780 big bore milage between rebuilds?

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snowprophet1

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May 2, 2003
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Waterford, Pa.
I got 1 1/2 months of riding on my 780 bb setup (a little over 3,000miles). Great gas milage, and power was raised. The last day of my trip up north, it sounded clattery, idled like crap, and started hard when warm. When we got back, sure enough compression went from 127ish psi down to 110, 70, and 80. I tore it down last night. The piston coatings are like brand new still. Temps on gauges were always running the plugs a beatiful safe brown. Water temp ran usually 160*F, but saw 180*F for about 5 minutes of riding. (regular temp light never came on). The cylinders are still in good shape, but the skirts of the pistons shrunk .005"! The second rings down were all stuck, the top rings were all slopping up and down around the exhaust area, (with one ring even being cracked!), and the slop was because the top of the piston near the main exh were actually starting to get raised! No melting. No seizing. Piston skirts were in nice shape, but like I said, they shrunk. I ran a piston to cylinder clearance of .004", and had the piston tops ceramic coated and the skirts w/the friction coating. Is this an every year thing with the big bore kits, or do most people get years out of the same rings? Any stories on longevity? I just want to know what to expect next year. :cry:
 

As accurately as I can possibly measure it, the openings are (#1) 2.020", (#2) 2.020", and (#3) 2.040" wide, and have a good radius to close on the ring instead of a slamming flat to snag it on. Chamfers look good (not too small). The bore is 2.877", so 70% that would be 2.014"? I guess technically they are more than 70%, but extremely close. The #1 piston had the least distortion (but still junk), #2 had the most distortion, and #3 (the widest span) was less damage than #2, but more than #1. Taking pictures of the tops of the pistons w/my camera are too hard to see the distortion, but the ring slop from it is easily noticable. When the pistons are at bottom dead center, #1 is about flush w/the bottom of the port opening, #2 and #3 are actually higher, because of too much material actually taken out by accident (real men admit their mistakes occasionally). I ordered a pound of devcon today to change the flow on a couple intake tracts, but I don't know if I could use it on the exhaust side. If I built up the main exh port floors to keep the heat from (possibly) concentrating on the piston edge, maybe that could help, but it doesn't seem that the pistons necessarily melted (egts were safe) but at the same time, the port openings don't seem to try and catch any rings either. After 1,000 miles I tore it down, and everything was perfect. If it were a porting profile causing the distortion, I think it would have done it by then. Wiseco rep says that I got a good amount of milage out of those pistons and that I should be happy...it may just be the nature of the beast using pistons every season. I'd rather find a cause and correct it. I'm open for ideas or theorys. :roll:
 
Piston skirts will always collaspe when they get to hot. Was there carbon track under the piston crown. With the intial .004" clearance combined with the piston skirt collaspe of .005' and running 70% exhaust width to bore diameter thats not at all surprising that these rings simply wore out. Piston rocking in the bore combined with a "huge" exhaust port. If I was you I would be extremly happy with this lasting 3000 miles. I wouldnt do the devcon in the exhaust port. I would weld it or leave it alone. Do you have a iron liner??? or plated aluminum bore? Did you do the big bore yourself?
 
By saying "carbon tracking" under the piston crown, if you mean that the underside of the piston crown turning brown and having black crap coating it like an old oven, then yes. The rings are actually not worn at all near the exhaust port or anywhere else, except for one ring actually broke but didn't cause damage. The bore is nikasil like stock, and I did all the work myself except for the piston coatings and the nikasil. From what you said, I'm coming to the conclusion that the piston rocking from the shrunk piston skirts (having a large exh opening) probably let the rings sort of catch in the main exh ports, thus jamming the second rings, and lifiting the top ones to a sloppy fit. I'll bet if I tried to drive it one more day it may have had a catastrophic failure! I think I got lucky! The motor never saw much more than 180*F, but I may try and keep that from happening again in the future. I'll think of something this summer. I don't know if the "cool heads" would make a difference, but I may look into getting the water passages lower into cylinders to keep the piston skirts cooler. Thanks!
 
big bore

It's not your water temp that shrunk your pistons it's the exaust temp
you where probebly running to lean
 
I ran the sled for a very long steady speed at 1200*F on the egt's (minutes and miles) and hit the kill switch and pulled to the side and checked the plugs. They were a very nice safe brown color. The egts were rarely above the 1200*F during the trip for more than a second or two (I have lights that yell at me if I do). Before my trip, I had it at an airstrip, where I leaned it out step by step and ran the heck out of it and kept checking plugs, and the color was still safe. When I finally ran it up to 1400* the last run, the plugs were finally running light, and I definately wouldn't run it on a lake or anything like that. Of course I richened back up after that just to be safe. A week later I tore everything down to look at it, and the pistons, cylinders etc were all still in the same condition as when I put it together. (Checked the cylinders w/dial bore gauge, and mic'ed the pistons). The skirts of the pistons measured exactly what they did when I put them in originally. On my trip, is the only time the motor (water) temp got over the mid 160*sF. I'm not saying that running lean and heat soaking the pistons couldn't do that, but I really don't think I got the pistons got that hot from being lean. The color and piston wash looked excellent. I guess the pistons could have been heat soaked though from being run constant miles upon miles at about 80mph though, but the egts said the mixture was safe. I guess it's a possibility. I never could run that fast that long here at home!
 
big bore

what kind of pistons?did you have were they wiseco?
 
big bore

Thats why wiseco pistons are only good for bought 3000 miles.
 
Back in 1999 I bought a complete 780 srx from Maxximum Performance. It could be guaranteed that at approximately 600 miles of running the motor would go down. Same problem as you had piston skirt shrinking causing catastrophic failure. After four rebuilds trying Wiseco, Arctic and lord knows what other kind of pistons Maxximum Performance and I gave up on it and put it back to stock bore, stock piston, ported motor. I have ran this motor for 8000 miles with 0 problems. Sense then Wiseco has built a better piston with a heavier skirt that is suppose to solve the problem. I aways loved the way the 780 ran when it was right but could not live with the reliability problem. When you ride 2000 miles in most years I don't want something that has to be rebuilt every season. All that being said I was a ready to build another 780 from CPR until I read your post. CPR uses the same piston as you did in your kit. They do weld up the webbing in the bottom of the cylinders for more strength which is suppose to help. My main consideration is a heat soaked motor shrinking the skirts causing failure. I have a M-10 in this SRX so when there is not a lot of snow for the track to pick up my water temps run as high as 187 on my Avenger. Are the 780's realiable for over 4000 miles or is that just a dream? Please give me any input on this matter because I would like to build one more 780 before I get a RX1.
Thanks
Sleeper
 
I debated on coating the pistons again since they didn't last one season (the coatings did though!), and CPR said they don't coat theirs for their kits. Since I ran a tighter tolerance than most, I guess I'll give it one more shot w/the coatings, and I will try and improve the cooling for next season. Around here, you don't maintain such a high speed for any length of time compared to what I did on my vacatoin in Cochrane Ont. I never had any shrinkage (ha, ha...shrinkage), in the first 1,000 miles when torn down, and the egts were much higher before that teardown than after, so I'll see if I can get more milage out of 'er next season(s!).
 
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