Have seen a few piston issues lately. Of course proper breakin including AFR are important but first you have to give the assembly a good chance by properly checking assembly. So here are some tips. Feel free to add more here since there are many.
SLEEVE
Fit of piston:
Measure fit. If you don't have bore guage take it to a shop or a feeler guage will work. These sleeves are machined in a fixture so it won't be perfectly true or round but go by manual and follow the specs in it or provided by manufacturer. Here is a quick check. Stock piston and stock sleeve. Piston will fall right through but if you turn piston it will not fall through. I won't even bother measuring since it's no good and very good chance skirts of piston will sieze to liner. Maybe not permanently. But piston and liner will be scored causing loss of compression and performance. Measure you ring gap always.
In this pic stock piston and stock sleeve piston will not even fall through. I am holding in the air. Not good. Sleeve can be fitted but would require Hone.
[attachment=7328:Piston_s...tips_008.jpg]
Reason this happened is because some sleeves aren't machined properly. LOOK FOR CHAMFERS. Every corner on every surface should have a radius or chamfer if it doesn't pay attention it is THE BEST indicator of improper machining. If no chamfer or too small in corner indicated you MUST put one there. Grab a file and five minutes your done. Does not need to be as big as the other stock chamfer but won't hurt if big like that. It Must be bigger than the case chamfer. Can also cause case cracking etc.
[attachment=7329:Chamfer_Comparo.jpg]
The Dimple
If too deep sleeve can crack. Just tip of drill or none at all is best
[attachment=7330:Dimple.jpg]
Piston and Sleeve Tips
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cannondale27
- Posts: 0
- Joined: Wed Mar 10, 2021 10:41 pm
PISTON
Get a good one. ASSO or Timbo CP don't even waste time with anything else. You have alot of effort in building a motor. Just go with what works and lasts.
When you get one mark it as shown. I guarantee if in past you built other brand motors you will wonder if you put the piston in backwards. This allows you to look in plug hole and confirm it's in the right way. Industry standard is arrow towards exhaust. Do this immediately when you recieve piston so nothing is taken to chance.
[attachment=7331:Piston_s...tips_001.jpg]
[attachment=7356:Cam_Timi...tips_011.jpg]
Get a good one. ASSO or Timbo CP don't even waste time with anything else. You have alot of effort in building a motor. Just go with what works and lasts.
When you get one mark it as shown. I guarantee if in past you built other brand motors you will wonder if you put the piston in backwards. This allows you to look in plug hole and confirm it's in the right way. Industry standard is arrow towards exhaust. Do this immediately when you recieve piston so nothing is taken to chance.
[attachment=7331:Piston_s...tips_001.jpg]
[attachment=7356:Cam_Timi...tips_011.jpg]
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cannondale27
- Posts: 0
- Joined: Wed Mar 10, 2021 10:41 pm
THE CASE
Yes the case is also critical to piston and sleeve. Remember the chamfers. They must be on every corner if not stop, think why isn't it there? What will happen? Fix it. Here is two cases. I would run the one with crappy big chamfer but this can cause case cracking since sleeve will not fit all the way down unless chamfered (file) to clear. Huge pressure from torquing head will also distort sleeve if it's not properly seated.
Sleeve must fit ALL the way down.
No nicks, No silicone. The two aluminum surfaces MUST meet each other here and at head/case junction.
Also the rough chamfer is going to be a stress area where a crack could easily start. As a machinist the tip to look for chamfers has saved my butt so many times I can't count. It takes time to measure everything but only a second to look for chamfers and radiuses. Almost all machining processes where two surfaces meet a chamfer or radius is used. Lack of them indicates something wrong in the machining process. Visual check can save your butt.
[attachment=7332:Bad_Case_Chamfer.jpg]
[attachment=7333:Good_Case_Chamfer.jpg]
Yes the case is also critical to piston and sleeve. Remember the chamfers. They must be on every corner if not stop, think why isn't it there? What will happen? Fix it. Here is two cases. I would run the one with crappy big chamfer but this can cause case cracking since sleeve will not fit all the way down unless chamfered (file) to clear. Huge pressure from torquing head will also distort sleeve if it's not properly seated.
Sleeve must fit ALL the way down.
No nicks, No silicone. The two aluminum surfaces MUST meet each other here and at head/case junction.
Also the rough chamfer is going to be a stress area where a crack could easily start. As a machinist the tip to look for chamfers has saved my butt so many times I can't count. It takes time to measure everything but only a second to look for chamfers and radiuses. Almost all machining processes where two surfaces meet a chamfer or radius is used. Lack of them indicates something wrong in the machining process. Visual check can save your butt.
[attachment=7332:Bad_Case_Chamfer.jpg]
[attachment=7333:Good_Case_Chamfer.jpg]
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desertbound
- Posts: 0
- Joined: Wed Mar 10, 2021 10:41 pm
Wow! Someone else actually does teh arrow on the piston thing. Everyone thought I was crazy for doing that. I even did it on the 5 valve yfz pistons.
I'd say a major thing that most overlook is gapping the rings. I know cp always claims that their rings are pre gapped, but I never trust it. It takes 2 minutes, and is very easily done. A proper ring gap will produce more power. Also, having the gaps in the rings placed in the proper position on the piston is important also.
I'd say a major thing that most overlook is gapping the rings. I know cp always claims that their rings are pre gapped, but I never trust it. It takes 2 minutes, and is very easily done. A proper ring gap will produce more power. Also, having the gaps in the rings placed in the proper position on the piston is important also.
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cannondale27
- Posts: 0
- Joined: Wed Mar 10, 2021 10:41 pm
Heck yes.Nothing worse than having to wonder.Would be different if a guy could do whole motor at once so you could remember exactly what you did.Other one is that cylinder o-ring.I always am paranoid trying to remember if I put it in.Good practice is to put all o-rings and such your changing in a box.Throw ones you remove in another box far away from bench.When good box is empty you know you did it.Worst thing is to find a part on bench or floor and then wonder.We have all been there.No Pocket parts 
Of all CP's I have done only one was pregapped.
Of all CP's I have done only one was pregapped.-
speedracer
- Posts: 0
- Joined: Wed Mar 10, 2021 10:41 pm
QUOTE (Trouble @ Feb 27 2009, 07:40 PM) <{POST_SNAPBACK}>
a piston isnt perfectly round,a lot of people dont know this.
It seems sleeves can be not round also.