Performance Tech Tip; 350X Big Bore questions answered

[MR ATC]

ok i think the question of big bore 350X's is one of the most popular. it seems like people always ask how big to go and what kits are out there. while this is not a 100% complete list as there will always be a engine builder doing his own thing. it is rather extensive. first i will give the basic displacements. then i will discuse options and differances

piston...................68mm crank (stock).............74mm (6mm)stroker



81mm (stock)................350.4cc........................381.3cc
81.5mm ........................354.7cc........................386cc
82mm............................359.1cc........................390.7cc
83mm............................367.9cc........................400.3cc
84mm............................376.8cc........................410cc
84.5mm.........................381.3cc........................414.9cc
85mm............................385.8cc........................419.9cc
85.5mm.........................390.4cc........................424.8cc
86mm............................394.9cc........................429.9cc
87mm............................404.2cc........................439.9cc
89mm............................423cc...........................460.3cc
89.5mm.........................427.8cc........................465.5cc
90mm............................432.5cc........................470.7cc
90.93mm.......................441.5cc........................480.5cc
92mm............................452cc...........................491.9cc
95mm............................481.9cc........................524.5cc


as you can see thier are many different sizes to choose from.

one other thing you may notice is there are two ways to achieve the same displacement. however although they will be the same cc and have the same peak H.P. they would be completely different in there charecteristics. let me explain by using the stock 81mm piston with a 6mm stroker that achieves 381.3cc vs just boring the cylinder for a 84.5mm piston to get the exact same displacement.

the 81mm/74mm bore/stroke combo is a more "square" design. by that i mean the bore and stroke are closer to the same size as the 84.5mm/68mm bigger bore set up. what does all that mean? a squarer motor will produce quicker and more revs and have a broader H.P. range. meaning it will produce more H.P. at the lower RPMs then the 84.5mm bore only motor. this translates into a more usable powerband with the bore/stroke combo. wereas the bore only will have a slower reving powerband.

this is why all the new four strokes have faster reving motors then the older designs. this produces a better "race" motor. before four strokes were mostly trail bikes/quads and a slower reving motor was more desirable for "play" use.

now i know there are more displacement options for a 350X. some going up to 550cc. and some have used old XR500 top ends to increases displacement.

lets discuse the XR500 option

basically a stock XR500 uses a 89mm piston. if you look at the chart you will see this as a option for a 350X big bore. however ( and i have discused this with Micky Dunlop formerly of Powroll and now owner of Four Stroke tech. he was the first person to use a XR500 top end on a 350X and has since abandoned the practice. ) the XR500 has a less advanced head design. plus the cam does not have the added oil passages of a 350X head. what does it mean? a 350X cylinder/head with a 89mm bore will run better then a XR500 cylinder/head.

some things to note. cylinder sleeving will be required on the larger displacements as will case machining to accept the larger cylinder.

and this is just a list of piston/stroke combinations. there are over dozen differant cam profiles and severall carb options as well. not to mention oversized valves and head porting. each also depend on your displacement, type of power you want, and your wallet.

hopefully this will be of some help to those wanting to go BIG on there 350X's

this is why i never have a direct answer to the "how big" question

with all these options it depends on more of your wallet and how you ride.

[360X]

Great info Bill:D

[Troll]

Nice write-up.................................:thumbsup:

[3WheelsForever]

Mr atc what all has to be done to the xr 500 cylinder and 350x bottom end for it to work. What about using the xr 500 cylinder just to gain the 89mm bore and then using the 350x head and opening up the ports for more flow? What kind of work would this take?

[MR ATC]

honestly i never used a XR cylinder. Micky Dunlop talked me out of using them years ago. i just install a XR sleeve in a 350X cylinder. its cheaper for me this way since i'm a WISECO dealer and they sell sleeves.

as far as work to be done...the 350X center cases have to be machined out to accept the larger cylinder.

[3WheelsForever]

So you can intall the xr 500 sleeve and only have to make the cases to fits the cylinder? And then you can also use the xr piston? What year sleeve must be used?

[MR ATC]

79-82 XR/XL 500

[Rivertoys]

WOW! What an awesome thread!!! Thanks.

Remember, I'm new to ATC's....What should the ballpark cost of having this kind of work done be (Dropping off an engine, and picking it up, ready to install)? I'm most interested in the 400.3cc Stroker option.

Of the combo's you listed, is there an option that's more common or "standard"?

When you stroke these engines, is there any issues with breaking a rod due to the steeper angles it's being pushed through?

Lastly, would the tranny need to be beefed up to handle the extra horses?

Thanks!
RTJas :D

[greenhuman]

You can use the head off a Honda FT500. They do require a small amount of mods to work but you can use your standard 350X rocker box and 350X cylinder and most people would not notice the difference. The FT head has valves much larger than the 350X but externally it is almost identical to 350X head. They have 35mm inlet and 30mm exhaust valves with larger diameter stems than a 350X. You can run a high compression FT500 92mm Wiseco piston and a 74mm Powroll stroker crank. Powroll can shorten the 350X rod an extra 3mm so the piston does not have the top cut to fit. This combination is very reliable and will produce a serious amount of horsepower.

[NOS_350X]

What kind of modifications do you need to do to fit the Ft or xl or xr 500 heads? I'm probably going to do this to my X this next time i tear it appart.

[armyvet]

A xr 500 cylinder WILL not work without extensive work.The cylinder studs holes will need to be drilled out to accept the 350x studs,the cylinder bolts on the cam chain side of the cylinder bottom dont line up with the 350x,if you still think you can make it work like i did once you get the cylinder on the cases and the head,you'll find the studs to be about a 1inch and a half too short.Looks like i'll be going the xr500 sleeve in my 350x cylinder route.