Stroker crank question.

[proy]

So I'm sure someone on here has covered this in another post but I can't for the life of me find a definitive post on what the best way to install a stroker crank into a qmb139 is. I'm confused on weather machining the piston or a spacer under the jug pared with a longer timing chain is the better option.

If anyone can link a specific page on a post or a proper wright up on a successful stroker build it would be highly appreciated.

Now my inner grease monkey wants to try and build a full blown race engine starting with a 50cc block. Realistically though would I be better off just buying a stock 150cc and fab a new set of motor mounts since fabrication isn't a big deal for me?

I just find it hard to justify the cost of a full blown race set up on a 50cc block when I can just buy a stock 150cc and remain reliable and get the power I'm looking for.

[GrumpyUnk]

May be I am confused. A 'stroker' crankshaft has a bit longer throw(radius) on the rod journal. It will travel in a longer circle.
If you change the crankshaft, the piston and connecting rod are GENERALLY also changed. In most automotive designs, people do not shim the cylinder head, or much around changing the block when a stroker crank is installed.
The rod & piston, if left the same, WILL push the piston higher and lower(also further fore and aft) as it moves.
The rod comes installed on the crankshaft when it is assembled. Unless you are getting a kit that you press together yourself(good luck with that, a couple dial indicators seem necessary).
I would expect that the connecting rod installed should be designed so a standard piston(I know of no stroker 'specials' on the market) should be installed, and the piston would travel from BDC to TDC and stay within the bounds of the cylinder.
If you are uncertain, contact the vendor. If the vendor speaks a different language... try the TRANSLATF function to communicate your concern.
Off the top of my head, I would expect the con rod installed to be shorter in length, center to center between big end and wrist pin diameters. I dunno how else it could be, though wrist pin placement within the piston can be modified also. I'm sure you will get comment from someone who has mucked about with these.
tom

 Added... I stand corrected.  I looked at racingplanetusa web site.  The Naraku indicates standard length con rod installed.
 That means the piston would extend past the top of the cylinder at TDC unless the install is modified.  A different piston could keep the same deck height, and leave the cam chain geometry unchanged from factory.
 I poked around a bit more, and did not find anything from Naraku that would enable use of their stroker using a modified or specialized piston.  Kind of leaves things totally to the customer.
tom

[proy]

Oct 31, 2020 14:12:34 GMT -5 GrumpyUnk said:

May be I am confused. A 'stroker' crankshaft has a bit longer throw(radius) on the rod journal. It will travel in a longer circle.
If you change the crankshaft, the piston and connecting rod are GENERALLY also changed. In most automotive designs, people do not shim the cylinder head, or much around changing the block when a stroker crank is installed.
The rod & piston, if left the same, WILL push the piston higher and lower(also further fore and aft) as it moves.
The rod comes installed on the crankshaft when it is assembled. Unless you are getting a kit that you press together yourself(good luck with that, a couple dial indicators seem necessary).
I would expect that the connecting rod installed should be designed so a standard piston(I know of no stroker 'specials' on the market) should be installed, and the piston would travel from BDC to TDC and stay within the bounds of the cylinder.
If you are uncertain, contact the vendor. If the vendor speaks a different language... try the TRANSLATF function to communicate your concern.
Off the top of my head, I would expect the con rod installed to be shorter in length, center to center between big end and wrist pin diameters. I dunno how else it could be, though wrist pin placement within the piston can be modified also. I'm sure you will get comment from someone who has mucked about with these.
tom

 Added... I stand corrected.  I looked at racingplanetusa web site.  The Naraku indicates standard length con rod installed.
 That means the piston would extend past the top of the cylinder at TDC unless the install is modified.  A different piston could keep the same deck height, and leave the cam chain geometry unchanged from factory.
 I poked around a bit more, and did not find anything from Naraku that would enable use of their stroker using a modified or specialized piston.  Kind of leaves things totally to the customer.
tom

Thank you for the info. From all I can see from researching parts most stroker cranks available for the 139QMB block all need something to be changed as far as piston to valve/head clearance. Nothing as far as a "kit" or a working list of parts that don't require some adjustment.

As far as what route im going to go, moving to the 150cc to keep the reliability of a stock engine.

As much as I want a 50cc to be able to do 45 around town and be able to do a safe speed up the hills in my area that is asking a lot. At least without pushing the engine to a point where it begins to be too unreliable.

Re reading my original post I see why you were confused about me mentioning using a 150cc block. I was just trying to say using a stock 150cc engine instead of trying to push the 50cc engine so far.

[proy]

Won't claim that I really know what im talking about. Still reading into it and trying to troll through other peoples posts and see what is involved in doing a stroker build.

As far what I am comfortable saying I know what I'm doing is really only top end work. I do a lot of big bore kits and carb tuning for a lot of local people. I have done a few oil pumps and crank replacements.

I'm just looking to expand on what else I can learn to do on the 139QMB platform over the winter with another project and was considering trying to do a stroker build.

[scooter7878]

I know this is a old post but I just installed a ssp-g stroker with a 50mm airsal domed piston and a big valve head and 47/17 gears and I did not need any extra gaskets or any mods to case everything installed no issues. It's fast as hell I'm hitting 55 flat ground and 0 to 40 mph in 11 seconds and I can feel clutch slipping once I get a good clutch I'll probably get a tad faster. Just make sure u check ptv clearance with playdoo and be sure ur cam timing is correct one tooth off and u will hit. I had no issues though and my scooter runs better than ever I'm glad I did it it's as fast if not a tad faster than my buddy's stick 150cc