Author Topic: Refresh motorcycle cylinders  (Read 1099 times)

Offline AlexS

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Refresh motorcycle cylinders
« on: January 20, 2018, 01:00:26 PM »
I have a couple of Rd250 (2t) cylinders that needed to refresh. Cylinder diameter is +- 54 mm.

Does someone have experience with boring of motorcycle cylinders by your self?

For my homemade model I have turned them into the lathe. But I had to refine the surface with sandpaper for a better roughness, and then to be honored. For a model engine I think it is ok, but for a motorcycle two stroke? I know it is also a matter of experience, but maybe have someone here some hint and tricks?

I can take them away. But I thought I have a lathe and milling machine, so why not doing it at yourself?

Thanks in advance!




Offline stevehuckss396

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Re: Refresh motorcycle cylinders
« Reply #1 on: January 20, 2018, 02:16:38 PM »
Depending on how much material needs to be removed, you may only want to hone them. If they are badly out of round or many thousands need to be removed, then boring would be the best option.

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Offline Vixen

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Re: Refresh motorcycle cylinders
« Reply #2 on: January 20, 2018, 04:14:22 PM »
Alex,  I would find out what oversize pistons and rings are available first.

Then you will know how much material will need to be removed. Probably a cylinder hone will be enough.

Mike
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Offline wirralcnc

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Re: Refresh motorcycle cylinders
« Reply #3 on: January 20, 2018, 04:40:21 PM »
I have bored and honed many motorcycle barrels. If your going oversize pistons you will need to bore and hone. If the cylinders are good condition and correct size then a quick hone to clean and reinstate the cross hatching is enough, then fit new rings/ rings and pistons depending on condition. Remember to deburr the edges of all ports before fitting. If your going down the route of new pistons, I would purchase first so you can measure to give correct clearance. Then the all important running in. ;D

Offline gbritnell

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Re: Refresh motorcycle cylinders
« Reply #4 on: January 20, 2018, 04:52:33 PM »
Unless you can gaurantee that your machines will bore a concentric and straight hole I wouldn't even consider it.
Having worked in a motorcycle shop I have used a true cylinder boring machine which is extremely rigidly made for that specific purpose.
The cylinders need to be set up on the mounting face to get the bore square which requires using a face plate with clamps. Most people don't have a face plate large enough for this type of setup.
gbritnell
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Offline Admiral_dk

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Re: Refresh motorcycle cylinders
« Reply #5 on: January 20, 2018, 06:58:52 PM »
I had a few done over the years and there where quite a number of places that did this in my youth and really many skilled machinist, there where only two persons that any performance / reliability conscious biker would use here and now there's only one left - and he is not far from retirement. There are also only two places left in this part of Denmark that has the equipment to do it at high quality and the really funny thing is that the best by far is the local Harley shop and they never had anyone there who new how to operate the gear, so they always persuaded the last one from the competition to come to them once or twice a year and do a bunch  ;D

There's only a few um. - micrometer, microns etc. - between a great runner and a bad one - especially the old air-cooled Yamaha RD's are sensitive - then again, the new hard chromed requires even much smaller tolerances.

My nephews Yamaha YZR80 has piston sizes for every 4um. - so first size is 47.460mm, next up is 47.464mm, then 47.468mm and so forth.

Offline AlexS

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Re: Refresh motorcycle cylinders
« Reply #6 on: January 22, 2018, 09:26:35 PM »
Thank you all for your advice and info.

Because it are cylinders of a two stroke and so the tolerance must be more accurate. So I had taken them to a specialist.
Fortunately, there are here a couple of people that could do the job. Given the wear and tear, the owner of the shop has advised that the cylinders only need to be honed. Lucky me, the new pistons need to be the first excess.

Alex