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Hi all

I am interested in modifying my standard TY250 engine to a 320

I would like to buy a replacement cylinder and head so that I would always have the original parts to fall back on if needed.

Would any of the TY twin shock cylinders be ok to use for the conversion?

There is a cylinder on ebay at the mo but the guy does'nt know what year bike it came from.

From one of the posts it said to use a DT360 piston, I have only looked on ebay but they only look available in the USA.

Does anyone know of a supplier in the UK?

The Majesty site wants £150 for a piston kit but it looks like I could get a DT360 kit for approx £100 from the USA.

On another post one guy asked if he needed to buy the DT360 liner for the barrel but the advice was he didn't need to.

Does this mean that the person tasked with fitting a liner would be able to modify something that is readily available?

In another post "feetupfun" said that the head would need welding and machining, would you be able to explain this in more detail?

I beleive you also need to fit a spacer under the barrel and "Woody" said that this should be approx 2mm.

Would this need to be played with or is 2mm a general standard?

Does the standard 250 jetting need changing once the modifications are complete?

Any help would be appreicated

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Any of the barrels can be used, it doesn't matter which

If I was having it done I'd give the barrel and new 360 piston to PJ Motorcycle Engineers in Wolverhamption who will make and fit a new liner. It needs a new liner as the original hasn't enough thickness to be bored to 80mm. The 320 isn't ported any differently from the 250.

I'd only use a genuine Yam piston. Patterns can be poor quality and will rattle soon after fitting. Yams don't rattle if bored properly and run in properly when using original pistons. The DT piston is better as it has windows cut into the inlet skirt rather than the YZ which has an arch cut out of the skirt. Windows give a better bearing surface and longer life to the piston. You can use a DT400 piston which will give 340cc instead of 325cc. No difference in performance, they both pull like a train.

If you're buying from USA remember postage costs and you may have to pay import/customs duty. The latter is the luck of the draw. If you have to pay it you'll be close to £150.00 anyway. If buying from UK make sure it is a genuine Yam piston.

For genuine pistons in UK try Moto Ward, Moto Link, who both specialise in old YZ parts. Or Nigel Birkett who may still have standard size pistons

Can't help you with what to do with the head as I haven't a clue, but with 10mm difference in the bore size, the original 250 obviously won't work.

The 2mm spacer is to restore the piston/port timing as the 360 motor uses a 70mm stroke whereas the 250 is 65mm. Therefore the 250 piston is a different height from the 360/400

The crankcase mouth needs opening up to accomodate the bigger liner which has a much bigger OD with the 320 liner. This can be done with the engine built using a flap wheel (how Shirt did it apparently) but obviously you have to protect the bottom end from swarf etc.

Jetting is similar to the 250 but you have to go bigger on the main jet from memory. However, be warned, that they are a pig to jet and get to run properly and very difficult to make rev out cleanly. They have loads of bottom end torque but don't like to rev out.

Ask yourself is it worth the effort and cost for what gain? I'd estimate you're looking at around £500-£600 to do all this. You may be better off speaking to Nigel Birkett and asking him if he can port your 250 barrel. He never used the 320, just a 250. With a properly ported barrel and a lighter flywheel weight, the 250 will go very well. Even just removing the weight altogether and nothing else makes a difference. Which is something else to consider. The 320 had a custom made weight to keep it tractable, a fair bit bigger than the heaviest of the standard weights fitted to the 250 motors. It's unlikely you'll find one of these now and the 320 with a lighter weight can be a bit lively.

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Totally agree with Woody comments and suggest you have a ride of one before you go ahead. If you can't find one to test ride, that should tell you something

The TY250 head will be the wrong shape inside because the bore diameter is different. It may also need machining to get the compression ratio right.

The different model TY250 cylinders can be identified externally.

If after all the advice, you still want to ride something with too much capacity, did you know you can make a 360cc TY250 motor? You can use an RT2 or RT3 or DT360 cylinder, head and conrod and lengthen the stroke. The downside is that it needs more machining of the crankcases than the 320/340 motor due to the stroke and conrod differences, and will not fit in a Yamaha based frame Majesty due to the extra height of the cylinder and head. It also needs a flywheel approximating the weight of a Volkswagen Beetle to make it rideable

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Thanks for the repies guys, and for going into the subject with so much detail.

It really does give me something to think about so I may take my time and ask for a go on one first.

I will also give Nigel Birkett a rind and ask for a price on the porting.

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The problem with the 320 is that there are good and bad. Even if you try one, with no prior experience of riding them you won't really know which you're trying. They are infinitely tuneable to make them soft, sharp etc. and it is possible to get the 320 to rev clean out (I had one but it was like it when I bought it) But, the difficulty is getting it how you want it as they do seem to finicky in set up, especially carburation.

I've had three and they were all different and only one would rev until it screamed, the others flattened off before top revs. I've heard of other people complain of the same problem, yet I've also ridden others that have hit top revs. This is what I mean by finicky to set up to run as they should. On the last two I had, I did wonder if the electronic ignition was stopping them hitting top revs, but never put them back onto points to check. When I bought the one mentioned above it was on points and screamed, but after we rebuilt it, it never revved out properly

What you have to consider is what type of trials are you riding it in. There are no classic club trials that need the power of a 320, a standard TY175 is usually enough for 99% of classic club trial sections and more often than not, 100%. The 320 was introduced as an expert bike for national standard riders who were riding big sections. You'll encounter nothing like that in a classic club trial. They can be very good, but they are pokey - do you need that power. Is it going to get you in trouble.

Money spent on the 320 conversion and then the time to get it running right, is probably better spent on sorting other areas on your 250. Decent shocks (steel body falcons are by far the best value for money), good tyres, brakes shoes relined with softer material for efficient brakes, front fork set up and get the 250 motor running properly.

You'll have a bike that works well and more than enough for classic club events.

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I bought the bike in question last April from Bavins of Diss.

It is a Majesty replica and he had done all the frame mods, the engine craddle has been moved upwards, the steering angle altered and the rear shocks angled.

I initially found the engine very flat and I was struggling on climbs, on short climbs it was like the power was coming in at the wrong time instead of making your speed first the power was coming in when I would usally be wanting to back off and find grip.

I looked through this site and there was alot of advice on lightening the fly wheel.

I bought a spare fly wheel and had the weight skimmed by approx half of the original, and this did make a difference, I have never tried it without a weight.

I also noticed that on uphill sections the front wheel was way too light.

I measured the wheel base and it was approx 50" so I bought swing arm extensions and had these welded in.

This made the bike much better on uphill sections.

I contacted Craig from the Majesty site and he said that 52" is about what I should be aiming for.

I do have TY Mono forks to try once I find a Mono front wheel and yokes.

At a trial I let a guy with a Majesty 175 have a go on it and he said that the gearing needed changing and the damping from the rear shocks was poor.

I changed the gearing and bought Rock Shock rear shocks and Magical front spring from Craig.

All these things have improved the bike and it does turn and handle very well it is just that I thought that the 320 mod might be the ultimate mod for the engine

I had a go on a TY175 and that did rev quicker than my 250 but at 6ft 2" and 15 stone I wonder whether it may be a bit small for me.

I have also spent a fair amount on the bike so I would like to stick with it.

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Sounds like you have spent time on sorting the bike and have most/all of areas I mentioned covered already.

The way you described the engine response initially is how they are as standard, pretty flat. It takes a while to get the flywheel mass turning and then just as you want to power off it gets going. So the next thing to try is run it without the weight altogether.It will run fine without the weight fitted and you won't lose any tractability without it, but it should improve pick up again.

Try advancing the ignition a little. I can't remember what the static setting is for the TY but you can advance it to 3 or 3.5mm BTDC which,along with removing the weight should help it pick up quicker. Something to note though, is that if you fit electronic ignition, they have a built in power curve which runs the ignition slightly retarded for the initial response, advancing as revs increase to give a more controlled and progressive throttle response which will keep the pick up on the lazier side. Not really needed on the TY250 as they aren't a sharp engine. More useful on snappier engines. It will probably pick up quicker on points, with it advanced as suggested.

If you still want to try the 320 route after all else has failed, If I was doing it I'd look for a complete engine to convert. Once you've opened up the crankcases for the larger sleeve, I'm not sure how well it will work as a 250 again if you refitted the 250 top end if it turned out you didn't like the 320. There would be space around the bottom of the liner due to the smaller ID of the original. This may affect crankcase compression or transfer of fuel up through the transfer ports. I'm guessing, you'd need to check this out from someone with better knowledge on that than mine. Loads of TYs have been broken, getting hold of an engine shouldn't be too hard I wouldn't have thought.

The Yam is a relatively short bike anyway and they are a bit light on the front on climbs, the 320 exaggerates this even more with its increased torque. The fact that the steering was steepened when they did the Majesty copy would account for the even shorter wheelbase you measured. Majesties didn't have their steering altered (generally, some did) Also, what did they do with the footrest position. If this was moved, it may also exaggerate the light front end.

If you fit the mono forks, you will move the front wheel forward as they're leading axle and this will increase wheelbase further, now that you have fitted extensions to the swingarm. You may want to try a standard swingarm again when you have fitted them. The extensions will help stop the bike lifting on climbs but the addition of mono forks may not help the steering. It's all trial and error once you start modifying frames and footrest positions etc.

Edited by woody
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I took the flywheel off my TY 250 after a few rides on it , I was a little afraid that I would loose traction easily in muddy section, but with the tire and suspension that are avalable now, I never had any problem . I'm not a top rider and my throttle control is not the best, but I never considered replacing the no-weight flywheel with the spare one I bought just in case.


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