2012 RMK cylinder scoring

[Ethan Paulson]

Two summers ago I decided i was going to do some engine upgrades to my 2012 rmk assault that has 3500 miles on it figuring it was due for a new top end to get some power back. I went will all bikeman parts such as their billet head, fix it kit, power commander, i already have their pipe and can, clutch kit and other modifications. After riding one weekend and 55 miles my pistons and cylinders were scored very bad(intake side much worse than exhaust). Intake side is rought to fingernail and exhaust is just about there as well) After doing some research i thought maybe it was because i ran bikemans low compression head at high elevation with the fuel controller so it ran rich and possibly caused this. so this year i took it back apart, purchased a replated cylinder and rk tek pistons so i wouldn't have to use the spacer which rk tek claims isn't as good as his fix. It got new gaskets and wrist pin bearings and everything that could go to properly rebuild a top end. After 2 heat cycles i took the head off to put my high compression domes in before heading west as i did heat cycles with low compression dome and there is already scoring on the intake side. I have the oil pump turned up and oil added to the tank for breakin. It appears that the scoring is perfect vertical lines along the head bolts. I am pretty sure the crank is just fine it seems tight but i am lost at this point on what it could be. Pictures are of after 55 miles. Any insight would be appreciated.

 

[BeartoothBaron]

I had very similar wear marks - but maybe not as bad - after doing the top end on my '12 Pro. I've got a long rod conversion, so it uses a spacer with standard height pistons. It came with OE pistons (to the best of my knowledge; I'm not sure how to confirm the markings), and something caused a locating pin to fail or a ring to break and eat a cylinder. I'm not sure if I ran them too long, or if it was caused by some fuel pressure issues I discovered after the fact (never any det warnings). Anyway, I had the cylinders welded and re-plated and put in Wossner pistons that I ran for a season, then out of an abundance of caution I took it down again to have the crankshaft gone through, and that's when I found the marks. It looks like the same thing, although my pistons looked cleaner when I took them out. I wish I'd run AXYS pistons (I chose Wossners partly after not being able to find '15 OE pistons - since discontinued) when I did the top end. OE pistons seemed like too much money with iffy cylinders, so replaced the Wossners with SPI Hyperdrive pistons. I also hand-honed the damaged areas with some fine grit sandpaper, and have the oil pump cranked plus 100:1 pre-mix. 3-400 miles since then and so far, so good.

My best theory as to what's causing it is ring flutter. Wossners, and it looks like Wiseco as well, uses a different style of locating pin than OE (which most cast replacements copy). I posted about this when it happened, and I heard some mention of similar wear with forged pistons. Meanwhile, others swear by the MTNTK fix kit, which uses a custom Wiseco piston. As best I can tell, both RK TEK and Bikeman use Wossner. Going back to when I blew a piston, the good cylinder looked almost perfect; if I had a time machine, I could have done the top end with OE pistons and avoided the headache. I'm hoping there's a good explanation here - is it specific to Wossner pistons, or has it happened with Wisecos too? I can't think of a thing I did that might have caused it, but I'm just an amateur when it comes to rebuilding these. To answer the obvious question, I did set the ring gap to spec. If this has ever happened with cast pistons, nobody here has ever mentioned it. Personally, I'm once bit, twice shy. I'm not touching forged pistons again until I know for sure what's going on here. I won't speculate on Wisecos, but this would be another bad report on Wossner pistons - maybe they just screwed up on a run of piston rings, but who's to say? I do think there's merit to running a taller piston, or ideally, a long rod with OE pistons (but that's more work and expense).

Well, forgive the long post here, but to the OP, my recommendation would be to buy SPI's fix kit, or find the one that uses Cat pistons if it's still available, and then run good oil through it and plenty (maybe turn up the oil pump even more, and keep running 100:1 pre-mix). If it works well for you, buy a leak-down gauge and monitor it. I'm sticking with cast pistons until I learn more, but even with "fix kit" pistons or long rods, you should expect to replace them periodically.

 

[RHODESIE]

i have ran quite a few rk tek pistons. wossner and now wiseco. over the counter wossner and wiseco also. i have installed dozens as well. they all streak like that and the more miles that are put on the less noticeable they are. its not been an issue for me. i always thought it was ring chatter also because the ring is a little loose in the groove. after a half tank of fuel break in period i let 'er buck and tell customer's to do the same. i like the rk tek pistons because they have thicker skirts and weigh exactly the same. still lighter than stock. seems like less handlebar shake. cast pistons seem to be 2-3 grams different from each other.

 

[BeartoothBaron]

That's interesting @RHODESIE, and I've heard similar reports. No disagreement, and you're not the only guy running Wisecos and Wossners (either their product or MTNTK, RK TEK, Bikeman) with good results. I guess what concerns me is that what I experienced (and it seems the OP too) is real damage, however slight. If it were something I couldn't feel, I don't think I'd worry about it. I was always someone who thought the hate for forged pistons was old wives tales, but having run a set and seen premature wear, I guess I've changed my mind. Part of it is, I'm pretty sure you can cause failure with a forged piston if you get on it hard with a cold motor, or stop long enough for the thermostat to fully open, then start it up and hit it. I can live with that: I always warm it up fully, and let it idle a minute or so after stopping to let the temps stabilize - even with cast pistons, where "you don't have to do that." But, having seen how clean the cylinder looked with cast pistons even with high miles (the good cylinder when I first did the top end), I'm convinced that no matter how careful you are, cast pistons are easier on the bores. You might have to replace them more often, but if you told me I can do the top end with the SPI pistons I'm running every 1000 miles and be guaranteed to blow something else before any top end issues, I'd be in.

I'm not saying "don't run anything but cast," just that's what I'm going to stick to for the time being. You do get a bit more vibration, that's true. Also, either way, I think you get out of it what you put into it: run cheap oil and gas and pull the rope and get on it like you stole it, and you're going to create problems. Also, I hope this is an issue the can be fixed: cast pistons, even OE, can lose the locating pin and fail (seems to be on motors that see long WOT runs), and there's the weight advantage. It just seems to me like something's not right if you're seeing this crop up though. If they fix whatever's causing it, or if someone can tell me the magic formula to prevent it, I'll be first in line to try forged pistons again.