Top end

[Terrapista]

Hi. After more basic info. How do you decide on piston size for a rebuild? Like choosing between 1st oversize 0.25mm or second oversize 0.5mm. Also when is it time for a "sleeve ". And another... there is a process of honing the piston when is it time for that. Just to lessen decision time and know what to buy for what is required.
Is it the case where you have done one thing and its not fixed so the other is required because it happened with my clutch rebuild. If its too long just pointers to web sites will be appreciated.
Cheers

[pokey]

you need to actually measure your components and not with a ruler or tape measure. this is engineering we are considering.


 I sense you are fairly new to the mechanical side of things . one of the most informative places you should read is Dans

http://www.dansmc.com/mc_repaircourse.htm

lots of good info for the many questions you will have. read it and save up to buy good quality tools.

[TTezza]

Also found Ichiban Moto on YouTube has a lot of useful information. ?

[KTM47]

It all depends on where you are, what brand of bike and who you trust.

For myself as I'm just North of Brisbane I would take my barrel and old piston to Alan at the Serco workshop.  He would measure it and then workout whether, a new piston is enough with a hone or if it can be bored to the next oversize piston or not.  Serco sell and stock Woosner pistons.  They only go up in 0.20 thou increments.  OEM pistons go up in ).10 thou increments.

Most workshops will check sizes and give advice.

[Terrapista]

Yes very fresh to vintage or any dirt bikes. Thanks for the info. Have measuring calipers and all tools at the ready. Was asking for a leg up for this task as i would order stuff for clutch and ready to install then something else would be needed and the cycle would repeat far out mate took ages to finish! but satisfaction when completed.

[LWC82PE]

there is a process of honing the piston when is it time for that

You do not hone pistons. you hone barrels.

I highly recommend getting a  factory/oem and aftermarket (you can never have too many manuals for your bike, what one does not cover, the other will) workshop manual for your bike. They really are very helpful and answer most questions. Sadly these days many people can not be bothered to pay $30 for a hard copy manual or even $5 for a download PDF manual and just want a forum or Facebook to answer all their enquiries/questions and be the problem solver.

Most jap bikes 1970's or newer allow up to 2mm oversize using aftermarket pistons. After that it usually means you need a re-sleeve or new cylinder/barrel or replated bore.

[Terrapista]

Happy new year all.
Couldnt agree more about the manuals and an element of truth seeking a solution also.
However your comments have placed a process to this work in my head in addition to the measuring. Manuals and elbow grease will make, at minimum, piston kit replacement possible. Heck i will be able to talk to riders at the tracks to socialise a bit too! Admittedly I have shortcutted as all you guys have found out through time, money and effort back in the day for your craft. Im starting at 44 so respectfully grateful for the knowledge given.
Cheers
Leon

[sleepy]

With your cylinder I would recommend taking it into however you decide to do your rebore, the actual engineers that do it not just your local bike shop and get them to measure it up before you buy a piston kit. I have had quite a few cylinders turn up with an OS piston supplied by the owner only find the bore is to badly worn to clean up at that size. Some suppliers will swap to the next size but most end up with a piston that can't be returned.
I normally on a cylinder that is a bit questionable bore it to the next size allowing the .003" honing allowance to see if it cleans up before ordering a piston.

[80-85 husky]

pistons are designed to wear out before the bore but if the rings are not serviced regularily the piston skirt can wear a "slap groove" at the lower end of the bore and the rings can wear a lip into the very top of the bore. once this wears out, a new same size slug (piston) will have too much clearance and will wear out prematurely as well as being down on power, requiring the bore  to be opened up (rebore) to take the next oversize slug so the engine is all back in spec. (correct tolerances). if there has been a catastrophic event...airfilter fail or the worn rings/ piston fail and gouge the bore, it may take two or more oversize machinings to clean the gouges out of the bore. if the bore is too badly damaged, the liner can be pressed out and a new liner pressed in.

so take the piston and the cylinder to the machine shop and get a measure done. determine (don't purchase) what oversize pistons are available. get the shop to bore to the  next oversize and check the bore is now serviceable. if not they will needto machine it out to the next "next" oversize then you will know which piston to purchase. once the piston arrives, the shop will then hone (very fine rebore) out the cyl to give the correct piston clearance and finish the bore with crosshatch marks for better lubrication.

a tip.. the piston ring grooves have pins that locate the end of the rings...make sure you get that all lined up before slipping it back into the cylinder. its to stop the ring ends moving around to the exhaust or inlet port and then being grabbed and torn out...very exciting I did it once

[Terrapista]

And there you have it..pokey and ktm's post did not register until the last two posts were up. Wow it is definitely an engineering thing without proper tools and machinery. Thanks for the insight everyone.

[pokey]

Thought I heard a penny drop.

Try and slide the piston all way thru the bore and it should stick on the skirt at the top. The bore is not parallel and you will probably find its not even round. It needs to be measured at three heights and again at 90 degrees so that’s six measurements just to get that far.  What size it needs to be is also dependant on the type of piston being used as a forged piston needs a bit more and for that info you need to check with the piston manufacturer..
 The absolute best things you can do is start reading and reading and then read some more. Is always something to learn but also watch someone who actually knows as they will teach even more like how to tell the top and bottom of a ring and how much end gap on the rings ..

For high revving engines you really do need to be spot on with the specs or its not going to last and end up costing you more time and money and will end up putting you off learning. Good luck with it as it is kind of fun when you get it right.

[Momus]

Terrapista, if you are working on a 2 stroke the ports need to be chamfered after reboring. This eases the piston rings back into position after they have sprung slightly as they pass the port, particularly the exhaust port, and is vital for ring life and helps performance.

At some places this reasonably time consuming finishing is not considered part of the job and is neglected. Read up on it and make sure it is done or that you are able to do it.