Strange Discoveries when changing the piston - couple questions

[robwbright]

I'm in the middle of a top end rebuild - the first time I've changed the piston since Eric built the motor in early '06 (I was not riding for over a year with wrist problems - I had changed the ring once before).

Here are two pics of the top of the piston - with flash and without. It looks pretty good to me:

http://i175.photobucket.com/albums/w134/robwbright/DSC07988.jpg

http://i175.photobucket.com/albums/w134/robwbright/DSC07989.jpg

There's a small score in the side of the piston - it's the only mark you can feel with your finger and it's right below the ring. Any ideas what might have caused it? It's only about 2-3 mm long.

http://i175.photobucket.com/albums/w134/robwbright/DSC07990.jpg

Getting some definite blow by on the exhaust side:

http://i175.photobucket.com/albums/w134/robwbright/DSC07991.jpg

Awhile back I was working on jetting and found that a small piece had broken off my carb slide. The piece could not be found, but the motor ran fine and there seemed to be no bottom end issues. We all just assumed that the piece fell off when I pulled the carb slide out of the carb. Not.

Here's what the piece did to the crank - the picture's a bit blurred (I was in a hurry because light was getting low outside and dinner was ready. . .). The scores are into the metal - there is nothing rough sticking up. The "marks" at the very top middle/right and bottom right of the pic are reflections/shadows. The rod has a small amount of side to side movement and zero up and down movement:

http://i175.photobucket.com/albums/w134/robwbright/DSC07993.jpg

Since the damage has already been done and there doesn't seem to be a bottom end problem, I suppose I can just keep running it. . .? It's been about 10 hours seat time since I discovered the carb piece had broken off.

Here's a photo of the inside of the cylinder and the head:

http://i175.photobucket.com/albums/w134/robwbright/DSC07994.jpg

Now here's a question. I don't recall what the intake side ports looked like when I changed the ring almost 2 years ago. Here's what they look like now - that's actually an "indentation" in the plating - would that have been Eric's work in enlarging the ports - or is that a problem area? I hope I don't need a replating. . .:

http://i175.photobucket.com/albums/w134/robwbright/DSC07998.jpg

Finally, the cheap plastic clip which attaches the power valve actuator arm to the power valve was broken - thus the power valve was not working and there was a lot of thick black gunk in the power valve area. I'm guessing that screws with your jetting. . .?

Thanks

Rob

 

[rpm12505]

You do know EG offers free diamond honing forever as well as diagnosis.
The only thing that would concern me would be the detonation marks in the head, though the piston looked ok.

We had a piece of carbon fiber reed put a slight score in the cylinder and EG was able to hone it out.
Well good luck and I'm sure somebody that knows what they're talking about will chime in soon.

P.S. the mark on the piston almost looks like a crack.

 

[Pete Payne]

That indentation between the 2 boost ports is most likely where thew open end of the rings is statring to wear the plating . . This happens when the ring gets rater worn an ther is not as much ring in the ring land supporting it .
I would do some good measuring to see how deep it is and address this problem now .

 

[flying flea]

Should there be a pit in the piston like that?

 

[robwbright]

That indentation between the 2 boost ports is most likely where thew open end of the rings is statring to wear the plating . . This happens when the ring gets rater worn an ther is not as much ring in the ring land supporting it .
I would do some good measuring to see how deep it is and address this problem now .

I don't know much, Pete - but I doubt that's it. In looking at it closer, the indentation is pretty deep (1mm maybe). It appears to have been cut in there with a grinder - the ports have that same appearance all the way around - and the grinds are angled in towards the opening.

As I said, I don't know much about this stuff, but it looks unlikely to me that the ring could cut that far in in only one spot and only 1mm in length on the port.

On top of that, I would be highly disappointed in the plating job if it's worn out after about 10 races and (relatively) not that much play riding/practice. My races are on a track with 1:03 lap times and only 4 or 5 lap heats and mains - and I'm no expert, so I'm not killing the motor.

But maybe I'm wrong. As I said I don't recall what the ports looked like when I got them back from Eric and when I changed the ring the last time.

 

[robwbright]

Should there be a pit in the piston like that?

Pit? Where? Do you mean the score on the side?

 

[flying flea]

Theres a perfect circle in the center of the piston, is that where your spark plug fires or is it just your camera?

 

[robwbright]

Theres a perfect circle in the center of the piston, is that where your spark plug fires or is it just your camera?

It's on the piston - but it's made that way.

 

[robwbright]

And BTW, what's the deal with these A, B, C, D piston sizes? My old piston has the serial number and "Eric-Gorr.com" stamped in the top. There's no other letter anywhere.

The new piston has the serial number (846M05800) and 6428C stamped in the top.

The outside of the box has the serial number and 2284CS and CW15 and S668 AND 042208/S/J/6428 and says Clear: 0.0020(0.051mm).

I checked the Wiseco site and there doesn't seem to be an option to order A, B, C, or D size pistons in that serial number.

Is it going to work? The last thing I need is a seizure from the wrong piston.

Thankfully, this new piston has the oil holes already drilled in it.

 

[Rich Rohrich]

Rob - Wiseco uses one size for all, and the expectation is the bore will be finish honed to the correct piston to wall clearance. Eric hones his cylinders himself after they are plated to make sure they will be sized for the standard Wiseco piston clearance.

The lettered piston sizes is to account for manufacturing variances at the OEM level. New barrels are measured and if they fall within a certain range they are marked to ensure that a matching piston ends up in an engine and the correct clearance is obtained.