YZ250 Gearbox Upgrades - Broken 3rd or 4th gears on a Two Stroke?

This is a direct cut and paste from the start of this conversation in the other thread...
 

40 wrote:

1. Do you need to replace the YZ250 mainshaft and countershaft with YZ450F shafts when performing this retrofit?

2. Do you need to replace the YZ250 shifting cam with the YZ450F cam?

3. Do you need to replace the stock YZ250 shifting forks with YZ450F forks?

4. Are any YZ250 crankcase modifications required to accept the wider YZ450F gears?

5. Can you elaborate in detail on how the stock rigidly staked YZ250 shifting forks are changed to a slide-over-pin design per your modification? The rigidly staked YZ250 forks are an abysmal engineering oversight, allowing the pin/fork combined assembly to be weakly supported in the loose crankcase pocket while sliding into the tight opposing crankcase pocket.  Ideally, your modification would employ longer pins which allowed for equally strong support in each of the opposing crankcase pockets while the modified fork slides over the now stationary pins.

1) Yes, both shafts have to be replaced and the YZ450F main shaft must be modified to work with the 250 clutch.
2) No, I use the two stroke cam & ratchet mechanism.
3) Two of the forks have to be replaced with the four stroke parts (Left & Center).
4) The left case needs precise machine work and two special bearings to accept the new gears and shafts.

5) Abysmal... I totally agree and made pretty much the exact changes you described to that system.

I use pins from a 2005 - 2021 YZ125 engine and set them up to span the width of the cases.
A small rubber o-ring in each case bore can prevent the pins from rattling axially.  Many gearboxes have a spring inside the hollow pins.
These pins make the transmission a lot easier to assemble.  Just that change without changing gears and shafts is a major upgrade.

The YZ250 and early model YZ450F shift forks are not actually staked, they have spring tension pins (roll pins) that can be driven out using a punch.  I use a 10mm hone in the shift fork bores to allow a sliding fit on the YZ125 pins.  If the bore is honed oversize or if it has a lot of taper, it will rock and bind instead of sliding across the pin.

You end up getting wider gears with four dogs on all of them instead of three.  The "4th gear pinion" also gets a wider bearing surface on the main shaft, so it is less likely to seize or gall under high pressure.

Several years ago when I started exploring solutions I thought I would make a kit by modifying some new OE parts and machining shift forks.  I still have some prototype 4340 shift forks and the fixtures to make them on the CNC, but I've been busy with so many other things in my shop.  The old photo has an unfinished early prototype on a 450F main shaft next to the equivalent two stroke fork and gear.  The large boss eventually got machined down to become a cam-pin and I milled the corners off of the stiffening ribs to reduce weight.   I may revisit this stuff in the future.


...More cut and pasting...

 

40 wrote:

Helpful post and thanks for your response.

Can you provide more detail on the YZ450F mainshaft modification required for the YZ clutch?
Can you provide manufacturer and part numbers of the required special bearings?
Can you provide more detail on the required left crankcase mod? And are you able to modify customer cases in your own shop?

Thanks again from a fellow YZ gearbox sufferer.

Can you provide more detail on the YZ450F mainshaft modification required for the YZ clutch?

The threads and the splines for the inner clutch hub are too long. 
First, the splines must be cut down so that they are just below the surface of the hub to allow the lock washer and nut to properly clamp and hold the hub.
Second, the threaded end of the main shaft is too long for the two stroke clutch.  The throwout bearing assembly will hit the threaded end of the shaft and prevent full engagement as the clutch begins to wear.  Trim the threaded end down so that it only has one exposed thread beyond the outside surface of the nut.
Third, the left end of the mainshaft will protrude beyond the modified (narrowed) bearing in the left case.  The straight portion of the shaft on the left end must be shortened so that it is slightly shorter than the width of the custom 6004-RS bearing.

All of the shaft modifications involve hard turning or grinding, some of it is an interrupted cut.  A universal tool grinder with diamond wheels is the best way to do these three modifications to the main shaft.

Can you provide manufacturer and part numbers of the required special bearings?
I use a tool and cutter grinder to reduce the width of the OE Yamaha main shaft bearing.  I think it's a 6004-RS
I switch the original countershaft bearing for a part from the XR400 or Trax400EX.   I like to joke that I use quad bearings.

Can you provide more detail on the required left crankcase mod?
The left crank case needs more room for the teeth on 2nd gear pinion and 2nd gear wheel.
(this requires you remove and discard the bearing retainer tabs and screws for the countershaft bearing.
You also have to mill the pocket for the 6004-RS main shaft bearing about a millimeter deeper.

And are you able to modify customer cases in your own shop?

Presently, I'm not set up for it.  The YZ stuff was on the backburner for a few years until I built one for my buddy last summer.   I do each case one by one and dial them in on the machine individually.  To do a bunch or to do them efficiently and with repeatability I would have to build a fixture.   It's not out of the question, but handling customer parts one at a time is a pretty inefficient (expensive) business model.   I have also considered machining new cases that allow 450F gears to drop in without any special machine work.  The advantage to that is that I can add a fitment for a hydraulic clutch slave cylinder option.

Thanks again from a fellow YZ gearbox sufferer.
I feel your pain.

It's a ton of work.

For me it beats the heck out of replacing stock transmission parts every 30 to 40 hours and constantly feeling like I'm going to find "neutral" on a jump face.

Cryogenic treatment or re-heat treatment of the standard parts can improve durability a bit, but ultimately the design is weak and asks too much of the materials used.   Also, improving gear strength does nothing to fix the problems with the shift fork design and the fact that there are only three small dogs per gear.

I have considered making a small run of the 3rd, 4th and 5th gears, plus improved shift forks and pins as a drop-in kit for the YZ250.   

I've considered machining new pairs, (or just the left side) engine cases to allow dropping in the 450F parts too. 

Each solution has manufacturing advantages / disadvantages as well as pros / cons for the rider.
The 450F gear swap I described above is proof of concept enough for me to have confidence in what is wrong with the stock transmission design.
 

I don't do any social media, except an Instagram account (@dialedmfg) that I started for my retail parts business and I've barely done anything with that.  I am only familiar with the Magura "Jack" hydraulic clutch conversions and cut/weld setups like what Factory Honda HRC racing was doing on the previous generation of CRF450R race bikes in the USA.

I've ridden the Magura conversion on a friend's KX500 and I didn't like it.  It fixes the super hard clutch pull, but it has a damping feel, almost like the hoses or fittings or components have some serious flow restriction.

I have done some work on a conversion, starting with my 1990 KX500 and also keeping my YZ250 in mind but I think it's a unique approach and potential product, so I don't really want to share that publicly yet or promise anything to anyone.


I will make an Instagram post today, though... I did some work this week on a vintage YZ250 project for a local friend and I should probably share more of that kind of stuff.

I agree with that statement.  Starting the retail business for my suspension kits has also set up an avenue to sell other parts like these that may not be worth starting a business for, but would be a great addition to help establish my brand.   Most of all, I don't want to sell low quality crap and I don't think anyone riding/racing these bikes wants to buy junk.

From your post here, I think you have the basic lay of the land.

The modifications and changes described above aren't available as a product or service for the reasons you pointed out.
A drop in kit is the best solution for a product as far as I'm concerned.  Anything else is inefficient (expensive) and could be a big hassle for everyone.

I wonder if anything in that Fantic is true to the original YZ250, or if it's all new...
My experience with a 295cc big bore was that the transmissions last longer.  Most likely wear and tear was reduced because I was shifting a lot less.

I have everything in my shop to cut the gears except a few custom cutters to match the tooth profiles used in the YZ.
I also have a tumbler to run them through after heat treatment.

I try to squeeze in a few side projects per year between my wholesale orders.   I will complete the design work so that this can be one of those side projects when I have some free time on the machines.

I'm typing this at 11:15pm, Sunday night... I'm up late running two machines.
 

I hate to be a disappointment, but currently I'm not offering this as a service or a product, but if I do, I will come back here and make it known...
I have a working setup that is proven to be very durable, but lack a good business / production plan that makes sense for me and for potential customers.

More than likely, I can come up with a shift fork upgrade kit this year.

I did some simplifications and improvements on the gearbox upgrades a year ago while building some bottom ends for a pro racer from the northwest. I have his 2004 bottom end and another (2023?) bottom end on the bench right now for assembly... I've been too busy to assemble those for about six months, but I'm finally caught up to my wholesale orders and have time to work on a couple of these projects.

I streamlined a lot of the modifications, but I can't get away from doing some CNC work on the left engine cases. Currently, I haven't identified a practical business plan for processing the OE parts and keeping them in stock in an efficient way without the cost being very high. I have considered machining new left cases to my own specs.  That would be about the same cost as a modified OE case, but would allow me to offer other things like a hydraulic clutch option.