maximum piston diameter on a stock 68mm stroke?

[sfab]

i got a few questions for the knowledgeable guys on this site. ive already built a 61x bigbore and found it has amazing low end punch with nice all around power as well. i like to experiment a bit and was thinking about maximum pistons diameter that can go on a stock 68mm SJ stroke. i know theres already the BB sleeve that can be bore to 86.5mm but i was thinking if it was possible to go with 88mm XP951 seadoo pistons with a seadoo sleeve modified to suit the SJ stroke.
theres a 10mm difference in stroke but im sure the ports can be lowered to match the ones on a 61x. With some transfer ports job (enlarging) could it be possible to run those pistons? im a machinist so i have access to almost any tooling for fabrication. what would be the limitation in that kind of design (im sure theres some...) and how reliable it would be if it was possible to build? since the 61x will offer no support if it was bore to fit those sleeves, they will probably sit in a billet cylinder that will be similar to the 61X with some modifications for the transfers and exhaust ports. i also think that i might have a problem with the wrist pin diameter but i have a few ideas to fix it.
am i just too ignorant or it could be a possible project? :biggthumpup:
thank you!

 

[Matt_E]

While we're both waiting on the knowledgable guys to chime in, why not just get a Lamey cylinder, or an ADA cylinder?

 

[SuperJETT]

88 and 90 bores have been toyed with on a 61x cylinder before with custom sleeves, but didn't work out well enough to market.

The best option is to go ADA or Lamey, but $$$

 

[sfab]

While we're both waiting on the knowledgable guys to chime in, why not just get a Lamey cylinder, or an ADA cylinder?

well, i actually hate my job, but i freakin love to build performance parts for my SJ!!! :biggthumpup: id like to build something really big that do not need to be stroked with no major cases mods. the ADA engine price is just to high (2000$ cylinder + 1600$ stroked crank and other extras..) while a big block of T6-6061 aluminum, some stock XP sleeves and pistons and lots of time is cheaper in my book. :biggrin: but more seriously i think it would be a great project and if it worked, will be the cheapest option to get maximum displacement.

 

[Matt_E]

You will have a hard time increasing the cylinder size to accomodate larger bores.
As you noted, the stock cyl is not beefy enough and doesn't have enough transfer volume past certain bores and sleeve diameters.
Great, so just increase cylinder block size, right?
Unfortunately, that will not be easy either. You cannot increase the size right between the jugs - so you're limited on size.

 

[sfab]

You will have a hard time increasing the cylinder size to accomodate larger bores.
As you noted, the stock cyl is not beefy enough and doesn't have enough transfer volume past certain bores and sleeve diameters.
Great, so just increase cylinder block size, right?
Unfortunately, that will not be easy either. You cannot increase the size right between the jugs - so you're limited on size.

ahhh i see. but how ADA manage to do this? they still use the same crankcase i assume, with 118mm piston center to center so maybe theres a way to expand the transfers without breaking through the other side... i know there is some limitation with that kind of setup but im sure theres also a lot of advantages of running bigger pistons... im not looking really to make money out of this project but simply to experiment a bit. im no expert in 2 strokes engine but close to one as a machinist :bs2: :haha: thats why i need input as if its a good idea to try or a wrong one... :smile:
thank you!

 

[Matt_E]

You could reduce the cooling jacket space between jugs to get some transfer volume.
Billet aluminum probably allows for thinner walls than casting.

 

[SuperJETT]

ahhh i see. but how ADA manage to do this? they still use the same crankcase i assume, with 118mm piston center to center so maybe theres a way to expand the transfers without breaking through the other side... i know there is some limitation with that kind of setup but im sure theres also a lot of advantages of running bigger pistons... im not looking really to make money out of this project but simply to experiment a bit. im no expert in 2 strokes engine but close to one as a machinist :bs2: :haha: thats why i need input as if its a good idea to try or a wrong one... :smile:
thank you!

Contact Paul (Jr.) at LPW, he could probably give some insight.:wink1:

 

[crammit442]

well, i actually hate my job, but i freakin love to build performance parts for my SJ!!! :biggthumpup: id like to build something really big that do not need to be stroked with no major cases mods. the ADA engine price is just to high (2000$ cylinder + 1600$ stroked crank and other extras..) while a big block of T6-6061 aluminum, some stock XP sleeves and pistons and lots of time is cheaper in my book. :biggrin: but more seriously i think it would be a great project and if it worked, will be the cheapest option to get maximum displacement.

That's about right on the cylinder price, but remember it comes complete with a girdled head and domes. I don't know who you're getting your crank price from, but that's about 300 high. Crankworks is only 13ish retail.

Charles

 

[Speedfreak]

If your a master machinist then get you a block of aluminum and some pictures of an ADA or lamey cylinder and build one on your own. You will have to bore the cases to match and then throw a set of 90mm pistons in there. Heck who knows you might stumble onto something that no one can live without.

 

[sfab]

If your a master machinist then get you a block of aluminum and some pictures of an ADA or lamey cylinder and build one on your own. You will have to bore the cases to match and then throw a set of 90mm pistons in there. Heck who knows you might stumble onto something that no one can live without.

thats what i wanted to do, experimenting a bit... i might give it a try in a few month. right now, my 61X bigbore is more than enough so no need to rush this project...

 

[Cross]

As a fellow machinist with experience in billet cylinders, I too have thought about making a billet cylinder. Then I decided just to go out and ride during my free time. Plus by the time I was done designing, machining, porting, testing, ect. a cylinder I could have bought a couple Lamey and be done with it!!! My time is worth $$$.

Brian

 

[Jr.]

The OEM 61x cyl casting has several limiting factors. the first & foremost is the casting it's self. The overbore can really only go only as far as the typical BB allows, the reason is the material that is currently there.
once that is cut away, the sleeve will be exposed & subject to the cooling elements, That & compounded by the lack of alum for both support & the damping it provides, the cyl will have a very short lifespan.
The second is the volume of the transfers, this is limited. For you to keep increasing the bore Dia, you will effictively reducing the transfer volume........... ?

The DASA & ADA cyls have increased volume due to added transfers & port area.......... A much better option........ Both mechanicly & Dollar wise!! THese can effictively use the 88-90mm bores

To answer your origional question, I feel the cyl started loosing potential beyond 86.5mm
I have built them to 89mm......... they didn't work!!

 

[LegDragger64]

I am running 86mm weisco pistons in my re sleeved superjet , is there an 86.5 regular dome , non flat top piston available ?

 

[QJS]

I'm with Paul on this one, you can get quite a bit more transfer area if you are machining a cylinder from scratch I have one on my machine as we speak and the transfers are quite a bit bigger. It is also safe to assume that larger than stock pistons will need this extra volume, you will trade bore size for transfer volume and at some point will start going backwards ie you don't have enough transfer volume for the size pistons therefore you will reduce power. The question is at what point does this occur? I am going to stick with smaller 85-86mm pistons so that I can keep the transfer area up,
If you go ahead with this project good luck but make no mistake it is a massive job to make it work.
Chris

 

[sfab]

thanks all for the replies.
i think i will give it a try this winter. it will be basically a XP 951 copy that will fit the 62T case, with power valves . the cylinder and porting will be build around the NW BB sleeve that came fully ported (p/n 1431B). im just unsure about the stroke of the crank... i want it to be almost like the XP (78mm) but im not sure if its too much. Crankworks have this stroker in stock tho... the ADA bigbore kit use a 8mm, maybe theres a reason.. i will need the crank before finalizing the drawing to get the exact height on ports and cylinder. tons of job to do but also tons of funs :biggthumpup:

 

[#ZERO]

thanks all for the replies.
i think i will give it a try this winter. it will be basically a XP 951 copy that will fit the 62T case, with power valves . the cylinder and porting will be build around the NW BB sleeve that came fully ported (p/n 1431B). im just unsure about the stroke of the crank... i want it to be almost like the XP (78mm) but im not sure if its too much. Crankworks have this stroker in stock tho... the ADA bigbore kit use a 8mm, maybe theres a reason.. i will need the crank before finalizing the drawing to get the exact height on ports and cylinder. tons of job to do but also tons of funs :biggthumpup:

That will be an interesting winter project. :scratchchin:

The stock 951 has an 88mm bore with a 78.2mm stroke and produces a 130 hp with dual 46mm mikuni carbs.

I'm sure that you could get a little more power from those pre-ported big bore 951 northwest sleeves.

 

[BombThreat]

power valves... yummy