Two stroke piston showdown.

[dbf498]

Ya' know, there are probably as many if not more procedures to break in a new piston as there are choices for pistons. A process I've used in the past on full rebuilds is a series of three hot-cold cycles. What that means is run the engine for about 10 minutes, idle no load, shut it down then let it cool down for for 20 minutes to complete one cycle, then repeat the process again two more times for a total of three times. For the break in process use the same gas/oil mix you ride with. After the hot/cold cycles, I'd run it for about an hour under a light load varying the speed avoiding WOT; no more than 2/3 throttle. Once you've completed the hot/cold cycle and the light one hour ride you should be good to go.  Even though the method I describe is one I use for a complete engine rebuild it should also work for breaking in a top end job as well.

One thing to keep in mind is the importance of a good warm up before hammering it. Usually a lap on the track or anywhere from 2 to 3 minutes on the trail to allow the piston to come up to temp should be all you need before you start getting into it. I try to be a little on the conservative side and so far the method I described has done me well. Once you get your bike back together and broke in I'm curious to hear your thoughts on the Wossner piston.

[2T Institute]

You won't have a break in with a piston and bore the same size you will be lucky to get the piston in the bore. "Size for Size never fits" is the old machinists axiom. There is no piston to bore clearence. An "A" size piston will be a 53.94 or 95mm you should be trying one of those first.
Can you post a pic of what you measured the cylinder with? Because 53.96mm is a mighty odd cylinder size to plate to. Measuring cylinders is a precise art where 0.01mm makes a big difference.
A quick and easy way to gauge piston clearence is to place a feeler gauge the same thickness as the stated clearence. If you need to push the piston with more then 2 fingers to pass it through the bore there is not enough clearence.

[Jeram]

most confusing thing that I find is that vertex and wosner sell pistons based on the actual piston size, whereas Wiseco and other  manufacturers sell the piston based on the bore size.

some time ago, it made my first piston purchase a little confusing.

[gpnewhouse7]

I've always been taught to use OEM only in my engines as they are what the engines were designed for so if you can't get Honda one's then just try and get as close as possible to the standard piston.

[TotalNZ]

Ya' know, there are probably as many if not more procedures to break in a new piston as there are choices for pistons. A process I've used in the past on full rebuilds is a series of three hot-cold cycles. What that means is run the engine for about 10 minutes, idle no load, shut it down then let it cool down for for 20 minutes to complete one cycle, then repeat the process again two more times for a total of three times. For the break in process use the same gas/oil mix you ride with. After the hot/cold cycles, I'd run it for about an hour under a light load varying the speed avoiding WOT; no more than 2/3 throttle. Once you've completed the hot/cold cycle and the light one hour ride you should be good to go.  Even though the method I describe is one I use for a complete engine rebuild it should also work for breaking in a top end job as well.

One thing to keep in mind is the importance of a good warm up before hammering it. Usually a lap on the track or anywhere from 2 to 3 minutes on the trail to allow the piston to come up to temp should be all you need before you start getting into it. I try to be a little on the conservative side and so far the method I described has done me well. Once you get your bike back together and broke in I'm curious to hear your thoughts on the Wossner piston.

In my opinion that method as described is a good way to glass your bore and get alot of blowby. That 10mins idling from startup won't bed your rings in.
I've never understood the need for heat cycling and noones every givin a reason for me to do it.
I start the motor and DON'T let it idle at all, give it small blips on the throttle till it's up to temp then ride it with light load to around 1/4 maximum rpm. I usually short shift it while dragging the rear brake. You don't want to lug it or over rev it but it's real important to get load on straight away. I'll do that for maybe 5 mins then stop and give it a good look over for fluid leaks and whatnot. Then back on and run it up to 1/2 max rpm (till it's just about to come on the pipe ) with bit more load. After that i kane the F**K out of it for another 25 to 30 hours. no dramas ever doing this and also no blowby. Ring seal is crucial and you won't get that glazing your bore idling for 10 mins

[2T Institute]

I've always been taught to use OEM only in my engines as they are what the engines were designed for so if you can't get Honda one's then just try and get as close as possible to the standard piston.

Hate to be the bearer of bad news but there are only a few piston manufacturers in the world. The Dover group own Wiseco, Vertex and CP , in Japan ART make most OEM pistons ProX is ART's retail brand. Asso Werke , Elko and Wossner are still independantly owned (I think ...............not sure)

Yes that be a good run in method I do 3 X 5 mins 1/3,2/3 then go for the doctor throttle, at the track it's 2 slow laps tootle back to the pit and then go for the doctor.

[dbf498]

Here are three methods. One from Eric Gorr, two from Clymer (1997-2001 CR250 and 1981 to 1988 CR125). TM has their own run in (break in) procedure at http://www.tmukonline.net/Page/133/faqs.html along with engine starup up procedures http://www.tmukonline.net/Page/131/engine-starting.html.

As far as glazing a cylinder I haven't had one glazed yet and I've used the method Eric Gorr states in his book for many years. One might say I've just been lucky, maybe, but ya' know, if it works why change unless there's irrefutable evidence to the contrary.

http://i1205.photobucket.com/albums/bb431/dbf498/messagepartguidmessagepart3AINBOX2F.jpg (Eric Gorr, Motocross & Off-Road Performance Handbook (third edition pg 129))
http://i1205.photobucket.com/albums/bb431/dbf498/messagepartguidmessagepart3AINBOX2F-1.jpg (Clymer, 1997 to 2001 Honda CR250)
http://i1205.photobucket.com/albums/bb431/dbf498/messagepartguidmessagepart3AINBOX2F-2.jpg (Clymer, 1981 to 1988 Honda CR125)

[SachsGS]

Back when Husqvarna was still a Swede they had very comprehensive manuals and in the manual detailed information was provided about breaking them in. Husqvarna recommended 4 heat cycles and I think this is where this procedure came from.

[TotalNZ]

Heat cycling goes way back but can anyone actually explain the need or benefit for doing it?
I belive you need load on straight away to get the rings to bed properly. As you'll know gas from the combustion process forces the rings out against the bore, if you don't get that light pressure on straight away you risk excessive blowby i reckon.
I can't think of anything worse than letting my motor idle for 10 mins with a brand new topend in it.

[dbf498]

Not allowing a proper warm up isn't good either. If not allowed to warm up properly the piston will come up to temp faster than the cylinder and if not allowed to equalize could potentially cause a cold seizure (four corner seizure).

[2T Institute]

Not allowing a proper warm up isn't good either. If not allowed to warm up properly the piston will come up to temp faster than the cylinder and if not allowed to equalize could potentially cause a cold seizure (four corner seizure).

Warm up and break-in are 2 different things.

[dbf498]

2T Institute you are correct,  warming up and break in are two different things. However, I was responding to what TotalNZ stated that he couldn't think of what was worse than letting an engine sit idling for 10 minutes with a new top end; I gave my opinion of what I thought was worse.

When Stusmoke asked about what break in process to use I gave him my opinion and caveated that with there are many opinions as to what the best break in practice is. I later provided evidence from published sources, so, take them for what their worth. Until someone can provide any evidence, not an opinion, to prove I'm doing something blatantly wrong or catastrophic to my engines I'm going to continue with the process I've been using for many years.

Here are two additional break in procedures; one from L.A. Sleeve and the other is from Trinity Racing...take these for what their worth as well.

http://www.trinityracing.com/files/Engine-Break-In.pdf - Trinity Racing although an ATV company they build some killer Banshees.

http://www.lasleeve.com/downloads/2-stroke-break-in-procedures.pdf - L.A.Sleeve - really no need for an introduction.

I've provided at least six different methods of breaking in an engine...each different...are they wrong...no...are they right...it's a matter of opinion.

[gpnewhouse7]

I've always been taught to use OEM only in my engines as they are what the engines were designed for so if you can't get Honda one's then just try and get as close as possible to the standard piston.

Hate to be the bearer of bad news but there are only a few piston manufacturers in the world. The Dover group own Wiseco, Vertex and CP , in Japan ART make most OEM pistons ProX is ART's retail brand. Asso Werke , Elko and Wossner are still independantly owned (I think ...............not sure)

Yes that be a good run in method I do 3 X 5 mins 1/3,2/3 then go for the doctor throttle, at the track it's 2 slow laps tootle back to the pit and then go for the doctor.

Thats not really bad news to be honest, but as far as me using OEM pistons and basically all OEM engine parts that is what my mechanic (works for a very reputable local dealer and is a good friend aswell as an absolute genius with the spanners) has always told me, he's always said if you can afford it buy OEM or for cheaper stuff buy Pro X but never us high compression pistons unless you've already upgraded the crank and gearbox so as to avoid putting too much extra strain on parts that weren't made to take it. (He races LTR450's with fully tuned RMZ engines in them in the British championship so he knows what he's talking about).

[Jeram]

the only advantage that cast pistons (including OEM) have is that they expand less under use and are softer so if they seized there is less of a chance of them taking the bore with them. doesnt matter whether OEM of Prox, they do the same job at this

Forged aftermarket pistons are lighter (than OEM), harder and stronger which makes them more resiliant during times of adverse conditions (hot/lean/detonation). Less chance of melting a piston. Unfortunately the forged pistons expand more than the cast piston so need larger bore clearances. And due to being harder and stronger if the motor seizes there is a greater probability of the forged piston damaging the bore.

For me, I need high temp resistance for long preiods (SM/road racing) so I always run forged. If I was doing MX, Id perhaps consider the cast pistons

[Stusmoke]

Holy crap guys... I don't look at this for a few days and you drop knowledge bombs. Thanks and thanks again this is all extremely helpful.

You won't have a break in with a piston and bore the same size you will be lucky to get the piston in the bore. "Size for Size never fits" is the old machinists axiom. There is no piston to bore clearence. An "A" size piston will be a 53.94 or 95mm you should be trying one of those first.
Can you post a pic of what you measured the cylinder with? Because 53.96mm is a mighty odd cylinder size to plate to. Measuring cylinders is a precise art where 0.01mm makes a big difference.
A quick and easy way to gauge piston clearence is to place a feeler gauge the same thickness as the stated clearence. If you need to push the piston with more then 2 fingers to pass it through the bore there is not enough clearence.

I can't work it out hey. My measurements said 53.96 and my book says 53.96 its just plain weird. However it also says the service limit is 54.006 mm. Anyway I have doubts that Honda manufactured a cylinder in which no aftermarket piston would fit in so I've just ordered the 53.96 mil piston. The piston that is in there at the moment had .251 mil clearance to the cylinder. But theres really not much carbon build up on the piston it looks relatively new. I also emailed Wossner pistons and they also said that the stock bore size is 54. So I don't know whether theres a misprint or something but I'm dreadfully confused and fairly confident I've made a mistake somewhere. I've ordered in the 53.96 and will be paying for it today.

Thanks everyone I really appreciate the help. And thanks for the break in procedure advice too.