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426, HC stage1 build, several questions

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Hi All, long time lurker on TT, especially the XR and Beta sections.

 

I've had my 03' XR400 for about 5 years and it still puts a smile on face every time I ride 

it. The truth is, I think it's so darn good that I'll most likely never sell it, even if I add a newer mate to the stable.

 

Of course I'm always looking to improve things so I've been thinking of doing some motor work. The bike already has a Mikuni pumper, a Pro Circuit exhaust, and the suspension has been resprung and revalved for my weight.

 

There's so much info regarding big bore kits and would like to run my plans through you more experienced XR folks and get hopefully some feedback. I see there's some build threads going on, but every situation is a bit different. The XR runs great at the moment but smokes a bit at startup when stopped for a week or more, but for less then 5 seconds so it doesn't concern me much. I still want to keep that in mind when I tear into the engine.

 

My plan is to go with a 426(88mm) 10.1:1 piston and an HC stage 1 cam. Replace the timing chain with a CRF one while apart. I would like to go with this compression because I'm in CA and we have crappy gas here. I've heard of people getting detonation with 11:1 pistons with premium fuel and I want to stick to pump gas. I don't have a set budget for the build so if something makes sense for this build I'll do it. I mostly want some more power, but want to keep it as reliable as possible. I don't want to go into the engine again for several years besides valve adjustments and clutch plates when needed.

 

These are my main questions in no particular order(this will be my first engine build):

 

1. Is it recommended I change out the valves while inside the engine. I've read a lot about the stainless steel valves. I'm guessing the stock ones will be in good shape. Is going oversize 1mm worth it?

2. If I do valves, do I do springs as well as guides and seals?

3. Is a 3/5 angle valve job worth it?

4. Is a stage 1 cam the best option with the 426 and a pumper or would a stage 2 work better? I like the low end and mid of the bike, that's why I think stage 1. The things is it already has great bottom and mid.

5. Is the stock connecting rod adequate or should I replace with stronger one. If I replace the rod, do I need to split the cases?

 

I would like to start buying parts soon but won't tackle this until August/September.

 

Thanks so much and I'll update this same thread when I start te build. I probably missed several things as well :)

 

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Hi All, long time lurker on TT, especially the XR and Beta sections.
 
I've had my 03' XR400 for about 5 years and it still puts a smile on face every time I ride 
it. The truth is, I think it's so darn good that I'll most likely never sell it, even if I add a newer mate to the stable.
 
Of course I'm always looking to improve things so I've been thinking of doing some motor work. The bike already has a Mikuni pumper, a Pro Circuit exhaust, and the suspension has been resprung and revalved for my weight.
 
There's so much info regarding big bore kits and would like to run my plans through you more experienced XR folks and get hopefully some feedback. I see there's some build threads going on, but every situation is a bit different. The XR runs great at the moment but smokes a bit at startup when stopped for a week or more, but for less then 5 seconds so it doesn't concern me much. I still want to keep that in mind when I tear into the engine.
 
My plan is to go with a 426(88mm) 10.1:1 piston and an HC stage 1 cam. Replace the timing chain with a CRF one while apart. I would like to go with this compression because I'm in CA and we have crappy gas here. I've heard of people getting detonation with 11:1 pistons with premium fuel and I want to stick to pump gas. I don't have a set budget for the build so if something makes sense for this build I'll do it. I mostly want some more power, but want to keep it as reliable as possible. I don't want to go into the engine again for several years besides valve adjustments and clutch plates when needed.
 
These are my main questions in no particular order(this will be my first engine build):
 
1. Is it recommended I change out the valves while inside the engine. I've read a lot about the stainless steel valves. I'm guessing the stock ones will be in good shape. Is going oversize 1mm worth it?
2. If I do valves, do I do springs as well as guides and seals?
3. Is a 3/5 angle valve job worth it?
4. Is a stage 1 cam the best option with the 426 and a pumper or would a stage 2 work better? I like the low end and mid of the bike, that's why I think stage 1. The things is it already has great bottom and mid.
5. Is the stock connecting rod adequate or should I replace with stronger one. If I replace the rod, do I need to split the cases?
 
I would like to start buying parts soon but won't tackle this until August/September.
 
Thanks so much and I'll update this same thread when I start te build. I probably missed several things as well :)

 

The low comp perfect,stock valves are fine for xr4,but not for xr250. new valves,valve seals.I like the web cam stage 1 cam even with my 440.Stock rod is perfect as long as it is not worn out.Stock valve job good,You dont need 1mm over size valves.Our 440s been running way over 10 years.I would put Barnet HD valve springs in it.Make sure your cam rocker arms are in perfect cond,Or will eat new performance cam. BTR

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On all of our Xr4s never replaced any tensioners or guides. On two Mexicans bikes here ,had to replace all of it WHY. Because they keep riding there bikes for years,with cam chain shot. Is your bike a high miles.?

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Bike is not high miles.

Got the following ordered:

Wiseco 10:1 piston

Hot cams stage 1

CRF450 timing chain

Cometic top end gasket kit

Kibblewhite valve springs

Barnett HD clutch springs

That's it for now. I'll update once I start the process. Local shop will do cylinder bore and check valves and seats. They're most likely fine.

Thanks

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I just added to the parts list.

Viton valve stem seals

Kibblewhite black Diamond intake and exhaust valves.

I'm thinking of doing the valve guides too since everything else on the head is getting replaced? Decided to get new valves and if the current ones are good I'll use them on.my brother's build who's XR4 is older.

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