My first thread and I hope to get it right, if not, forgive me and I will try to do better, anywayz some background on a bike I'm doing for a friend, it's a 1992 FLH 80" EVO with carb, he had no background info other than it was black and it ran, the bike ran good for a long time, many miles on it and most of them running hard down the highway, he'd still be going now, smoke n all, if his other buddies wouldn't have said they weren't gonna ride with him "no more" anywayz, on teardown I get to the rear cylinder and I didn't even get the cylinder off all the way b4 the piston pin came sliding out at me, so, yeah, those in the know, know what happened, it's pretty impressive when a wrist pin keeper gives up the ghost, it broke on 1 side beat the crap outta the piston a little then decided to make its way thru the pin to the other side and really raise hell, parts of the circlip ended up in the cylinder stuck impaled in the head, other parts from that dreaded clip went on an adventure thru the bottom end, thru the breather and all the way into the oil pump but amazingly didn't make it up into the gears (5 pieces in the pump and 3 stuck in the breather). So far I've gotten the bottom end redone mostly, rods rebuilt, Jim's crankpin, stock stroke (4.250), Sifton oil pump, I still need to get a breather gear and am going with the Andrews EV27 (bolt in) or equal of, the original cylinders turned out to be bored .030 over, the rear cylinder was unsavable, my buddy isn't rich and wanted me to reuse as much as I could and keep it oem as possible so heres where I'm at, my indie shop that does what I can't, set me up with another oem cylinder and bored the 2 cylinders out to .040 over and fit Keith Black hypereutectic pistons (8:5:1) to each cylinder, I find these pistons aren't your average piston and require extra care in the ring gap department, as far as the KB ring gap factor goes in my research I've seen different charts with different #ers or wording like "street naturally aspirated light bike" would have a .0065 gap factor x 3.538 bore =.022997 (.023 top ring gap), this would be the gap for a "light bike", the bike I'm working on is far from light (full bagger) and the owner and his lady are far from light also, I just see the potential for this bagger to carry more heat on the top ring than a "light bike" and my thinking says the gap should be bigger, but what exactly I'm not sure, if i go like .002 over the .023 gap and go to.025 would that be sufficient to keep the rings from butting on a bagger that can get pretty hot sitting in traffic on a hot summer day and it just seems to me with these hypereutectic pistons that you have to run a wider gap than usual, but what is to much and where is the sweet spot for a heavy bike that can get road hard. Thanks for putting up with me this long!

 

Bigger is better. +.002 is fine.
The factor for high compression [11:1] is .008 so you're right there.
With those pistons it's better to err on the high side.

 

I would strongly suggest not using the KB hypeutictic pistons.they are not called " Ka Boom" for nothing

 

By people who don't know what they're doing maybe. i know guys that have used them with no problems ever. Your clearance on my pistons/cylinders certainly would've been a problem with them. But no worries, they're being fixed. My head on the other hand is being replaced. F'n hack.

@ wheelz76 I'll send you a PM and I'll put you in touch with someone that knows those pistons real well and is a rep for them. He'll help ya out. He's never had one go ka boom either.
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Wow!

 

What I don't understand about Hypereutectic Pistons is that if they expand so little in the cylinder and you need less than .002" piston to cylinder clearance, why would you use a ring gap so big? Makes no sense to me... I've always set ring gaps 4.5 times the bore and never had issues.. AND.. in all honesty I don't think I'd use a Hypereutectic piston only because the minimal amount of clearance required to run the Piston.. I'd rather run three to four thousands clearance..
Inquiring minds would love to know...

 

Ask most reputable engine builders & most if not all will say not to use them.i didn't make up the "Ka Boom" ****,it's well known all over the industry.

 

I know K-Bs automotive hyperutectic pistons have a raised top ring groove, which is closer to the heat of the chamber and that is why a larger gap is required. I don't know why they're raised, but the rings expand more (not the piston) and can butt the ends.

 

I can understand that. So they have a very narrow top ring land.. in that case then I would not think they would be good for high compression high temperature build..

 

I've seen KB pistons break the ring lands,break the skirts off, completely shatter (no gunner60,not on any build I've done cause I won't use them) this is what I've seen over the years on certain motors that have come in my shop.the majority of builders I know won't use them