Best 2 stroke drag engine builder

[Vintagetz]

A lot of good suggestions but the dynamics for a drag bike based on a 400 crank are not good, although it could work in the dirt.  it would be a long stroke motor given a 62mm bore and a 56.25 stroke = 308 cc's

There is a drop in Yamaha piston for that build, it is a YZ125 (period) piston, stock bore is 56mm so a first over would put you right there.  you will need the right pipes to make it work and as I said in the dirt it might work.

The alternative is to send a crank to Crankworks, they will stroke a crank at a reasonable cost. 

you can use a 2mm oversize YZ piston @ 58mm and get the stock crank stroked to 58mm which is a +4mm from stock and is a common stroker crank for a banshee /RZ, it is known to work well.

On a drag racer, I am not sure I would even lengthen the rod, I would use a  2mm spacer plate to even out the additional stroke.   A 115mm rod would increase the crankcase volume a little and 250's run well as a long rod configuration. 

Given you are drag racing there is not a lot of benefit to getting later model European 250 cylinders, they have longer fins and larger heads that are as big as the 400's and help with a road racer.  They run billet heads on drag racers with no issue given the short run time.

The advantage to stroking a 250 is that you don't get into a torker motor (long stroke) configuration

58mm bore X 58mm stroke yields 306 cc's

On a 62mm stroke (400 crank) with a 56mm bore you end up at 305cc's, the issue will be RPM and piston speed.

The reason I say it might work is hookup in the dirt.

I am not sure how big you can get a YZ piston BUT at 60mm you are at 305 cc's and there is PLENTY of meet to re-sleve  a 250 to get it at a 60mm bore.
This is the easiest way to get to the class limit and you know it works its just a slightly smaller bore than the 350.  This is the way I would go.

I just checked and there are 60mm YZ pistons available from multiple sources.




It will be EXPLOSIVE as coming on the pipe,  MORE than porting you will need the right pipe design.

[Nkreig]

How far are you from mount pleasant? Eric Thomas (the Kaw Shop) bored my cylinders and did some other work for me. He drag races an H1. He may not be able to do the work you're looking to have done, but I bet he knows who in PA could.

+2 on Eric, he bored my 75 350 and will do my TZ replica here soon as well.  Great guy and does top notch work.

[Vintagetz]

I have been thinking about this and have the following suggestion IF you have the budget. 

1. I have built a full race (what some would call a stage 5) RD350 motor using 400 cylinders and it did very well.  I built it for Mark Morrow one of the fastest RD road racers for the past 20 years.  He is a MULTI MULTI time national champion.  He wanted to try and run an RD 350 against the 400's

Specs were stock 54mm stroke, 115mm 400 rods and a 66mm blaster piston displacement = 370cc's

I decked the top and bottom of the 400 cylinders to make them match up with the 54mm stroke.  In doing this it greatly reduced the port work because the 400 ports are already Taller because of the longer stroke so they had advanced timing on the 350 bottom end,  115mm 400 rods.  No spacer plate is required.   Because I used the blaster piston and the 115mm rod I took very little off the bottom of the 400 cylinders and enough off the top that it removed the top fin, this was simple machine work.

I had to reduce the 400 head CC's to get the compression right.  Mark won many races on it against the 400's but he had to work hard against the additional torque and power of the 400s (his riding skill did a lot to help).   He has raced many RD's both 350's and 400's, his words, This is the fastest 350 I have ever raced.

On to your project, if you find a builder I am happy to share details,  I would do as you hypothesized,  use the 400 cylinders sleeve them down to run a 60mm YZ 125 piston (it's like a blaster piston shorter pin to the crown than a 350/400 piston), I think its the same as an RD250 piston as they are also shorter pin to crown. 

If you can get a set of European 76 - 78 RD250 heads they are the same size as the 400 cylinders and the smaller combustion chamber will make it easier to machine them for the 310 CC displacement (larger than the older US-spec, I have 2 RD250 Daytona heads that also match the 400 cylinders, I might let one go).

I think this will yield the best possible results with the easiest path, all the machine work is very simple and straightforward, and it will be easier to port because of the 400 cylinders.  I would opt for aluminum sleeves with nickasil bores...


Fell free to reach out to be if you are intested in discussing this

[JR]

Hey VintageTZ thanks for all your input and expertise on my project. John Ritter will be my builder, and I'm sure that any and all ideas will be appreciated. Hopefully when completed, we will have one seriously nasty drag bike!!!! JR

[Vintagetz]

John has his own idea's good luck with the build.