Crank Case Mods for a Freestyle Big Bore Stroker build.

JKMike

JKMike

Location
Annapolis, Maryland
Hey yaw,

I’m working on finishing up the last of my research before I start ordering parts for my winter motor build. This will be my first real 2 stroke motor build, but I have built and tuned numerous other high-performance motors over the years. My plan right now is to build a big bore, power valve, +10mm stroker motor in the 1100 cc range. Still haven't quite decided on which cylinder I want to go with but I have it narrowed down to either a ET, TPE or Dasa cylinder. Anyway, I would like to do all the case work myself in the interest of saving money and because, as crazy as it might sound, I enjoy working on my skis and the challenge of figuring stuff like this out almost as much as I do riding them. I should also tell yaw that I'm a mechanical engineer/CNC machinist and through my job I access to a full machine shop with some serious big league toys where I can get away with the occasional personal job such as this. (Could never do production though!!!)

I have been able to find some basic information on what all needs to be done. But it seems like a lot of this type of info is usually pretty guarded, which I can understand. Nonetheless, I believe I have a pretty good grasp on all of the case mods required, at least from a conceptual standpoint. So porting the intake track to improve flow to the transfer ports, matching the transfer port ramp area on the top of the case to the cylinder (sry for the poor terminology there), filling all the voids with an aluminum infused epoxy such as Devcon to help with breaking through the case when porting, boring out the case for the larger sleeves in the BB PV cylinder and epoxying/reinforcing the bottom of the case and then trenching the bottom of the case for stroker crank clearance. With all of that said, I do have a few questions I am hoping someone will be willing to help me with.

Have I left out anything above that needs to be done?

When boring the case for the larger sleeves, how much clearance should there be between the sleeve and the bored out case? Numbers would be awesome but at the very least, are we talking press fit, slip fit or loose clearance fit? My plan right now is to simply duplicate the factory fit.

As for the trenching, how much clearance should there be between the crank and the bottom of the case and how wide should the trench be?

I realize getting the requested info above is rather unlikely, but at the very least if anyone is willing to either post, or PM me pics of cases that have been bored, trenched and ported I would greatly appreciate it. Specifically focusing on the bore, the trench, and any porting that has been done.
 
T

te4250

Location
Western NY
I’m wondering a lot of this stuff myself, as I was also planning on building a big bore stroker this winter. the only thing I can say to help is in all the different two strokes I’ve dealt with I have never seen a sleeve be a press fit or even tight slip into the cases, I’d go for an easy slip and err on the big side.
Also I think for trenching just measure the throw of the webs of your stroker crank and give a bit of clearance on top of that, with no experience whatsoever in trenching cases .005”-.010” radial clearance comes to mind lol. I do think you want as little crankcase volume as you can get so you get the most primary compression possible. You can always add crankcase volume later with reed spacers or something but would be hard to take crankcase volume away once you’ve built the motor. Hoping someone with actual experience with these motors chimes in
 
hornedogg79

hornedogg79

dodgin' bass boats
I had the same questions a few years ago. Hard to find much help here. There are some helpful older posts but you have to dig deep and know who's words to trust. You can run up to a +4mm crank without trench so a +10mm will need 6-7mm trench the width of the rod end. Make sleeves fit like stock. Pistons will be traveling 5mm lower so cut deep enough so the skirts don't make contact. Search hashtags of similar motors on Instagram. Pictures of others case work helped me more than anything.

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JKMike

JKMike

Location
Annapolis, Maryland
It makes sense that you would only want a slip fit for where the sleeves protrude down into the cases, as I'm assuming that only the skirt of the piston travels down that far, so that portion of the sleeve doesn't have to contain the forces from the combustion process. But I haven't torn one of these motors apart yet, so Im not certain on that yet. My plan was to simply make the sleeves fit the same as stock while making sure I have enough clearance for the increased stroke, like hornedogg79 said.

I had heard that you can run up to a +4mm crank without trenching, but I hadn't been taking that into consideration when trying to determine how deep to trench the cases. My plan had been to simply duplicate the stock clearances but clearly that would have been overkill. Good catch.
 
iQRTurbo

iQRTurbo

JKMike said:
It makes sense that you would only want a slip fit for where the sleeves protrude down into the cases, as I'm assuming that only the skirt of the piston travels down that far, so that portion of the sleeve doesn't have to contain the forces from the combustion process. But I haven't torn one of these motors apart yet, so Im not certain on that yet. My plan was to simply make the sleeves fit the same as stock while making sure I have enough clearance for the increased stroke, like hornedogg79 said.

I had heard that you can run up to a +4mm crank without trenching, but I hadn't been taking that into consideration when trying to determine how deep to trench the cases. My plan had been to simply duplicate the stock clearances but clearly that would have been overkill. Good catch.
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As you will find out, the trenching required for a 10 mill stroker is basically going to be cutting your cases in half. Almost certainly will be breaching daylight in between the webs in the bottom. Depending on the rod used, will depend how much daylight you will see. Some builders bolt and epoxy a plate to the bottom of the case to replace what was removed, a lot of work...
 
hornedogg79

hornedogg79

dodgin' bass boats
te4250 said:
Does anyone weld cases instead of epoxying?
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They warp too easy from the heat. Here's a pic of a 10mil. You can see where it pokes through the bottom. Both it and an 8mil dasa I worked on had disaster plates. I'm guessing it's right around that point it becomes a necessity.
e32f2a8b83e00b0836c44f44a68dc4f2.jpg


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T

te4250

Location
Western NY
Gotcha, nice pic, I feel like I'd want to just make billet cases at that point lol.


DylanS said:
Lamey has good instructions/pictures on his website for crank case porting! For sure worth a look.
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Where do I find this website? I did not have good luck googling lamey, dan lamey, lamey jetski, lamey yamaha. lol
 
J

Jarrett1999

Vendor Account
Nobody uses weld. You will need a disaster plate. Most use jb weld for the epoxy and jb quick weld to repair imperfections later if it doesn’t cure perfect or if you take to much out when porting. Also if your intake porting isn’t Damn near spot on you’re 1100 or 1000 isn’t gonna run much better than a 701
 
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