Running a steady 80mph when the back cylinder dropped. Then I found this... what would cause this? Keith black hypereutectic (or however it's spelled) pistons. Happened after 1200ish miles of running fine

 

Well....I have heard they can do this under certain circumstances....or you might have an intake leak.

 

Looks like detonation to me. Have a set of those Keith Black pistons out in my shop ,( For a shovelhead) that blew a half-dollar sized hole in the top of the piston because of detonation. Hypereutectic pistons have a high silicone content for low expansion so you can fit em tighter. But they are also very brittle & won't take a lot of abuse, like a forged piston will. I know this , cuz I use to work for Dynagear & they made hypereutectic pistons. Very abrasive material. You can see all the little b-b's on top of your piston & then it blew the chunk out of it & then you lost power. Been there , done that. Your going to have to split the cases to get all that crap out of there. Good luck.
By the way, I'll never buy another set of Keith Black pistons.

Ride Safe,
Harold

 

What's the head look like?

 

Ouch - that doesn't look nice! Funny thing about KB pistons. The indy who built my 89" stroker would only use them because they were the only brand he could get that were dimensionally accurate and consistent and would also hold up in his drag bikes! They were fine for me, pre-107" days.

 

I would say detonation or they just simply got to hot! In drag cars, we NEVER use them! Nothing but problems!

 

K black pistons have always been a money maker for them as they dont work and people buy them anyway

first as said above the timing was probly wrong too much

the fit is too tight piston to wall ( the spec they give you ) and the alloy they are made of is for water cooled motors soooo

plus you bought the 10 to 1 set and with that small cam the static compression was way over the top ( again the timing was wrong )

put a flat piston in it and use cometic 020 steel base and the .030 head gasket and drive it 50,000 miles trouble free

anything else with the gas we now have you will be doing it again the end of this year

 

I wouldn't be so quick to blame the piston. I'd pin it on the mechanic who replaced those pistons.

At 1,200 miles you should not have all the scuff marks and blow-by that I see in the picture. I'm going to lay the blame on not precisely checking bore against piston dimensions as it looks like the piston rattled in the bore and that cause the top segment to crack.



My Indy just installed these based on experience and previous use. He was very certain that the machinist had the most precise matching there could be. I just rolled over 1,200 mile on the new rebuild and pray I don't ever see this.

There are so many "precise" factors needing to be addressed. Experience is the greatest teacher! We had to wait for machinery and my jugs to be at a proper temperature before boring. We did the rebuild over last year's extremely cold winter and waited for the temps to do the job. Also the strict method of break in started with my Indy doing warming cycles starting at 30 seconds runs to properly seat the two different metals used. He did many "runs" like this before I was even allowed to have the bike back. And my break in was strict too. Nothing over 3 grand for almost 500 miles and an oil change. Then another 500 limited to 4 K rpms and under. I just hit the first 1K and have now been giving it full throttle and running it out to the limiter. At 30 over and 9.6:1 bump it's impressive with the EV27. Carb and pipes next on my list.

And best wishes getting your ride back on the road!

 

I have a friend that has put 100k miles on his 9.5-1 KB hypereutectic pistons.

 

I know of two reputable builders around here that will not use, or re-use KBs In a motor.