Skorpion sport cup rebuild

[twinlinemoto]

After trying to start my new bike, I found that there was 0 compression. After pulling the motor and top end off, I discovered a torched piston and cylinder, yay! I am currently unable to remove the piston from the rod, I will try a custom wrist pin puller but I may need a new con-rod/crank as well. UGH!

First question: What motor is the MZ 660 closest to? I know the cranks are different between the YFM660 raptor and the 660 rhino, but there is a lot of aftermarket support for both.

Has anyone used the big bore overbore (102.5mm) pistons? They are made by CP and apparently are a solid option for those who have roached their previous 686 big bores.
Any advise is appreciated!

[Srinath]

I ran a 102 and destroyed it. I wasn't told @ purchase it was 12.5 compression 102 mm bore. I replaced it with 10.0:1 102 wiseco. I am yet to finish it and start it up.
I was not that impressed with it TBH, the stock one I bought was a bit down on low end torque but that was all. Of course I ran teh airbox and jetted stock carbs.
Cool.
Srinath.

[edfmaniac]

I'm under the impression that you can't safely bore a stock cylinder over 102mm. I guess 1/2mm isn't really all that much difference but the walls are pretty thin at 102. Just buy a good used Raptor cylinder off of ebay and bore it out to the next size up. My 686cc 11:1 Wiseco setup is still running great after about 750 miles but I would have settled for a 100.5mm or anything in between.

Also, I have an aftermarket cylinder that came bored out to 102 if you want to give it a go. I'll drop in a box and send it to you for $50 shipped(within the cont. US). Can't remember the company that put the kit together but they sold hundreds of their big bore kits in Canada. I'll even throw in the flat top piston that came with it for free since I don't have the ring set anymore.

[Srinath]

LOL, my auto mechanic (who helped me with this) said - this is paper thin. So I wont even have bored it. Didn't yamaha bore this thing from 600 to 660. I think it started out life as a 600 - 94mm bore ???
Cool.
Srinath.

[edfmaniac]

It's a completely different casting than any Yamaha 600 that I know of. I don't think they made a water cooled 600 single. So it's probably got a sleeve originally designed for a 660 as well.

[twinlinemoto]

The plot thickens!
I pulled out my caliper to make sure I was on the right track with the bore and its 100mm! Somebody must have blown up the last 686 and put it back to stock. Now I can put it back to 686 and be happier.

New trouble though, I am having one hell of a time getting the piston off the rod... Im making a custom wrist pin puller, but I fear that the small end of the con rod might be shot as well. Can anyone tell me if a raptor crank will drop right in? Maybe a hot rods crank or other?

[edfmaniac]

No need to bore to 102mm on the first go around unless you already bought a piston and think it would be cheaper to have the extra machining work done. They will charge more to cut that much of the sleeve and you won't notice the difference in power from such a small increase in displacement. Good luck on the rod. You might try to order one from overseas since no dealership in the US will order parts for Yamaha engines that weren't sold here. The 99 and earlier XTZ660 has the exact same engine, as does the SZR660, but I have never been able to find an oem parts house for European models that is accessible from my US internet location.

[Srinath]

It's a completely different casting than any Yamaha 600 that I know of. I don't think they made a water cooled 600 single. So it's probably got a sleeve originally designed for a 660 as well.

Aaaa true.
Cool.
Srinath.

[twinlinemoto]

edfmaniac:
There are hundreds of us based retailers that sell aftermarket parts for this engine, as the top end is identical to the Yamaha 660 raptor (the bottom end is only different for the reverse gear I believe). My machinist does not charge extra if he has to bore 0.01mm or 2mm and I believe the small displacement increase and significant compression increase have a large effect on the performance of these engines!

Does anyone here have 660 crank experience?

[edfmaniac]

I'm just regurgitating what someone wise told me. If you concentrate on building the engine right(proper squish, etc.), you won't miss that 1.039% increase in displacement and you won't have to go looking for a new cylinder next time you want to put a fresh top end on her.

I didn't know comp ratio had anything to do with bore size.

[twinlinemoto]

Its more the other way around: bore size affects compression very directly. If you increase the bore, you increase the displacement. When you compress more air and fuel in the same amount of stroke and combustion chamber, the compression ratio will certainly be higher.

A bit facetious, but I was alluding that I would be putting an 11:1 piston in it too.

Now, all forum semantics aside, can anyone help me with my crankshaft dilemma? Are there differences between the xtz/szr660 and the raptor 660 crankshafts?

[edfmaniac]

Semantics aside, they take into consideration the larger bore when they build the various piston models. It's the piston design and final assembly, not bore size, that gives you final comp ratio. If you aren't measuring squish, you are just guessing. Now if you simply bored the cylinder and rebuilt it with a oem flat top piston and the exact same brand of gaskets, you will increase comp ratio, but it should be relative to the increase in displacement, 1.039%. Not worth it in my opinion.

I found one website from Europe that listed the Raptor 660 and SZR 660 as compatible with the rod they were selling. That probably isn't any comfort for you but it may be more of an answer than you will get around here with Bill gone and the lack of activity by all of the other MZ Skorpion specialists that have graced this forum in the past. Try emailing them. http://www.sandifordracing.co.uk/prox-c ... 7171-p.asp

[edfmaniac]

Woops. I think my math is off. The increase in displacement when going from 660 to 686 would be 3.9%, not 1.039%.

[twinlinemoto]

I agree that squish is important, and yes usually big bore pistons are revised to keep the squish adequate, but the bore size directly affects the compression ratio - hence the revision in squish design!

I just did some research on power gains from stock to 686 on a raptor and the 3.9% increase in bore is good for 2hp without any other modifications. If cams and carbs and porting have already been done and you do the big bore afterwards, the increase should be much larger. Unfortunately nobody has done a dyno run with a stock bore modified bike, then a bore kit run.

Thanks for the link, I emailed them. Ill post if they have any useful information.

[twinlinemoto]

I have received no reply, so I will be buying a raptor crank from hot-rods and hopefully everything works! Ill post pics when its done.