CR 80 eating pistons????

[Rcannon]

I hope you guys can help me. I have been working on a CR 80 for the past few months. To make a long story short, the bike eats pistons. Weisco or stock it does not matter.

The basic piston looks ok. The problem is in the exhaust port area. The CR uses a bridge in this area. The piston ends up looking like the one in erics book that says..."Bad crank seal" This happens within 8-10 hours of operation. The riding is closer to a race speed as the kid rides the bike rather quickly.

It looks as if carbon collects here and carves a line on the piston. If left long enough the ring catches the exhaust port and BAM!!!!. The weisco can take the abuse without shattering. The stock piston cannot.

Please tell me how I can confirm a bad right crank seal. I have a feeling this is the problem. The bike has done this since the day we brought it home. A local machine shop looked at the piston and said.."its an 80, what do you expect???"

Any help you can offer would be greatly appreciated. If this si normal for the CR 80/85 I would not touch one of these bikes with a ten foot poll. The engine is a very old design. The head gasket is poor. I woudl recommend the KX 80/85 or RM 80/85 hands down over this bike.

 

[Jaybird]

Just curious what mix, and at what ratio, you use?
What are you using in the clutch?

 

[DRZ_Dad]

Could be several things, plating wore too far on the cylinder, putting a D piston in an A cylinder,
Did you drill the holes in the piston per wiseco's instructions?

My son's CR80 was breaking the piston skirt on the exhaust side, after only a few hours run time, I sent the cylinder and head to E.G. he said the plating was gone, So I ordered the big bore kit from him........

 

[Rcannon]

The bike is running Yamalube R at 32:1. Basically the same as our other bikes. The elevation here is @4500ft so race fuel has never been necessary.

The bike has eaten these pistons from the day we bought it. The Weisco piston is available in one size only, so I cannot see that being an issue. We drilled the hole 1 time and skipped it the next time. Neither way made much difference.

The bike was replaced today with a 2005 CR 85. Hopefully this one does not cause any trouble. The cylinder looks different especially in the exhaust area.

We are going to do the big bore thing with the bad engine. The plating could be worn by this point. Hopefully Eric will be able to relieve something off this exhaust port brige .

What size did you go with on your big bore??? Does Eric only offer 1 size?

Were using his shop as he is the only one with a reasonable price for the work. The work I have seen from Eric is also top quality.

DRZ, the pistons in this CR 80 will break exactly as you describe. If you catch them soon enough you will see the line forming on the exhaust brige. I believe this wears enough to catch and shatter. The weisco piston does not shatter. It just starts running noisy and poorly at high revs. Weisco pistons have saved us huge dollars on lower end work on this bike.

 

[Jasle]

well to confirm a bad crank seal you plug the echaust and then make a plate to cover the carbe area. Install a car's valve stem in the plate and then pressurized the motor plug in place. you don't want to go more than about 10 psi so use a hand pump or regulated air hose. then you can spray something soapy on the seals and see if they bubble.
I don't know about the cr80 but on the kaw you had to remove a little from the exhaust bridge. It gets hotter than the rest of the cylinder since its thin and cannot dissipate heat as easily. So it expands and gets tight with the piston.
Have you measured the bore and piston to see the difference in measurements. Also are you checking your ring end gap when fitting the new piston?

When boreing over the typical size is .080 or 2mm. This is a very common size because it used to be allowable in AMA racing. I'm sure there are other sizes avail. Either way Gorr is one of the best.