Holy Grail of XS400 Big Bore Pistons

So I thought Id comment to bring this thing back up we to find over sized pistons for the sohc xs400. Doesn't have to be big bore just so we can rebuild it

Random eBay NOS xs400 pistons. That's it. No other bikes pistons will work without modification. Or fork up the money for Wiseco to make them, and go broke:shrug:
 
There was a german site that had oversize pistons/rings for these bikes. But the price with shipping and fees was something like $300-400 a set:eek:
 
I Don't Know man, seems with all the bikes out there something can work

You can make anything work. The problem is $$$. By the time you have something modified to fit, you could have had new pistons made, or just have bought 5 working motors.

Everything is a trade off with these motors. They just weren't/aren't popular enough for the aftermarket to sink money into.
 
I think they are becoming more popular there's a couple hundred memeber here and on other sites. Mikes has a couple things for it now includung a pamco which is pretty cool. Maybe soon hell have pistons
 
They even have standard rings for my 360. Converts to around $150 CAD though.

Not worth it yet.
 
As much as I hate to add fuel to this fire......

I am contemplating going a different route to increase displacement. Stroke. We use a local company to do crank regrinds at my work. I've talked to the guy that owns the place, and he said taking some material off of the rod journals is cheap as can be. I've run it through a few test in Solidworks and if I got the variables close, it 'should' hold up under normal use, but I can't be sure until I try. Making the rod journals a smaller diameter greatly increases the stress on the crank, and add to that the extra stress of the higher displacement. In my experience with doing this in 2 strokes, things tend to only last half as long as they do stock.

Benifits of this? If I take 1 cm off of the rod journal diameter, I can get about 480 cc out of this motor. I still have to take a look at the motor, but I dont think that at this stroke any modifying will need to be done to the crankcase. Standard pistons can be used.

The downsides? Increased stress as already mentioned. Custom rods and bearings need to be manufactured (still slightly cheaper than custom pistons and sleeves). Compression ratio needs to be kept in check, so either a thicker headgasket will have to be made, head chambers dug out a bit, piston dome material removed, or a combination of these. Or use race gas.

Bottom line is, this ends up being less work than going big bore, and slightly cheaper than having bigger sleeves and custom pistons made up. But at the same time, the motor will not last long. Not a big issue if it isn't driven 5k miles a year or raced each weekend.

Whenever I get a quote back from Crower on the rods, I'll post it here. Will probably be around $500 a pair.
 
Why not aim for a target compression ratio from the beginning to avoid having to modify piston crowns or dome volumes?
What ratio are you going for?
I don't have it in front of me but I believe Alexander Graham Bell recommended increasing compression ratio from 9.2:1 to 10:1 for our bikes.

Increasing the compression ratio to 10:1 would mean a displacement increase to 426cc and by your method a stroke increase from 52.4mm to 57mm.
Only a 4.6mm difference rather than 1cm.

Have them grind the radius down to meet that stroke and you shouldn't have to modify the cylinders, pistons or head, and the stock head gasket will work, while still maintaining the highest possible crank strength.

It's late though and my math could be off.
 
These bikes are only 391cc so it's even less. Just get a bigger/better bike:wink2: If I have to put 3-4k$ total into a xs400 to get drop the weight and make 45hp I would buy three very nice stock ones instead.
 
Because for the price of the rods, going from 400 to 426 ends up being quite pointless.
What's the point at 442cc?
If you're just doing it for the hell of it, then that's cool.

I didn't do the 10mm math last night. You might not need to do any modifications to keep the compression ratio under control.

Double checked and Bell recommends increasing the ratio to 10.2:1.
At 442 and without any further modifications your compression ratio will be around 10.4:1.
Might not run into any issues with it that high.
 
You guys want more performance out of these thing ? I don't think it's the bike for it just want pistons to be a little bigger for rebores. If I can increase the comp to 10:1 that'd be nice, but I'm not trying to go crazy with the bore, just rebuild. I love the way these little guys run stock
 
I didn't do the 10mm math last night. You might not need to do any modifications to keep the compression ratio under control.

Double checked and Bell recommends increasing the ratio to 10.2:1.
At 442 and without any further modifications your compression ratio will be around 10.4:1.
Might not run into any issues with it that high.

The problem with increasing compression over the stock 9.2 (or whatever) is that you start running into bigger problems. Someone might recommend 10 something, but in my experience, on non EFI motors with no dynamic timing, no knock sensors, no way to adjust fuel, etc, is bad news. You could jet richer, retard the timing, use higher octane gas and all, but now you are back to where you started performance wise. Maybe if you live in the rocky mountains its acceptable, but here at 200 ft, no way.

What's the point at 442cc?
If you're just doing it for the hell of it, then that's cool.

Well for $500 I'd rather have 440 than 420.
 
These bikes are only 391cc so it's even less. Just get a bigger/better bike:wink2: If I have to put 3-4k$ total into a xs400 to get drop the weight and make 45hp I would buy three very nice stock ones instead.

I'm already deep in the rabbit hole :doh:

No turning back now.

Also, my bike is going to see track days more than main roads.
 
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