twinport gearbox shims/clearance

[stump magnet]

can anyone advise on the clearance i should look for between the gear and shaft on the end of the two shafts in the gearbox?
have just fitted a modified single port gearbox into my twinport and have fitted the original thrust washers and have a clearance of around 0.3-0.35mm at the moment, still in dry build so can add shims if need be although the clearance seems ok considering everything will heat up when running what do you guys rekon?

[090]

I can't believe mr. Jikov hasn't replied here as he is an encyclopedia of cz knowledge.
I am certainly interested in what went into putting the side pipe gearbox into the twin pipe. Care to share?

[Bamford#69]

Hi,
If you haven't already done so , assemble the gearbox , without the crank, in the cases and tighten up the clutch shaft and countershaft sprocket nuts, assemble the selector rod and forks, and put the shift plate in , check the clearances between the shift plate guide slots and the selector fork pins in all gears and neutral , the shims are there to make sure that gear shift pushes the gears all the way into gear , you will soon see if you need to adjust the shafts ,to left or right to get the full engagement of the dogs, or to make sure you don't have a binding selector fork, when in neutral or in any gear,
this is the best way to check what size shims to use  when assembling any CZ  gear box ,
I have sectioned out the top cover of a 5 speed box , where the shift mechanisms are in the gbox cover, to allow for measuring  the shift mechanisms of the later 5 speeds with the cover in place . 
 I'm sure Brad would like to hear as well , how you get on using a SP box into the  Twinport, I know the they are similar but definately not the same , have you documented the engineering invovled? ,
 Its a pity the 250 twinport box isn't the same  as the 360 TP
cheers       

[stump magnet]

To fit a single port box into a twin port, right here goes, (deep breath) you need two single port gear sets, the reason is that you need the sprocket shafts from both sets as the clutch shaft is too long as it holds  the dry clutch in a single port engine.
you press the gears of the clutch shaft and off one of the sprocket shafts then press the sprocket shaft gears onto the clutch shaft but beware the smallest gear (1st) is too wide and will need reducing or ideally use the gear from a 5 speed set as its narrower, now the fun really begins, measure the length of the two original twin port shafts from the thrust face of the pressed on gear to the shoulder of the 15mm bearing dia and compare this to the single port shafts, you will notice the single port ones are longer by a few mm therefore the difference will need to be removed from the shoulder of the bearing dia by cylindrically grinding the shaft (effectively lengthening the bearing area of the shaft) then reduce the overall length of the bearing dia to obtain the correct width to fit the bearing so it doesn’t protrude.
now for the next tricky bit, you will need a primary gear (the BIG one that drives off the clutch) from a 360 twin port!! hens teeth eh, so instead get a knackered sprocket from a single port and machine it down so that you have the splined part plus a bit of dia left, then machine out the centre of the twin port 250 gear to accept the machined sprocket centre and weld the two together concentrically and accurately remember to measure the distance from the bearing side spigot to the inside face of the gear teeth on the gear before you machine the centre out so you can replicate this measurement on the gear you have made otherwise the gear will hit the cases or touch the back of the clutch.
because the single port shafts are 20mm dia as apposed to 17mm of the twin port, the bearings need changing in the cases for the primary gear end and sprocket end, the sprocket end bearing is a 3204BTV Double Row Angular Contact Ball Bearing the primary end bearing is a 6204. the other end of the shafts use the same bearing as the twin port (6302)
next you need to machine a spacer 20mm bore x 25 diameter x 1.4mm thick this goes onto the sprocket shaft between the new double row bearing and the gear this makes up the difference in width between the new 3204 bearing and the old original one.
now you can fit it all into the cases  for a dry run, use the original 15mm dia thrust washers, you need to then measure the shaft end float and aim for around 5-6 thou (0.13-0.15mm) no less, use shims to achieve this.
now you need the shift forks from a single port 4 speeder also as the twin port uses different ones and the shift plate as the gears are in a different order  otherwise you will go off the line in 1st  then into 4th  then 3rd then 2nd  that would be fun eh! You then need a sprocket of your chosen size for a single port, then a new shaft seal 38 x 28 x 7 ( you may need another 1.4mm spacer between the sprocket and bearing to bring the sprocket out a bit)
the beauty of this conversion is that the shafts are much bigger so less flex and the most important thing is the single port gears use 6 dogs rather than 5 so bullet proof and the gears are relatively easier to find and you will have a spare set as you needed 2 sets to start with.
Have just completed mine today and all gears select although I haven’t started or rode the bike yet, a guy over here has been using this set up for 10 years without problems.
Many many thanks go to Adrian Rampton for arranging and sourcing the parts for me.
The bike in question is on the front cover of the latest edition of classic dirt bike
hope this wasn’t too long winded let me know what you think.

[tony27]

Theres a twinpipe gearbox on ebay at the mo

http://cgi.ebay.com/ebaymotors/cz-motocross-250-cc-968-twin-pipe-parts-original-/250764968226?pt=Motorcycles_Parts_Accessories&hash=item3a62c1c122

[stump magnet]

ive gone through two original boxs already so im gona stick with the SP one as havnt broke the one in my 360SP yet

[stump magnet]

on closer inspection the dogs look very worn on 2nd and 3rd and the gear change splines are shot original not worth the bother, theres also a 4 speed single port box on there

[SAABCOMBI]

I have be thinking about buying a cz twinport 250 after ready about your problems with gearboxes, l have changed my mined about buying it.

[090]

I have be thinking about buying a cz twinport 250 after ready about your problems with gearboxes, l have changed my mined about buying it.

Yeah me too. Sell sell sell!

Very interesting stump magnet, thanks for sharing. At this stage both of ours have good gear boxes and we have some spares as well. Good to know though for the future.

[SAABCOMBI]

Please explain Brad.

[Bamford#69]

Hi, Stump magnet,
Congratulations on your preserverance and engineering skills , good to some Zen thinking about the gear box problems we face owning /racing a early CZ
Well done
cheers

[stump magnet]

my pleasure mate, wish i had known of this mod two years ago would have saved me lots of greif and cash

[stump magnet]

rode the bike yesterday and gearbox was faultless no more dropping out of gear worked a treat