Stihl ms660 54mm project

mrfubs

Member
Jun 13, 2013
306
10
18
michigan
Hi all,
It's been quite a few years sense I'v Been in these forums. I started a new project of mating a stihl ms660 cylinder to the 66/80cc crankcase. Just so you all know the 660 make 7hp at 9500. So far iv filled the transfer ports and existing bolt holes with alloy, drilled and tapped new holes to fit the new cylinder.

The next dilemma was how I was going to make the new 12mm wrist pin work with the connecting rod designed for a 10mm pin.
My first thought was to get the con rod bored out to fit a bigger bearing but then I realised there wouldn't be enough material left on the con rod to do so.
I found some bushings on amazon from china for $5 12mmx14mmx12mm oiless I wear resistant and they fit perfect in the con rod with out any mods. Just use a vise to press them in.

As of right now I will be deciphering how thick of a base spacer I will need to make and then i can continue with re balancing the crank back to a balance factor of 55%.

I'll be back with pictures soon
 

mrfubs

Member
Jun 13, 2013
306
10
18
michigan
Turns out with a 40mm crank and a long rod the piston was still at least a 1/4 from the squish band. With that said I had to come up with some way to deck the jug that much with out paying top dollar for a machinist to do it.

Believe it or not a sawzall worked...! I then sanded it flat on a belt sander and was only off by .004 if a inch on either side from the bottom to the squish band.

My new delima is I need a new 40mm crank because mine is in useable and cant seem to find a 40mm they're all 38mm.
Stay tuned...
 

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mrfubs

Member
Jun 13, 2013
306
10
18
michigan
I found me a 40mm zeda crank on ebay. I didn't like the con rod quality so I changed it. I rebalance the crank to 55% balance factor. I have some touch up work to do on it yet.

All I really have left to do at this point is make a header for the new pipe (2006 ktm sx85) and then I can put it together and dial in the squish band!
 

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mrfubs

Member
Jun 13, 2013
306
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michigan
I made a header for the new pipe with Hope's to eliminate blowing exhaust gaskets continuously. Turned out very well!

By the way the creator of this engine design made a dyno video of his 660 engine and it made almost 17hp!!

Here's the link

 

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Spare_Parts

New Member
Nov 5, 2017
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Newark Delaware
Don't know how much I would trust his dyno testing procedures. Seems to know how to use the dyno's flywheel mass to his advantage either to "make" or hide power.
Not saying this engine shouldn't make big power.
 

mrfubs

Member
Jun 13, 2013
306
10
18
michigan
It sounds accurate to me. You have to consider most of your 125cc 2 stroke dirbikes such as the yamaha yz 125 has a 54mm piston but has a longer stroke and different porting but makes 30-35hp so this number is very believable.
 

mrfubs

Member
Jun 13, 2013
306
10
18
michigan
She lives! It fired right up on the first start! I am in the tuning process still and also am trying to get the clutch to stop slipping. I have the shaft with the big spring almost as tight as it will go and it still slips once it hits the power band, it's pretty awesome!

I am running a 36t sprocket fyi

Here's a short vid
https://streamable.com/6ruiu
 

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snorks

Member
Jan 16, 2009
41
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8
Africa
Very refreshing! Nice to see. Im slightly concerned about the bore not being square to the cylinder base flange. A few thou can rob power. What size carb are you running? The zedafrank vid showed a little blow back from the carb? would you consider a reed setup or not enough space? Some of the sthil bores used to be chromed I think. Not sure if this is the case anymore. Keep up the good work.
 

mrfubs

Member
Jun 13, 2013
306
10
18
michigan
The bore is square enough considering what kind of motor were working with here.

I am running a cheap oko 26mm that cost all of $26. Seems to be running okay with a 115 main and a 45 pilot
This is a cheap cylinder so it is chrome plated. The OEM cylinders a much more expensive and are nikasil plated.

I have blow by too but I wont worry about a reed valve until I am looking for more power.
 
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Atom

New Member
Aug 3, 2018
1
0
1
32
good tips, I was wondering on the wristpin, now I know lol. im currently in the process of 3d printing a fan system similar to the one on the actual chainsaw that will run off the crankshaft. these will overhear very fast without it. once I get it right ill send yall a link to buy the kit
 

javy mcdees

Active Member
Jul 30, 2018
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good tips, I was wondering on the wristpin, now I know lol. im currently in the process of 3d printing a fan system similar to the one on the actual chainsaw that will run off the crankshaft. these will overhear very fast without it. once I get it right ill send yall a link to buy the kit
20 mph cruise is a pretty good fan. more important is a clutch that will harness these mods. there is a new design clutch with large surface pads out now, should be interesting. Now time to buy some parts!(p)
 

Valerii Ostapenko

New Member
May 12, 2019
4
0
1
22
Hi all,
It's been quite a few years sense I'v Been in these forums. I started a new project of mating a stihl ms660 cylinder to the 66/80cc crankcase. Just so you all know the 660 make 7hp at 9500. So far iv filled the transfer ports and existing bolt holes with alloy, drilled and tapped new holes to fit the new cylinder.

The next dilemma was how I was going to make the new 12mm wrist pin work with the connecting rod designed for a 10mm pin.
My first thought was to get the con rod bored out to fit a bigger bearing but then I realised there wouldn't be enough material left on the con rod to do so.
I found some bushings on amazon from china for $5 12mmx14mmx12mm oiless I wear resistant and they fit perfect in the con rod with out any mods. Just use a vise to press them in.

As of right now I will be deciphering how thick of a base spacer I will need to make and then i can continue with re balancing the crank back to a balance factor of 55%.

I'll be back with pictures soon