Big bore kit reliability?????

I scratched the inside of my cylinder wall somehow this last spring. I intended on having the cylinder re-coated and sticking in a Wiseco piston/rings. Stroker offers a 420 big bore kit for $400. For those of you that have a big bore kit (Clark and MotoGreg), how is the reliability, vibration, bottom end, etc?


I got my kit through Eric Gorr. I sent him my cyclinder, he had it bored, coated and then milled the cyclinder base to get a reasonable squish band. After the work and cleanup he sent me the cyclinder and Kit (piston, rings, pin, and top end gaskets). I believe he charged me $325 which is a good price for this work and kit. I believe Eric normally charges $375 and if you also send him your head for porting the price is around $450. Check with Eric for current pricing.

I believe that Stroker uses a pressed in liner and not a bore and recoat on the cyclinder. I prefer the bore and recoat route as I feel it improves cooling efficiency. I'm sure Stroker and the Sleeve suppliers will say there is no difference with a properly installed sleeve. I disagree with this position.

I have approximalty 1800 miles on the motor since I installed the 417cc kit. I went with the 13.5 to 1 Weisco piston. This means I have to run a little race gas mixed with 92 to lessen the chance of detonation (I use 1.5gal of VP C-12 along with 3.5gal of Union 76 92octaine). You can run the kit with a 12.5 piston just like stock. The 13.5 gives better bottom and mid range performace at the expense of a few hundred RPM on top--more power and torque in the rpm range where I do the majority of my riding.

I have had ZERO problems in the 1800 miles since installation. I feel the vibration level is the same as the stock bike.

99 WR 400


Clark did you do any work to your lower end as far as the crank and big end bearings go prior to the big bore kit? I run a stock 99yz400 just had the valves adjusted along with throttle cables, anyway the mechanic who did the work mentioned that my lower end bearings were louder than usual and that they could go out at any time causing catastrophic engine failure. My question to you is how due they know it is the lower end bearings and not something associated with the clutch? I would like to go with the big bore kit but I am concerened about the lower end. What do you think and how long should the crank bearings hold up? Is there any telltale signs like shavings in the filter or something obvious like that?

thanks, huge

Hey guys,

I've been running the 420 kit with 13.5 to 1 comp for 1 year now with zero problems. My WR is on the MX track 98 percent of the time. The only problem I have on the track is with the gearing. I am thinking of going to a 01 YZ250 or a 01 YZ426.

I have done no work to the bottom end on my 99 WX 400. I have installed a Hinson Clutch basket and Heavy Duty Clutch springs as personal insurance against clutch problems. But I did this as a peace of mind kind of thing and not because anything was wrong. Remember the new 426 clutch has a larger diameter (gives you more clutch force area to work with) and 6 vs 5 springs as on the older WR's and Yz's.


I'm trying to do more MX with a WR400. How do you compare the stock power vs the big bore kit you have? What about suspension.

Thanks Clark sounds like your lower end is pretty stout. I guess i'll rebuild it rather than buy a new one.


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