High RPM engine building

[Vintagewannabe]

To put it simply, I don't have as much experience building two strokes, and I wanted to know what it would take to make my RD350 safe to rev a bit higher.
I have been told that the points get unhappy at 10krpm, and I have read a couple things about people having the cranks welded somehow to hold up better.
I basically want to get everything together for when I eventually do a total engine rebuild.

[m in sc]

well.. lighter pistons, (vertex a good option) lighter ignition (would recommend HPI as its the smallest one). TBH, the cranks are OLD, the weights come loose at this age, even repressing them is a huge PIA and risky, i've had them come loose again. I run my vitos crank up past 10 all the time. it has a pork chop crank (no weights, its cut like a car crank sort of) and IMHO safer. also they coem with really good japanese bearings.

another option is to change the primary gear to a 250 style. that means 250 clutch/primary/oil pump (if you are on the pump). this actually reduces the speed of the clutch and transmission overall, reduces windage drag, and the gearing can be 'made up' on the front chain sprocket

[Vintagewannabe]

well.. lighter pistons, (vertex a good option) lighter ignition (would recommend HPI as its the smallest one). TBH, the cranks are OLD, the weights come loose at this age, even repressing them is a huge PIA and risky, i've had them come loose again. I run my vitos crank up past 10 all the time. it has a pork chop crank (no weights, its cut like a car crank sort of) and IMHO safer. also they coem with really good japanese bearings.

another option is to change the primary gear to a 250 style. that means 250 clutch/primary/oil pump (if you are on the pump). this actually reduces the speed of the clutch and transmission overall, reduces windage drag, and the gearing can be 'made up' on the front chain sprocket

I had seen the vito's, and wondered if it was better. Thats good to know. I had also never heard that about the other primary gearing from the 250 until now, and yes I run an oil pump. How hard is it to source those pieces these days?

[m in sc]

not sure. probably pretty easy via ebay. you have to run a 250 pump though, the internal gearing is different. YES it will feed a 350 motor just fine, been there done it (on an lc conversion as well).

the only issue might be the 350 basket set up for a 6 disc clutch vs 7, but the replacement baskets they sell are 7 disc compatible and can be retro fit onto a 250 clutch gear.

[Vintagewannabe]

How crucial is the 250 setup? I don't plan to beat on mine constantly, and am not trying to squeeze every bit of horsepower possible out.

[m in sc]

not crucial at all, not required. just an option

[JKV45]

What pipes do you have?

That will also be a factor.

[IR8D8R]

To go further on this. Is there a reason you shouldn't weld an OEM crank Vs buying a Vitos? If those are the only 2 choices and you really want to spin it. Since we already know that old stock cranks may come apart after so many repeated trips above 10k? Sure you may not be able to build it again. You can grind out a weld if it isn't just blasted solid. Or buy the aftermarket next time you need bearings.

It only takes a tack weld or do you need to really lay a deep bead on it?

IR8D8R

[Vintagewannabe]

What pipes do you have?

That will also be a factor.

Wicked motorsports

[m in sc]

because stock cranks can and do have the weights come loose. i have a really nice welded crank that has maybe 2k on it (tig welded, its out of my white lc hybrid) that the weights came loose and started tapping the rod.  not worth it to me, and i build my own cranks. yes, i even pressed the weight back in on the rebuild. and sorry, the current vitos cranks are just better quality overall.
 

[RDnuTZ]

fwiw, I bought 1 Vitos and 2 from HPR on eBay. Have not installed any yet, but visual inspection shows top notch quality on both. Vitos uses the KOYO Japan crank bearings IIRC and HPR uses TPI Taiwan bearings. The HPRs are a bit cheaper at listed price, but I had Vitos on my watch list for months and they eventually sent me an offer for 10% off. HPR lists both a 250 pn compatible and a 350 pn compatible. I bought 1 of each and both came with wrist pin bearing for 350 so confirm whatever you will get either way you go.

https://www.ebay.com/itm/364763626092?

[JKV45]

What pipes do you have?

That will also be a factor.

Wicked motorsports

Can you get a dyno chart from them to see what the powerband looks like and at what RPM they taper off?

You would want to make sure they were capable of the revs you are going for.

[SoCal250]

What pipes do you have?

That will also be a factor.

Wicked motorsports

Can you get a dyno chart from them to see what the powerband looks like and at what RPM they taper off?

You would want to make sure they were capable of the revs you are going for.

[Yamanatic]

Just an aside: DS7/RD/R5 cranks are virtually identical to TZ250/350/750 cranks. TZ cranks live 90% of the time WFO above 10K, put out about twice the HP of an RD, and don't come apart or twist unless rebuilt many-many times ... and it's still a rare occurrence. A rebuilder can tell by the tons of pressure required and the 'feel' splitting and assembling if the pin-fit is too loose; Yamacranks are amazingly reliable as is.

RZ cranks are a different story - they tend to spread apart 'more than twist.' I've seen a couple spread enough to bind up the motor; usually it's the timing-side flywheel that comes adrift.

Myself and virtually every TZ racer/rebuilder puts the pox on crank welding; never have, never will.

Warren

[m in sc]

I agree. but they also don't see the run time a street bike sees. my r5 crank made it 27k before it started to spread to where it was an issue, but the weights were fine.  bu also a low rpm motor. imho comparing short high rpm use race bike cranks to street bike cranks are apples and oranges.