Piston slap Part 2 - What am I in for?

[Jfishsolevibe]

Yo yall! Just wanted to pop in with a quick question about my GT380. It started showing a slight piston slap a few months ago. I changed oils and it helped a bit, but now its gotten fairly bad.

It slaps on startup, and any time it revs beyond 5k while freewheeling (clutch in or coasting). It does not slap while under load.

My first instinct was to do the rings and check the small end bearings... is there anything else that could be causing the slap? My gaskets are all intact, no leaks, etc. I can tell compression isnt great just by a slight decrease in overall power, but aside from the knocking, it runs pretty well still.

Just wanted to see what I was looking at for repairing it once I open it up and see if theres any damage to the cylinder walls. Am I looking at a rebore/hone and new rings or potentially pistons?

Thanks for the help as always!

FISH

 

[my-79-750-k]

Rod bearings, crank bearings, journals could be out of round, too much carbon build up on your valves, scored cylinder, etc. I'd start tearing it down and replace ANYTHING that looks amiss. That and when you rebuild, use quality fluids always. (Goes without saying really)

 

[hillsy]

Rod bearings, crank bearings, journals could be out of round, [size=10pt]too much carbon build up on your valves[/size], scored cylinder, etc. I'd start tearing it down and replace ANYTHING that looks amiss. That and when you rebuild, use quality fluids always. (Goes without saying really)

Ummm......you do know we're talking about a 2 stroke here, right?

 

[teazer]

Cams could be worn.........

Piston slap is the noise a piston makes at or near TDC and or BDC as it reverses direction and that makes it slap against the barrel wall. The cause is excessive clearance and the only want to determine if you need pistons is to open it up and measure the pistons and the bores and compare the readings to specifications. Piston ported 2 strokes tend to make a lot of noise but the issue here seems to be that it developed recently. It may not be piston slap, but could be a number of other things.

What I would do is start with a compression check and then I'd do a leak down test. You want to see the seals hold 6psi for 6 minutes with positive pressure and the same with negative 6 psi. Unlike a Yamaha RD twin, the GT series seal each cylinder with rubber seals so each cylinder can be pressure tested. On old two strokes, crank seals tend to go hard and tear if they have sat for a long time.

Cranks can be rebuilt of course. We recommend Bill Bune in MN for crank work. They have stripped and built a few cranks for us now and their work is good, they are pretty fast and prices are good. And no, I don't have shares in that company or know anyone there. Just happy to recommend a shop that does good work.

BTW, I'm assuming that carbs are clean, the points are clean and set etc etc. If it needs a service, start there.

 

[crazypj]

Piston slap occurs because piston or cylinder has excess clearance.
As piston wears the rings will not maintain contact with cylinder walls, pistons can overheat and rings loose tension (making blowby worse)
If you run about 0.006" the skirts will break off and it will probably be 'game over' for the motor.