can get a good season out of that, (had been doing that with the standard KX piston a couple times ) Bought a YZ465 from a friend, and it had just been redone and the piston was making noise, and it had been sent to the same guy. He had this notion about giving extra clearance for forged pistons. That is not always the case, especially depending on the silicon content of the alloy, who made it and when.
Maybe if I was flat tracking it or was Marty Moates, then I could keep the slug swollen enough to stay quiet...I am going to do the YZ cylinder next if this one works out. (This one is for an F9 frankenstein project I am working on.)

Crude method perhaps. I am tracking it the whole way, so have averaged .00245" per step, not that I can measure to 5 places, that is an average of 6 steps. The skirt area is now within .0005" of the top area, so that is coming around. I am in the .060 over neighborhood now.....I have a .080 over piston on hand that I had in reserve for my F9 racer. I looked briefly for a .060 piston but no luck. There is still a crappy area in the skirt anyway, so I think I am just going to go on up to fit the piston I have. I will change over to fine stones when I get close.

Thanks for the input!
Author:  Retro 3 [ Tue Jan 11, 2022 12:05 am ]
Post subject:  Re: Boring cylinders, clearances etc
You’re welcome :thumbup:
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