racing gs425

[mark herbert]

Hi guys, new to the forum! I'm looking to build a classic race bike from ua gs425!
what I could do with is any info you may have of tuning the motor, i can go to500cc, so that would be 72.5mm bore max, does anyone know if there any aftermarket pistons available,or if any different bike pistons fit?
same applies to cams, any aftremarket, or do other gs ones fit with an improvement?
I will probably fit 34mm round slide carbs ( unless anyone knows better)
any exhaust specs would also be usefull.

cheers Mark.

[GregT]

Haven't done a 425 as I feel the 450 plain bearing crank is a better proposition for race use, however all the GS add ons will work on those too.
Any aftermarket GS big bore 2 valve pistons should fit - I'm pretty sure the gudgeon pin size is common. Cam profiles likewise - just got to ensure you use the appropriate buckets - shim under if over about .420in lift.
Pipe - I recommend a 2 into one - 1 1/2in OD headers around 27in long, tailpipe 1 7/8in OD at least 24in before the muffler.
Plenty of info on here about tuning the twins - and depending on your rules/age break, you may even be able to use the 400 8 valve head.

Good luck

[hampshirehog]

Hi Mark and welcome to the GSR. Out of the box I think GS1100G pistons will fit - that'll give you 72mm bore on standard size. Or GPZ1100 pistons might fit as well - you'll need to check out gudgeon pin diameter (2 different sizes that I know of on the GPZ) plus of course pin to crown measurement.

I don't think anyone does special cams for the 425 so you would have to get your own made up. It might be worth looking at 450 / 500 heads and see what swaps over as well.

[mark herbert]

pistons

thanks guys, I believe the 425 has 16mm gudgeon pin!? do the gs/4 cyl have same size? I was informed the 1000's were bigger?
but it could all be internet b-----t!

[hampshirehog]

That scuppers the GPZ pistons as well - they're 17mm and 18mm if I recall correctly. I think the GS450 uses 18mm gudgeon pins - but that's no good is it (thinking out loud....). I think this might be a tricky one to crack off the shelf.

[Suzuki mad]

Modern day GS500 pistons may fit and Kent cams http://www.kentcams.com/ (if they are still about) could solve the other issue.

[GregT]

450 and GS500 pistons are 18mm gudgeons....have to have a think about this one,,,,

[GregT]

From what i can find it boils down to two options IMO...

First - and circumstances might dictate this way - change the rods to either "big" GS ones dependent on the big ends being the same. Or have rods made to suit the 425 big ends with 18mm gudgeons. Carrillo aren't that expensive - and will do small number orders.Arrow in the UK would be worth a call too.
This will let you access all the big bore Suzuki pistons.

Second - There are 72mm big bore Honda 750 SOHC pistons around. But they use a 15mm pin. So if you can find a set, there's some tricky machining to do enlarging the pin bore - and doing a circlip groove.

Your call - but if your crank is marginal, new rods and big ends are needed anyway.
This is why I like the 450.....

[pete]

The 450 should be a close to straight swap into a 425 frame too... no kick start though if you want the electric leg gone.

[mark herbert]

Hi guys thanks for all your input!!
the guy I'm buying off has just let me know its actually a 450!!! that appears to amke life easier according to your listings, has anyone got specific info on this motor? i would like to get into the 50-60bhp area if possible, hopefully with bore/gas flow/cams/exhaust/carbs.

cheers Mark.

[hampshirehog]

You're away now. Check out this thread - there are several others as well:

http://www.thegsresources.com/_forum/showthread.php?t=182159&highlight=gs450+pistons

[pete]

Yeah everything I've seen says the 450 bottom end is a much better candidate... and I believe that's the basis they went forward with the GS500's on that they still produce now.

[BassCliff]

Greetings and Salutatoins!!

Hi Mr. mark herbert,

Sorry I can't help with your "go fast" project but I can dump a TON of information on you and share some GS lovin'.

I just stopped by to welcome you to the forum in my own, special way.

If there's anything you'd like to know about the Suzuki GS model bikes, and most others actually, you've come to the right place. There's a lot of knowledge and experience here in the community. Come on in and let me say "HOoooowwwDY!"....

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", "Top 15 Tips For GS Happiness", the Carb Cleanup Series, and the Stator Papers. All of these tasks must be addressed in order to have a safe, reliable machine. This is what NOT to do: Top 10 Newbie Mistakes. Now let me roll out the welcome mat for you...



Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff