I'm thinking of starting some motor mods pretty soon, and I was wondering what size bore I should get. I don't really want to go with a 440 just yet. So, I've decided I'll go with a 406, 416, or possibly a 426. What do you guys suggest? What's your experience with these bores? What are the pros and cons of each? The modifications I plan to do are below.

JE 10.8:1 or 11:1 piston
XR400 or HRC cam
Mild Porting
Boring
39mm FCR carb

 

Just depends on what you're comfortable boring to... With displacement, usually the more the better.

Personally I run 87mm, which is 416. We were able to lighten the piston to within 10g of stock, and the engine actually feels balanced better than stock.

If you think about going to 426 (88mm) be advised that the cylinder is getting REALLY thin. Personally, I don't know if I'd be comfortable going any more than 416 on a stock sleeve, as even that is getting pretty thin.

Let me know if you have any other questions....

 

I have heard that the 416 is the way to go on 400ex bikes. I have seen some of them that make more peak hp than a strong 440. The 416 is suppose to be a lot smoother engine than the 440s due to its superior bore/stroke deminsions.I have ridden a few 440s and they are strong but are really rough engines.The 416 kits are suppose to be able to rev a lot cleaner than the 440. I really dont have any experience with the 406 or 426 kits, but im sure that they would be good kits also, just stay from the 440s.

 

i wasn't real impressed with the 416 that i rode. but when i drag raced it kicked my butt. i don't think the 406 would be worth the cash. especially if it costs the same as a 416 or 426.

 

The 416 sounds like it's the best out of those. How thin will the cylinder be? More than a comfortable amount? I don't want to decrease reliability greatly. Also, with a 10.8:1 piston, will I basically get the same performance increase as an 11:1 piston? Now, on to cams. I've looked at the XR400, HRC, and GT Thunder cam. The local tracks aren't super-tight. Nothing like you would encounter on wooded trails. They are more open, so I figure I'll need more mid to top end power. I don't want to lose any low-end power if I can keep from it. Please tell me what you would suggest. Thanks a lot.

 

Here's what I would get:

416 10:8:1 JE Piston
GT Thunder Cam
FCR 39MM carb

No race gas needed.

Also maybe you should send in your head to get a mild port job for $50 at GT Thunder.

 

I echo LDJ's recommendation....

At 416 the cylinder wall is still at a comfortable level, I have had no overheating or reliability problems what-so-ever...

 

Im running the 416 piston but Ive also had my crank stroked so my total displacement is 440cc which I think works great(so far).FCR carb should be here Friday and the TCS 0 preloads next week I hope they're worth the money.

 

if you don't get head work done you won't see near as much power increase. the head work does a lot more than you would think, go ahead, while you have the case split, and get some 3 angle valve jobs and porting, etc.

 

How much would the other mods that bigred250x mentioned cost? Where could I have them done? Would they be worth it? I don't want to decrease reliability very much, but I want some extra power.

The piston, cam, and carb look good to me. I just want mods that are going to be worth the money. Please help if you can.