XV750 pistons

[howardsmed]

Just thought his snippet of info should be out there. This is for UK and European engines only, which have 533 con rod length ( 140mm centres) this is NOT applicable to 447 or 256 engines

Yamaha are as we know great lovers of repeatedly using the same dimensions on their bikes, I have a 2020 YZF front end on my 1978 SR500, the headstock spindle uses the same bearings and diameters, and lots else, this same methodology is echoed in their piston design too, the 533 piston deck height is 29mm, the XT500 deck height is 29mm, ( which is why you can build an 880 from OEM parts) also XV750 are 29mm. all with 20mm gudgeon pin size

This means that, with a suitable cylinder barrel casting and liners, ( so NOT a stock over--bored, or stock with 750 liners) realistically that means a Heiden big fin, with properly dimensioned liners ( we use 99mm OD) you can build an 800cc engine, the compression is not so high it wont start, in fact with a thin head gasket, and the top cylinder face surfaced, around 8.3:1

it involves NO piston machining at all, peeps in the USA have used them in 144mm ( long rod ) engines, but have had to reduce the piston headland by 4mm, IF you were building a 750, for very little more effort you can have an 800,

Using XV750 pistons i have a 908 rephased engine drawn up, ready to build ( bored and stroked)

piston speed is ok, Mach index is good with stock valve size, rod to stroke is acceptable,

Rick Smedspeed and I will start very soon, i may even publish the build

Howard

 

[Adamc]

Just thought his snippet of info should be out there. This is for UK and European engines only, which have 533 con rod length ( 140mm centres) this is NOT applicable to 447 or 256 engines

Yamaha are as we know great lovers of repeatedly using the same dimensions on their bikes, I have a 2020 YZF front end on my 1978 SR500, the headstock spindle uses the same bearings and diameters, and lots else, this same methodology is echoed in their piston design too, the 533 piston deck height is 29mm, the XT500 deck height is 29mm, ( which is why you can build an 880 from OEM parts) also XV750 are 29mm. all with 20mm gudgeon pin size

This means that, with a suitable cylinder barrel casting and liners, ( so NOT a stock over--bored, or stock with 750 liners) realistically that means a Heiden big fin, with properly dimensioned liners ( we use 99mm OD) you can build an 800cc engine, the compression is not so high it wont start, in fact with a thin head gasket, and the top cylinder face surfaced, around 8.3:1

it involves NO piston machining at all, peeps in the USA have used them in 144mm ( long rod ) engines, but have had to reduce the piston headland by 4mm, IF you were building a 750, for very little more effort you can have an 800,

Using XV750 pistons i have a 908 rephased engine drawn up, ready to build ( bored and stroked)

piston speed is ok, Mach index is good with stock valve size, rod to stroke is acceptable,

Rick Smedspeed and I will start very soon, i may even publish the build

Howard

Great post Howard.
Please continue with a build thread. I am sure others would be interested, as will I.
Adam.

 

[howardsmed]

I had a long chat with Rick 10 minutes ago, we will do the stroker crank first ( people have been using 84mm strokers for 40 years, so we are not claiming anything new) what i have devised is a method that doesn't require stroker plates, this makes the whole thing much easier, and to all intents and purposes it will look like any other XS with a big fin cylinder. The final engine will be 83 x 84 giving 908cc

We start the crank work next week. the rest of the parts are supplied by our all UK based after i have designed them

H

 

[Adamc]

I had a long chat with Rick 10 minutes ago, we will do the stroker crank first ( people have been using 84mm strokers for 40 years, so we are not claiming anything new) what i have devised is a method that doesn't require stroker plates, this makes the whole thing much easier, and to all intents and purposes it will look like any other XS with a big fin cylinder. The final engine will be 83 x 84 giving 908cc

We start the crank work next week. the rest of the parts are supplied by our all UK based after i have designed them

H

That sounds great Howard.
I am very interested. May even do likewise to my bike.
Cheers
Ads.

 

[howardsmed]

it wont be a cheap day out if you do the stroker crank too, that is a lot of engineering work,
However the 800cc big bore will be a bare Heiden big fin cylinder, oversize ductile liners and bored to suit, the crankcases will need boring, as the biggest you can get away with is 87mm OD

but with an XS1/Shell #1/Smedspeed type 2.5 it will pull like a train and run out of puff, right at the top end, we get 60-65 rear wheel hp all the time, and 800 would not be much more, but lots of midrange. The 880 we but had 65ft/lbs torque

 

[jay760]

The tops of the xv pistons look very similar to the tx750 pistons that some people used to use with 256 cranks, as in they are almost flat, should give a nice usable motor

 

[howardsmed]

Yes they are flat topped, which means its easy to set the deck height, and thus the squish clearance, we will have to get some 83.5mm head gaskets cut, but that's no bother, the nice thing for us now, we have dozens of people who make parts for us ( all UK made, not Chinese junk) so getting a slighly differnet liner, or cam, or head gasket is just a phone call away

i had some TX750 pistons, but the deck height is less than a stock 256 piston, the compression would therefore be very low, we still have the pistons in the boxes they came in.