350X Big Bore require trans upgrades?

[cduemig]

I'm rebuilding my 85 350X from the frame up. Hopefully I'll be getting on the list to pickup the 400X big bore kit from ATC-Eric soon. I've got two complete engines, one has output shaft welded on and chain damage to case, the other is currently in my bike, has chain damage to case but has been fixed using JB weld putty (reformed the case, not just filled the hole). I'm getting a bottom end with no chain damage to build up my 400X project. I figure while in there I might as well go all out and get an aftermarket midrange cam, stiffer springs, hardweld the rockers, and a buddy is doing a very mild port and polish job on the head. My goal is to build a powerhouse but keep it still trail worthy. I'm tired of eating dust from my brother's Rapter 660.

Anyway, really long story short, will a stock bottom end hold up? I know the trans is suppossed to be near bulletproof, but what about the rod / anything else I haven't thought of? I got a ton of NOS replacement engine parts including a new honda crank / rod / main bearings. I was thinking if the bearings feel good on the bottom I'm getting to just build it from there, but if there's a risk of breaking this thing I could upgrade the rod on this new crank and then split the cases (yuck!) and install the new crank. Any thoughts from those who've gone big? Any other suggestions?

[Xowner]

ask the person that your are getting the big bore kit from, they should be able to tell you exactly what you need!

[NOS_350X]

Your tranny will be fine. They are just about bullet proof.

[Dirtcrasher]

I haven't heard of ANY bad 350X trannys and many had upgrades, so you should be AOK.

It would be nice to run a NOS crank in there for a Big Bore......

[TheRealFatShady]

I actually heard from a guy that was selling a top end that his tranny exploded or something of the sort. This was earlier today. I have a spare 350x engine but the valves dropped and got smashed on the piston/head. Head doesn't look repairable.

[Billy Golightly]

Only thing I've ever heard of happening is clutch slippage when running the big FT500 cylinder heads and making A LOT more hp than stock.

[TheRealFatShady]

Only thing I've ever heard of happening is clutch slippage when running the big FT500 cylinder heads and making A LOT more hp than stock.

I may look into that. Know how much work was involved in the swap?

Edit: Thanks for the info. Maybe a full ascot 500 engine can be used, as there wouldn't be issues of the top end oil situation, and the rocker arm dilemma. Unless they are way different?

[Billy Golightly]

Check in the mad scientists lair. The post was done jason85atc250R and SWIGIN from what I can recall.

[SWIGIN]

I have a ascot head swap thread in the mad scientist lair.

the trannys do hold up on a majorly built 350x motor but you'll more then likely only get a few years (maybe 5) out of a tranny. The first thing you'll find out is the clutch wont hold up, I cover that in my thread.

If your clutch holds up with normal HD springs you have nothing to worry about.

[cduemig]

I've heard quite a bit about the ascott swap. I'm not going quite that far. I bought honda fibers and Barnett springs so hopefully that'll do the job.

[SWIGIN]

Thats the best you can do without going to custom springs.

[jensenracing77]

the stock bottom end will be fine. X2 on a nos crank. the honda crank will hold up better than aftermarket ones will.

[big specht]

I have heard they will hold up untill you get around 500cc then they will start braking shafts in the tranny 2 guys I pull bikes with have 350s that are around 496cc area and haven't had and tranny problem the trannys but we don't shift when pulling or use any gears above 2nd