Piston slap on fresh rebuild.

Scott Pepple

Member
Mar 1, 2015
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Ok guys, I bought a blaster that needed a new top end....long story short I decided to rebuild the whole top and bottom end. new crank, bearings, seals and new top end. I had to go out to 67.25mm bore. Wiseco piston. I lubed up everything really good. I'm almost certain I had the rings lined up in the notches like they should've been. After running some heat cycles I started getting piston slap. So I tore it back down. I took the jug and piston back in to my shop and he shipped it out to millennium technologies to have it checked. They got back with him and said the bore job by napa was perfect. While they were at it they rebored it out to 67.5. When I got everything back I got looking at the 67.25mm piston and looked at the rings. The top ring had no brownish coating on it like it did when I first installed it. the bottom ring still had the coating on it. I'm thinking that top ring didn't seat and it got past that notch in the piston to hold it in place. So anyhow I've currently got the jug installed along with the new piston, and I made positively sure the rings were in the correct spot. Can anyone here relate to this problem at all? anything else I should maybe look into? The piston to wall clearance was perfect from what the shop said.


Secondly when I do fire it up again. My setup is obviously wiseco piston. FMF powercore fatty pipe and silencer. K&N filter w/o airbox lid. I bought a dynojet kit. It's saying run a 360 on the 3rd clip down at 2 turns without their plastic spacer. Just out of curiosity has anyone ever used those kits and how reliable their jetting information is?

Any and all input is greatly appreciated.
 
Welcome to the site bud.

There is a few things that aren't going to work for you from what I just read.

A 67.25 piston in a 67.5 bore? There are tight piston to cylinder tolerances that need followed, or you'll get things like piston slap, shattering of pistons, etc. I can't throw you those numbers cause I'm no machinist, but I'm sure someone will chime on on that. I wouldn't run it untill you get a bore matched to a piston.

The jet kit you mentioned we all usually tell people to avoid them, ad they rarely have the nets you need to get you jettes correctly. It's cheaper, safer, and the jobbwill get done right to buy individual jets from a dealer.
 
No, the first go around was 67.25 bore....when I had the jug sent back out they bored it again with another piston, 67.5mm piston to match the new 67.50mm bore this time. Both times it was bored it the bore was matched to the piston. Sorry if I confused you guys a little.
 
No problem. I think the way I read it was confusing.


The jetting instructions dynokit gave you don't sound too good. Make sure the carb is clean, float levels set. Needle clip middle (3rd position). Start off with a 32.5 pilot jet, airscrew 1.5 turns out from lightly seated. Adjust the airscrew once the bike is warmed up untill you get the highest idle (it takes a little bit of time for the engine to respond to the adjustment).

As for the main jet, what elevation from sealevel are you? Temps and elevations play a big part in jetting, just as motor mods do.
 
When its time to do plug chops, I'd start with a 310 to be safe. Carb parts warehouse or jetsrus is a good place to buy jets. I'd get 310, 300, 290, and 280's. Get 5 or 6 B8ES plugs too so you can chop them to make sure your jetted right.
 
just for reference, I had put a 340(one of the mains that came in that kit) in and set the needle, and it seemed really rich, so that does seem more in the ball park of where it needs to be.
 
If 340 felt rich, you need to go lower.

Those jet kits sometimes don't have genuine Mikuni jets either. So what they say is 340 might be ???

Start off with Genuine Jets in those sizes I mentioned above, and you should be Ok.
 
Alright update guys...I had taken the jug back in and my shop made it right and shipped it out to a shop. Its running great...Friday night I swapped main jet down to 280 after 310 being way rich. Yesterday I adjusted clutch because it was slipping a bit. Also plug chopped it yesterday. Plug looks perfect, its shifting perfect...and it wheelies really easy. Thanks for the help.

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