big bore wiseco pistons"WTF"

[Nutwood 472]

Hey everybody,well went rideing the other day for about 15 min,tore up a couple downed corn fields and cruzed up the road,idled through town"small town" and motor begins loseing some power so we pulled into a buddys yard and it dies and takes about 10 kicks to start,so we toe it back to the shop,I'm running a 472 super cub,with 140 compression,35pwk airstryers 52 pilots and 220 mains on 93 octan,noss cool head and custom cut domes and big bore cpi's,rest pf mods are in the sig,so do compression test"95 on right side and 130 on left side"the tester i used didnt have the spark plug extension in it,so comp is a little off,but everything is brand new,4to5 tanks of gas ran through it already,so took the head off and the right side piston lip above the top exhuast ring broke off inbetween the main exhuast port,it was a little smaller than the exhuast and blew it out,did not hurt the cylinder walls at all"thank God'and the left side started doing the same thing in the same spot,the left piston started bending up the top lip right in the middle of the exhuast port above the top ring the squish was around 46 or 47 cant remember exactly,and the ring gap I set at 18 and the pistons and plugs didnt show signs of overheating at all'the jetting was real rich and pistons were blackened on top and pugs were black, So why are the pistons doing this carp and wondered if anybody had the same problems I'm haveing?

[matt1]

Sounds like there might not be enough chamfer on the ports and caused rings to catch.

[Larry's Shee]

If I read this right it broke between upper and lower ring at exhaust port? Was port properly champhered? Beat me to it I type slow ,lol

Edited September 26, 2010 by Larry's Shee

[bigborebanshee]

I would say the chamfer was not enough,the piston clearance was excessive or the engine is being run hard before enough expansion can take place.

 

Some exhaust ports widths are close to being too big,as a result a sloppy or unexpanded piston can cock enough to cause the rings to snag the port

[Nutwood 472]

If I read this right it broke between upper and lower ring at exhaust port? Was port properly champhered? Beat me to it I type slow ,lol

 

 

the piston broke above the top ring

[mailman]

I know exactly what you are referring to. Port chamfer is definitely a possible cause. Although I have seen this issue first hand even though the port chamfers were excellent as well as the port shape. An overly square vs. oval port can cause that also. Other possible causes could be excessive heat and or rpm. How much rpm does the motor turn? The acceleration forces of the rings get exponentially greater when the rpms are high. There is a huge difference between say....turning 9K rpm vs. turning 11k rpm for a trail bike. This combined with too much heat and/or a poorly designed piston could all be possible causes.

Sorry I don't have any definite answer but can only offer suggestions. I know the cost of those pistons can require a 2nd mortgage on the home. Take a look at the underside of the pistons for any charring. That can be a sign of too much heat.

 

Where did you get the domes? A poorly designed dome can cause detonation and some strange symptoms.

 

Mailman

[Nutwood 472]

I would say the chamfer was not enough,the piston clearance was excessive or the engine is being run hard before enough expansion can take place.

 

Some exhaust ports widths are close to being too big,as a result a sloppy or unexpanded piston can cock enough to cause the rings to snag the port

 

 

well I let my engine warm up every time I ride,so it does'nt cold sieze and its a long rod crank,and the porting was done by M&M and the clylider coating was done by milenium"miss spelled"and the pistons come from cp industries,so I cant see where the craftmanship would of been a problem'but I'm not a machinist

[Nutwood 472]

I know exactly what you are referring to. Port chamfer is definitely a possible cause. Although I have seen this issue first hand even though the port chamfers were excellent as well as the port shape. An overly square vs. oval port can cause that also. Other possible causes could be excessive heat and or rpm. How much rpm does the motor turn? The acceleration forces of the rings get exponentially greater when the rpms are high. There is a huge difference between say....turning 9K rpm vs. turning 11k rpm for a trail bike. This combined with too much heat and/or a poorly designed piston could all be possible causes.

Sorry I don't have any definite answer but can only offer suggestions. I know the cost of those pistons can require a 2nd mortgage on the home. Take a look at the underside of the pistons for any charring. That can be a sign of too much heat.

 

Where did you get the domes? A poorly designed dome can cause detonation and some strange symptoms.

 

Mailman

 

domes were cut by M&M ATV and every thing else was bought there and ported by M&M ATV I just put it together and tuned it,this is so frustrating,and your a bout the price of pistons,350.00 a set,thanks for the info Mailman

[Forcefed]

domes were cut by M&M ATV and every thing else was bought there and ported by M&M ATV I just put it together and tuned it,this is so frustrating,and your a bout the price of pistons,350.00 a set,thanks for the info Mailman

 

Another thing to consider

 

- when I see this damage ... it's usually from detonation.

 

What does the curves in your Dyna look/shaped like ? Timing ?

 

When the rings were gapped, did the tips get *45'd inner & outer. A burr can cause the ring to stick (amongst other things) & comprimise the seal at higher rpm's.

 

Ex duration ?

 

140 kicking comp with a ported cylinder on pump gas can be a factor... as well as MSV - measure again & confirm.

[Nutwood 472]

Another thing to consider

 

- when I see this damage ... it's usually from detonation.

 

What does the curves in your Dyna look/shaped like ? Timing ?

 

When the rings were gapped, did the tips get *45'd inner & outer. A burr can cause the ring to stick (amongst other things) & comprimise the seal at higher rpm's.

 

Ex duration ?

 

140 kicking comp with a ported cylinder on pump gas can be a factor... as well as MSV - measure again & confirm.

 

M&M programed the Dyna,so I dont know what it looks like,and yes I tapperd the ring gap and the inside of top of piston look brownish and the top ring looks like it was beating the top lip up and the bottom lip down squeezing the bottom ring,they told me on the set up I wouldnt have to gap rings and they were at 14 so I gapped them to 18 for being ported and didnt want to take a chance on a ring hanging up and i run big bore pipes cpi and run an pro design oversized radiator and high flow pump and never over heated or srayed out of over flow and plugs didnt show any signs of getting hot.it was ported for agresive trail and a little drag so I wuoldnt think they were fully ported,ported to specs is what it said on invoice and thepistons are 10018m 72.00mm wiseco 4805f is another number on piston top.cant measure squish took it apart already.

dont know exhaust duration M&M did the porting,so dont if they widen it or not.but its a super cub so duration for exhuast should be on cp industries web site,197 I think,but I dont know

[jbooker82]

I thought the cub was drag ported out of the box

The whole reson for the creation of the Severl cylinder.

[Larry's Shee]

Talk to M&M about it, it's their beast.

[Nutwood 472]

Talk to M&M about it, it's their beast.

 

 

yeah talked to Andy today and going to send him pics of the pistons,but i think I'm just going to ship them to him so he can get a good look at them,he's thinking its getting to much timing in the high rpms and could be having detination problems,but he needs to look at the pistons to see how they are breaking apart on the exhaust side,I'm sure he'll get it straighten out. thanks for the info guys

[Larry's Shee]

Let us know what you find out.

[RadarRacing]

Send Andy your dome too or both of them and mark which one was the broken side.