What Is Needed for Athena 421 Build From Scratch

[Calum]

Hello all,

Hoping someone can give me some guidance on this. I've tried searching, but you either get far too many hits with regards to builds or conversions. I'll try and keep this brief.

I'm looking to build an RD350 31K engine from scratch and use build an Athena 421 cylinder conversion.

My question is, what parts do I need to do this.

I want to use the Banshee 350 crank cases, as these can be had brand new.

• Banshee 350 Crankcases
  
• Athena 68mm Big Bore Cylinder Kit
  
• Yamaha RD 350YPVS Complete Crankshaft + 4mm Hot Rods

I am assuming this is all I need for the engine component list.

My question is, is there anything I am missing. I plan on using Banshee 350 crank cases, but am I okay to use RD350 YPVS 31K parts for everything else? Transmission/Clutch Basket/Misc Gears.

Edit:

**Pinning Useful Links for Future Reference**

Big Bore Primer

[JonW]

Also... pistons for +4, carbs/filters, pipes, ignition, head work, rad.... etc etc

[Calum]

Hi JonW ,

Yes the pistons are included in the big bore link.

Pipes/Ignition/Radiator will be made to suit as it's not for an RD chassis.

I'm not sure what I want for carbs yet, I don't want big/big power. I just don't want to run YPVS for both cost/complexity of build.

I have read that some ran stock 26mm carbs from their 350 to the 421. I was thinking more along the lines of two 28mm Flat Slides. The fuel tank on the bike isn't huge and I don't want too much power.

Head work I'll leave to a specialist as that's out of my skill set.

[JonW]

Have a bit more of a read around (on here and elsewhere) at the 421 Athena builds as that will give you info like; You need different pistons and headwork or a spacer to run the +4 crank. The Athena kit is 392, not 421 out of the box etc.

My 421 build may still exist on the wayback machine (try web.archive.org/web/20211126221932/http://2smoked.com/2_stroke_obsession_-_www.2Smoked.com.html ), it gives a bunch of info but I am not the only person to build one so have a good read on here, search works to find the builds i would think.

I also wrote a big bore primer and the link to the wayback machine is : web.archive.org/web/20190731230135/http://www.2smoked.com/Yamaha_RD_&_RZ_upgrades_-_RZ350_RD350_Big_Bore_Primer.html

Wayback isnt perfect but it has a lot of the info, just not very pretty and some pics are missing.

It should all be a good grounding in what youre looking at I would think?

[Calum]

Okay, brilliant thanks JonW .

I guess my main point of concern is, I am okay to use the Banshee 350 crankcase with all 31K running gear right?

I am already the first paragraph in and feel like this is going to be a good read. This ought to be archived.

Firefox has a great "Text Only" mode which clears up the erroneous HTML page.

[headcoats]

You can use ypvs crankcases if they have that wider seal area at the front
Don't know which particular models though

[LC_BOTT]

Speak to Norbo as well, he has most of the parts available including complete kits. Might save you a few £££s if you are buying all at once? Depends also on what you 'need' or what you 'want'

www.rdlccrazy.co.uk/html/for%20sale%208%20ypvs%20trick%20bits%20page.htm

[Calum]

LC_BOTT Perfect thanks.

JonW Perfect read thanks. That article was basically what I was looking for.

By the sounds of it, sourcing the bits independently isn't the way to go and I should probably speak to my builder as they have offered to source all the bits and assemble the engine. Your article reaffirms this. They also build these engines day in day out and have just done my brothers build. So again rings all the right bells. I was just looking to spread the costs a little, but sounds like this could cause more hassle than it's worth.

[LC_BOTT]

I went the opposite way and tried to source everything as cheaply as possible Not so easy at the moment, but depends if you are prepared to wait for things to become available. Saying that, its probably been 5 years since I started buying parts. You will definitely be finished before me too.

[midlifecrisisrd]

Save yourself some money and buy used cases

The later 1ua/1WW cases usually have the wider frontal cylinder mounting area like these ones

www.ebay.co.uk/itm/296315443610?itmmeta=01HT5PV366T8Y7KYAM37EX6GDY&hash=item44fdc6d99a:g:aoYAAOSwx8JlfNRP&itmprp=enc%3AAQAJAAAA0F9GANvj%2BogvBIr3S1oXRUvSMVPaVli1fPGcsiDsc0Ruz0tvFRtKuckmq5UTegIDe0TGbDbohzcLExxNwZXIlh7ZO0E260h46Gtv2sU%2Fzy6fjwvbpaDa5BXjfG%2Fh4vC%2FUCvc90zwOqcwmCzEcX%2Bz%2FpcQBMOOAdan0RgQ26XzOSCgrbUBCGDh%2FuQ%2FEtApIEsIfHk7thz8cRBFEW6SdmomVdx4S7tpd17EFH%2Fo3mRGcx05YjrtqSgpvCo95lokXzoomQ5pFpZuaTsbLrEbM0%2Bpgrk%3D%7Ctkp%3ABk9SR5az7LbRYw

The seller is on here as kasaprossa

Steve

[muttsnuts]

banshee cases have the wider frontal area

Building one from scratch isn't hard, but knowing exactly what parts are needed, is less obvious

I build loads for people and each one ends up with some tweak or other, unless you have access to a lathe and stuff, you'll end up either hitting issues or farming out some jobs to companies that can do the work, there are a good few "gotchas" as well that you need to navigate if doing it for the first time

Also, depending upon which way you build it up, you need to make a careful note of or port timings etc and other engine parameters like squish clearance etc as you'll need that to design some pipes to meet your criteria unless you have access to some test pipes or pipes that are known to work with your engines setup

You will also need a programmable ignition (ideally) and you need to decide how you are going to time your engine up allowing for the extra 4mm stroke as that changes the timing etc

[Calum]

JonW I hope you don't mind, but navigating wayback machine is a bit of a faff. I've re-hosted your blog on my website (which is kept backed up), and set it to text-only mode for ease of reading.

dt125r.co.uk/assets/uploads/files/YamahaRDYZUpgrades/Yamaha%20RD%20&%20RZ%20upgrades%20-%20RZ350%20RD350%20Big%20Bore%20Primer.html

It's a really insightful blog and I've decided just to build the engine from scratch using parts I can find on the internet.

So far I've got the gearbox, selector drum, flywheel, clutch covers and an assortment of other parts. I'll look to get the cases next month and hopefully the stroked rods the next. Then the bottom end should be able to be assembled to some degree and I will order the cylinder kits and a local shop near me has agreed to do the machine to 421, they've done my brothers and quite a few others.

Thanks again for all your advice people!

[Calum]

JonW I hope you don't mind, but navigating wayback machine is a bit of a faff. I've re-hosted your blog on my website (which is kept backed up), and set it to text-only mode for ease of reading.

dt125r.co.uk/assets/uploads/files/YamahaRDYZUpgrades/Yamaha%20RD%20&%20RZ%20upgrades%20-%20RZ350%20RD350%20Big%20Bore%20Primer.html

It's a really insightful blog and I've decided just to build the engine from scratch using parts I can find on the internet.

So far I've got the gearbox, selector drum, flywheel, clutch covers and an assortment of other parts. I'll look to get the cases next month and hopefully the stroked rods the next. Then the bottom end should be able to be assembled to some degree and I will order the cylinder kits and a local shop near me has agreed to do the machine to 421, they've done my brothers and quite a few others.

Thanks again for all your advice people!

[JonW]

May 2, 2024 14:33:42 GMT 1 Calum said:

JonW I hope you don't mind, but navigating wayback machine is a bit of a faff. I've re-hosted your blog on my website (which is kept backed up), and set it to text-only mode for ease of reading.

dt125r.co.uk/assets/uploads/files/YamahaRDYZUpgrades/Yamaha%20RD%20&%20RZ%20upgrades%20-%20RZ350%20RD350%20Big%20Bore%20Primer.html

It's a really insightful blog and I've decided just to build the engine from scratch using parts I can find on the internet.

So far I've got the gearbox, selector drum, flywheel, clutch covers and an assortment of other parts. I'll look to get the cases next month and hopefully the stroked rods the next. Then the bottom end should be able to be assembled to some degree and I will order the cylinder kits and a local shop near me has agreed to do the machine to 421, they've done my brothers and quite a few others.

Thanks again for all your advice people!

No worries Callum.... A thought: You might want to add some text at the top to say that this is lifted from my old 2smoked.com website and that you have my permission to rehost it... also its worth mentioning that the prices are old, I wrote that 10 or so years back.

Good luck with your build.

[Calum]

May 2, 2024 14:49:54 GMT 1 JonW said:

May 2, 2024 14:33:42 GMT 1 Calum said:

JonW I hope you don't mind, but navigating wayback machine is a bit of a faff. I've re-hosted your blog on my website (which is kept backed up), and set it to text-only mode for ease of reading.

dt125r.co.uk/assets/uploads/files/YamahaRDYZUpgrades/Yamaha%20RD%20&%20RZ%20upgrades%20-%20RZ350%20RD350%20Big%20Bore%20Primer.html

It's a really insightful blog and I've decided just to build the engine from scratch using parts I can find on the internet.

So far I've got the gearbox, selector drum, flywheel, clutch covers and an assortment of other parts. I'll look to get the cases next month and hopefully the stroked rods the next. Then the bottom end should be able to be assembled to some degree and I will order the cylinder kits and a local shop near me has agreed to do the machine to 421, they've done my brothers and quite a few others.

Thanks again for all your advice people!

No worries Callum.... A thought: You might want to add some text at the top to say that this is lifted from my old 2smoked.com website and that you have my permission to rehost it... also its worth mentioning that the prices are old, I wrote that 10 or so years back.

Good luck with your build.

Added a note to the top of the blog post. (Empty cache and refresh)