Holy Grail of XS400 Big Bore Pistons


XS400 Enthusiast
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Mansfield, OH
After hours of research last night. Here is what I have come up with for bore options on the XS400.

If I was to build this I would use either a 78 or 79mm raptor 250 piston, ream out the small end of the rod to 17mm and use the raptor 250 wrist pin as well. These pistons can be purchased with up to 13.5:1 compression.


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Ok, this is what I would like to know. I don't think the stock stuff will work for the sizes you are talking about. I know the are a few xs650 guys boring there's out to 900+cc but they doing major machine work to them also there is aftermarket blocks that can use larger piston. And the cranks on them are custom made. I just don't think the xs400 crank and clutch setups can handle the big power jump.:shrug: Even if you could shoe horn a bigger set in there would it not be easier to get a bigger bike setup for more power? Don't let me stop ya:laugh: Like to see it:thumbsup: You do know they made a DOHC xs500:)


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you make a great point xschrist. i have both motors, ill have to tear em down and see how close the studs are. i know the dohc has wider cylinder spacing. worst case scenario, i have the lighter xs360 crank on the way, ill mate that up with some 70-71mm pistons and be done.
That is what I was talking about. The sohc xs400 engine is very limited in what you can do with it. Which is ok with me I don't care about speed anymore just riding:) Have you weighed the xs360 crank to the xs400? I think they are the same. The xs250 might be a lighter one.
I know a lot of 1L9 prefix stuff is the same parts as the 2a2 just newer models. But with the 360 crank it looks( from seeing pics) to be different all together. Of course back then how many people where doing mods like this to xs360-400's:laugh:
i ebayed some pics of xs400 cranks and xs360 cranks. it does indeed look different from the angles as far as i can tell. now, the reason for all this? im building a land speed bike.
For a bike that wont see the highways it don't need to last for 30k miles. But for such a difference in design there must be some reason why they only made them for two years and then went to the 400's design:confused:
probably to offset the vibes

You're probably correct. Heavy flywheels reduce vibrations. They also use up potential horsepower. Having owned 4 Harleys and two BSA singles, vibrations ain't shit to me:D

Lighter flywheels result in more HP to the primary sprocket, and allow the engine to achieve peak rpm's quicker. That's why we used total loss ignition on some of our old school race bikes. I hope Aaron keeps us posted on this crankshaft business.
thats a DOHC block, GS500 1mm oversized pistons and liners

I got your PM with the piston info. Thanks, it's good stuff. I like the GS500 pistons because that is a fairly popular motorcycle that is still in production, or at least it was still in production very recently. That means good parts availability.

The stock GS500 piston is 74mm. 74mm X 52.4mm = 451cc. That seems about right to me. The tranny and clutch should be fine with that. (Yamaha used the same gear set in models with over twice the HP.) However, the wrist pin diameter concerns me. I'm not sure the XS400 rods have enough material to safely enlarge them 2mm. A top end coming apart at 9000 revs ain't pretty! Maybe you can take a good look at a rod, and measure the thickness of the material around the wrist pin hole when you get a chance.

We need a "Crazy Engine Builders" sticky thread:yikes:
Who ever does this needs to get the stock bike on a dyno and then put it on after to see what kind of numbers the changes make:thumbsup: