Whats needed when rebuilding a 426?

[viking20]

My friend wants to rebuild his 426 soon,what will be considered as normal "wear" parts in these engines ? He bought it second hand , and knows the previous owner installed a big bore kit of some kind , probably a 444 ? Isnt this the only "known" big-bore? Anyway, he is tracking down the first owner to cut down on the time needed for the rebuild.....It has about 70 hours on the engine , and he is a vet rider,never uses high revs at all.He keeps an eye on the valves,and is ready to change the piston , cam chain , and any valve work that might be needed will be done once its apart...Anything else ? Cam chain sliders , adjusters of any kind ? Any of you guys know the approx. cost of a bigbore piston , I would rather not disturb Eric before its time to order..
Thanks !

 

[Matt90GT]

My friend wants to rebuild his 426 soon,what will be considered as normal "wear" parts in these engines ? He bought it second hand , and knows the previous owner installed a big bore kit of some kind , probably a 444 ? Isnt this the only "known" big-bore? Anyway, he is tracking down the first owner to cut down on the time needed for the rebuild.....It has about 70 hours on the engine , and he is a vet rider,never uses high revs at all.He keeps an eye on the valves,and is ready to change the piston , cam chain , and any valve work that might be needed will be done once its apart...Anything else ? Cam chain sliders , adjusters of any kind ? Any of you guys know the approx. cost of a bigbore piston , I would rather not disturb Eric before its time to order..
Thanks !

Hard to say cause you dont state the year on the bike.

Basically get a service manual for a complete run down of what is needed. But here is my suggestion;

-aftermarket gasket kit (cheaper than OEM)
-yz450 exhaust cam
-wiseco 13.5:1 piston
-yz450 cam chain tensioner (shorter than the 426s)
-new valves if they are Ti

Other than that, do a valve job and replace parts that are worn.

 

[viking20]

Thanks for your reply , Matt....
Well since its a big-bore we will probably have to use aftermarket gaskets anyway....Off course we will replace everything that isnt ok , maybe I should have been more specific...What about the crank? If any of our sons were riding this bike we would probaby change the crank to be safe,but the bike is ridden by a vet rider with low revs almost all off the time,so I would think it would be ok to put more hours on it if it appears to be ok? When we order the parts from Eric , I will ask him if we should use the 12.5 or 13.5 piston....We can use octane booster , but race gas is not an option. Its very hard to buy any over here , and cost is about
20 US $/gallon....

 

[Matt90GT]

Again, check the specs against the service manual. That is the best advice as to what you will or will not need here. Most of the time, you will not know what you need until you tear into the motor. And that is not really a bad thing to say the least!

I have heard of many using 91/92 on the 13.5:1 pistons without issue.

 

[Rich Rohrich]

Viking20 - It's tough to say for sure on the crank but the 97mm (444) piston is heavier than the stocker and stresses the crank a lot hard, so it's cheap insurance to change the crank and the 2 main bearings while you are in there. YZF connecting rods have the unfortunate habit of breaking just below the small end bearing with no real warning. If that happens the cases usually get trashed.

The need for valves is fairly easy to determine once you disassemble the head, pull the valves and inspect them.
If you aren't planning on disassembling the entire head then a rule of thumb you can use is, if the clearances have tightened up more than 8-10% since it was new then it's probably a candidate for valve and spring replacement. Pulling it apart and inspecting it is still the best way to do it though.

 

[viking20]

Thanks guys, we will see what hes got inside the engine when its time, but most likely it will be the Wiseco 97 mm...
You never know though,no-one replates cylinders in our country , most people have it done in the UK or the Netherlands.... Its just that he obviously prefers as little down-time with the bike as possible , especially when ordering parts overseas.........Should have done it over the winter,maybe his sons bike ate all the money,LOL...