Holy Grail of XS400 Big Bore Pistons

[aaronxs400]

talked to a couple guys, from the LOOKS of it, the rod CAN be enlarged to accept a 17mm wrist pin. so now we can use the Raptor 250 pistons which was my plan all along.

Also, xschris, did you take the photos of the cylinders? if so, can you measure something for me? crazypj said if you measure distance between the studs DIAGONALLY, and subtract 6mm off, thats the max bore size.

 

[xschris]

Took a pic of the stud holes and the top of the sleeve sizes.

 

[xschris]

(og-rider)-The tranny and clutch should be fine with that. (Yamaha used the same gear set in models with over twice the HP.)

Do you mean the dohc models or the sohc ones. The sohc clutchs only have a 4 bolt/spring set up where the dohc has the 6 bolt/spring deal also a bigger basket.

 

[aaronxs400]

hell why cant we use the stock liners to 78 or so mm?

 

[xschris]

Because they are only 75.5mm thick and the case is only 76mm

 

[lcgtr]

I looked into doing something similar myself but I don't believe you can safely take out the pin 1mm let alone 2.
I think looking at New rods that are correct length is the only safe option. Plus unless your getting dirt cheap machine work done you may as well just get custom pistons made, Arias will do them. You could possibly do a group buy to get the per item cost down

 

[OG-Rider]

While I was out riding today, I became really intrigued by the lighter crank Aaron posted. I know from personal experience with race bikes that when you reduce the weight of the rotating assembly by a couple pounds, the performance characteristics of your engine change A LOT.

The weight difference between the 360 and 400 cranks is INSANE! The XS400 crank probably robs the engine of a whole bunch of potential snap: acceleration and net horsepower both suffer if the rotating assembly is to heavy.

After looking at Chris' measurements, I want 72mm pistons; nothing any larger. A guy at Wiseco already told me he can probably hook me up with compatible pistons from their existing stock, but first I have to know the wrist pin placement height, and I won't be tearing my engine down until about February.

Anyway, I was getting pretty discouraged. About ready to sell the XS400 and get something more amenable to hot rodding, but we finally got a break in the weather today, and I went for my first ride in about 6 weeks. I'm good now

 

[XS400Cup]

I use Yamaha XJR1300 rods and modified Kawasaki GPZ900R pistons (diameter 72.5mm).
The stock sleeve allow max. 73mm pistons.
You can also use FJ1100/FJ1200 rods, but the better way are XJR1300 rods which are lighter and stronger.

 

[OG-Rider]

I use Yamaha XJR1300 rods and modified Kawasaki GPZ900R pistons (diameter 72.5mm).
The stock sleeve allow max. 73mm pistons.
You can also use FJ1100/FJ1200 rods, but the better way are XJR1300 rods which are lighter and stronger.

What did you have to do to the pistons you used?

 

[XS400Cup]

The piston crown is modified.
The photos show the piston before and after modification.

 

[aaronxs400]

problem is finding those xjr1300 rods here in the states, we didnt get that model. BUT we can buy aftermarket FJ rods.

 

[XS400Cup]

XJR1300 rod Yamaha Part Number: 5EA-11650-00

http://www.boats.net/parts/detail/yamaha/Y-5EA-11650-00-00.html

 

[Greasey Fingers]

The piston crown is modified.
The photos show the piston before and after modification.

Hi mate! The piston crown looks totally different, what did you do and was it hard? Looks like a nice upgrade. What about carbies what are you running?

And how many horses are you getting from it after all that, whats the approx top speed?

Looks great my friend

 

[XS400Cup]

The piston crown looks totally different, what did you do and was it hard?

The piston crown looks a little bit funny but it was necessary to reach 10.8:1 compression.
Therefore i could remove material from the cylinder head to improve the airflow near the intake valve.
(Sorry i didn´t made photos.)

What about carbies what are you running?

Original Carbs.

And how many horses are you getting from it after all that

Not testet, no idea.

And how many horses are you getting from it after all that, whats the approx top speed?

115mph at 10500 rpm

 

[bentwrench]

115! WHOOOOOOO!

 

[XS400Cup]

The fairing is very effective.

 

[xschris]

115mph I would figure a lot more with those kinds of mods What sprockets do you use.

 

[bentwrench]

Wind resistance increases as a square of speed, and required horsepower as power to the third. Doubling speed quadruples drag, and requires an octuple in horsepower to overcome.

Or so wikipedia tells me...

But I always just listen to the school of Dr. Mad Max: Speed is just a question of money. How fast do you want to go?

 

[XS400Cup]

115mph I would figure a lot more with those kinds of mods

Anything is possible. Do it.

What sprockets do you use.

Sprockets are 16/37.

 

[Greasey Fingers]

Wind resistance increases as a square of speed, and required horsepower as power to the third. Doubling speed quadruples drag, and requires an octuple in horsepower to overcome.

Let me translate that into english....

Dont bother trying to get speed out of an xs400, lol.