gs400 cams

[herbie 1964]

Hi again guys, trying to dig out more info
I'm intending racing a gs400, & i'm searching for tuning ideas, would the 500 cams fit, or can a exhaust be used in inlet for any improvements?
Heard rumours about using gs1000 pistons? & cutting a gs1000 cam down, any knoledge!?

thanks for any advice, or other ideas/info you may have.
cheers Mark.

[burning1]

Hi Mark. First thing you need to do is figure out where you're planning to race the bike, and what mods are legal. 1000 pistons may fit - the heads are similar, and the pin diameter is probably the same. But you should double check. Planning to swap pistons on my bike, and found a general list of things to consider:

1. Pin diameter
2. Deck height
3. Dome diameter
4. Dome height
5. Valve size
6. Valve angle
7. Valve clearance

2 valve GS models are very similar, but there are some small differences that can bite you. E.g. GS450 pistons have a larger dome than 500 pistons, despite being smaller bore... And the difference causes some clearance issues.

[herbie 1964]

intend racing in the UK in a 500cc aircooled class for pre 81 bikes! so mods are open as long as the external appearance is per period bike.
cheers mark.
I'm aware of things I need to check,but wondered if anyone had done it so knew of easiest route,

[burning1]

Unfortunately, I haven't done that particular mod. My understanding is that GS1000 pistons will generally be a good fit for the GSTwins bikes, but I can't confirm that. The guys at APE Racing or the piston manufacturer may or may not be helpful at providing the right measurements.

Also, have you looked into installing one of the 4 valve heads?

http://www.thegsresources.com/ would be a great place to check if you haven't already done so. There's a lot more GS400 guys over there.