Four stroke options

[FourstrokeForever]

When it comes to doing a four stroke for racing I'm of the belief that it's much easier to fit the MX models gear to the four stroke chassis then a four stroke motor to a MX chassis.

Easier no doubt but not as good a result. The MX frame will always be better for MX than a trail bike frame in my humble opinion

Only while it is fitted with the original motor, take out the light weight, compact two stroke and fit a tall, huge lump of lead and the whole attitude of the bike changes...and not for the better.

I agree, sticking a big thumper motor into a light weight 2 stroke chassis will change how the once great handling MXer behaves. Weight distribution is a big issue with most 2 strokes having a front biased weight balance and the motor generally higher in the frame than a 4st. The problem comes when trying to keep the counter shaft as centre and as close as possible in relation to the swing arm pivot when fitting a fourstroke into a 2st MX frame.

It is much easier to use the original "trail bike" frame and modify it with MX geometry (rake, trail, offset and wheelbase) and firmer suspension with more travel. All the parts make up the sum of the machine so you need to start with the parts you intend using before you start chopping and changing the frame. Bolt on the new front end and swingarm with wheels and then start measuring.

[LWC82PE]

It is interesting that even White Bros thought that the stock Yam frame was good enough and you don't need a 'HL' type frame

[TT5 Matt]

theres alot of places on a 500 thumper you can save weight if you want to go to the trouble of machineing up alloy replacement components or fit yz parts
rear axle  factory drilled out yz400
swingarm pivit bolt   factory drilled out yz400
all axle spacers   machined alloy
oil return hose fitting/plate    alloy
yz400 cdi ignition   good for lots of revs
yz400 clutch
WP cam tensioner adjuster
drilled countershaft spocket
swiss chese drilled out brake shoes
drilled out brake arms and cams
and many more places if your keen to save a gram or 10
is it worth all the effort? nar still a heavy old dinasor just buy a big bore 2t of your brand choice

[LWC82PE]

So the axle is 17mm and drilled out, what size is the hole in the middle?

[TT5 Matt]

from memory the rear axle had a 7mm hole and was off of a yz400d, the much modded tt500d i got from dave basham and the frame was modded by a dude who worked for a pommey frame builder and the back shock mounts/frame tubes were a copy of a rm125s which gave it 8.5 inches of travel,it worked really well compared to the stock rear suspension which might be why ive got 5 herniated back disc right now

[GMC]

It is interesting that even White Bros thought that the stock Yam frame was good enough and you don't need a 'HL' type frame

I think they would have been more about selling products for small improvements, whether the stock frame was good enough or not I doubt would have come into it.

Forgive me if this sounds like a sales pitch but one thing about the HL vs TT frame is that the HL changes the whole ergonomics of the bike, width of set rails, riding position etc, there's more to it than just suspension changes. Of course all these things can be changed and it depends what frame (XR, DR or whatever) your modifying from as well. It comes down to what you want to achieve and how much work your willing to put in.

[Nathan S]

I've got a DR250F here - you know, the "competition" model with an alloy swingarm and all that...

It's built like a bloody Sherman tank. Ignoring the considerable heft of the motor, there's heaps of excess weight in the thing - stuff like minor frame rails that are double the diameter of any halfway decent enduro bike, brackets onto brackets onto brackets, and lots of steel where an MXer would use alloy.
Plus the japs were all obsessed with too-short swingarms on their '80s four strokes. Presumably this is so the gutless 4Ts felt like they had some grunt, but it makes them all handle weird. (Notably, the twinshock KLX seems to be the exception to this - and it's the one with the greatest reputation for being gutless. I wonder if the sensible swingarm is at least partly responsible for this).

Add in shit shock absorbers, shit ergos, steel tanks, lousy forks/shocks and all the rest, and I can see the appeal in fitting the motor into something else.

[LWC82PE]

Good point GMC. You know when i posted that page above i actually had thought that WB's comments were also about 'selling their product'

[GMC]

(Notably, the twinshock KLX seems to be the exception to this - and it's the one with the greatest reputation for being gutless. I wonder if the sensible swingarm is at least partly responsible for this).

One thing about the KLX is that it was somewhat based on the 79 KX 125 with the trailbike KL motor so it started life with 'MX ergonomics'
Pity Kawaskai didn't run with this theme during the following years.
Honda's XR range wasn't too bad but the XR motor in a CR frame I'm sure would also have been very popular


As has been said previously, the swingarm pivot position is the important thing to design around but most early 4 stroke engines didn't have the swingarm pivot through the motor but by the early 80's most 2 strokes did so not only do you need to keep the countershaft sprocket close to the pivot you also need to keep the height in mind.

[Nathan S]

xl59's XR200 powered RM125(C?) is a fantastic thing. I like XR200s, but the yellow one blows them away. It wouldn't have cost them any more to build these than what they did build...

[80-85 husky]

ol mate had a 350 xl shoehorned into a monty cappra (VB?) frame... I think and the whole swingarm pivot issue came into play just after landing from the first jump he tackled.. the chain doubled up into the countershaft sprocket and she stopped suddenly!

a lot more cutting and welding took place after that

[HeavenVMX]

ol mate had a 350 xl shoehorned into a monty cappra (VB?) frame... I think and the whole swingarm pivot issue came into play just after landing from the first jump he tackled.. the chain doubled up into the countershaft sprocket and she stopped suddenly!

a lot more cutting and welding took place after that

My personal opinion only not the clubs but...

That is sacrilege and blasphemy. I really don't think it would be taking it to far to say that taking a f&%kin XL350 engine and jamming it into and thus destroying a relatively rare Spanish bike is a crime against humanity.

A bit like taking a YZ250B roller and hacking it up to jam an '73 XL250 engine in it or butchering a TZ350 road racer roller to jam a CB350 engine into it. Bloody hell even if you hate Montesa bikes you must see that is just wrong FFS.

[TT5 Matt]

so is it "wrong" to do a engine swap if the frame and other different parts of all different brands of bikes if they all came from the wreckers?after all wreckers only keep frames/forks/swingers for so long and then sell them for scrap which then goes to china?and if you blew your monty gearbox and the parts were no longer available new you wouldnt buy a 2nd hand box knowing you'd help kill off another monty so yours could live again?lets face it not all bikes can be saved as the high costs and availability of new parts wont allow it, sometimes its way cheaper to buy a 3yo modern then rebuild a 35yo bike even if you do everything yourself and just buy parts/paint. ive just done a costing on a ts185 that im getting off the wrecker here who's closing down soon,its complete minus wheels/top end/carb and airbox for $50,ive known him for 40yrs hence the cheap price but to get it up to scratch ill tip another 6 or 700 dollars into it and ive got alot of the bits needed even then it will be rebuild just to make it a reliable bike for farm work and to go to the nestest town to get basic supplies to save putting my good xh van on dirt which i hate

[Nathan S]

There was a time when the Monty was just a shitty old dirt bike with a blown up motor.

[HeavenVMX]

so is it "wrong" to do a engine swap if the frame and other different parts of all different brands of bikes if they all came from the wreckers?after all wreckers only keep frames/forks/swingers for so long and then sell them for scrap which then goes to china?and if you blew your monty gearbox and the parts were no longer available new you wouldnt buy a 2nd hand box knowing you'd help kill off another monty so yours could live again?lets face it not all bikes can be saved as the high costs and availability of new parts wont allow it, sometimes its way cheaper to buy a 3yo modern then rebuild a 35yo bike even if you do everything yourself and just buy parts/paint. ive just done a costing on a ts185 that im getting off the wrecker here who's closing down soon,its complete minus wheels/top end/carb and airbox for $50,ive known him for 40yrs hence the cheap price but to get it up to scratch ill tip another 6 or 700 dollars into it and ive got alot of the bits needed even then it will be rebuild just to make it a reliable bike for farm work and to go to the nestest town to get basic supplies to save putting my good xh van on dirt which i hate

There were around 1900 VB Montesa 250(1500)/360(400) made in total how many TS185s umm a 1000000, not the same. The VB frame could have been reunited with a Montesa engine as parts are more plentiful than many think even NOS cranks, cases, cylinders etc etc. The point is destroying a rare bike to make a POS is not a good thing. That is my opinion and obviously not yours which is totally OK.

Give me the XH van because I wouldn't buy it. I'll chop it, slamin on the ground and shuv a Chevy V8 in it, that sounds sweet or alternatively use it for a smash up derby as is. The XH van was way more plentiful than VB Monties I would think, but has no real value to me. I am however very pleased that you value it so highly, enjoy it so much and preserve it lovingly. That shows that you know what I am getting at, umm unless you have already put a Chev or a Nissan/Holden Turbo 6cl engine in it

Nathan almost all of our bikes start as shitty old dirt bikes as you know. But that is all relative I guess.

As I said this is my opinion not the clubs so will not respond any more.