87mm Big Bore?

Highside

Highside

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Does anyone know if there's a big bore kit that's bigger than the 80mm 750cc kit offered by mikesXS?

They sell gaskets for a 87mm kit, but when I emailed them about it, they said the 87mm bore kits were unreliable.

Is there anyone else who sells big bore kits??
 
Highside said:
Does anyone know if there's a big bore kit that's bigger than the 80mm 750cc kit offered by mikesXS?

They sell gaskets for a 87mm kit, but when I emailed them about it, they said the 87mm bore kits were unreliable.

Is there anyone else who sells big bore kits??
Click to expand...

Search function saves the day!

http://www.xs650.com/forum/showthread.php?t=933

Posted via Mobile
 
jay760 said:
Not sure why mikexs sells the gasket for 87mm bore, Heiden do the kit but only for euro engine with 533 rods,

http://www.xs650.biz/p/28/2736/mo74-cg|22=5/#05-0881-880cc-big-bore-kit-533-type-105:1-compression

the 840 kit is 84mm, 880 is the 87mm, for the extra cost that's involved, there's hardly any noticable power gain over the 750
Click to expand...

In theory you SHOULD be able to get the same HP per CC, if that isn't the case, then what do you think is the most likely cause?

carbs too small? head doesn't flow enough? needs bigger cam? can't get enough compression ratio?
 
Heads, cam most likely. Some of the knowledge from the old guys that raced these when they were new (Shell, et.al) said it got really hard to keep the engines together above about 70hp... trouble with cracking cases and all that. They had a better head, too. There's only so much displacement these heads can support before you get to spend a whole lot of money to bring it all back into harmony.
 
Not sure the reason, I've had a 760 an 840 and a 710 and there is very little between any of them, not so you could say any engine was a vast improvement on the other,
 
An 880 big bore is on my to do list. Izuzu pistons have the right compression height for the stock 447 rods. You can find a set of four, with rings, for as little as $50. New sleeves would be the only big expense.

The redline will be limited to around 6,000 rpm because it uses the smaller stock pins with the big heavy pistons. For that reason it would actually make more sense to run a smaller cam. Something that peaks just above 5,000 rpm. Sure, it wouldn't have the high peak power numbers of a race-prepped 750. But I don't ride a race bike, I ride a street bike and this combo would be perfect for lugging it around town.
 
mrriggs said:
An 880 big bore is on my to do list. Izuzu pistons have the right compression height for the stock 447 rods. You can find a set of four, with rings, for as little as $50. New sleeves would be the only big expense.

The redline will be limited to around 6,000 rpm because it uses the smaller stock pins with the big heavy pistons. For that reason it would actually make more sense to run a smaller cam. Something that peaks just above 5,000 rpm. Sure, it wouldn't have the high peak power numbers of a race-prepped 750. But I don't ride a race bike, I ride a street bike and this combo would be perfect for lugging it around town.
Click to expand...

Interesting. Are the Isuzu pistons forged? I'd be inclined to chuck 'em up on a Bridgeport and see if I can get 'em down to the weight (or close) of a 80mm piston

Do you know where to get the sleeves?

How about porting the head for more HP?

Do you know how much power on of these things can handle?
 
About 70 horsepower or a little more before you start breaking things (like cases). If Shell Thuet couldn't do it on Yamaha's budget, Good Luck Brother.
 
hi you could try looking up gs1150 or bigger and see what size they are,,, and they have a good name ... regards oldbiker
 
Highside, if you want a 75+ hp engine get something that has it to start. Building the XS to those levels isn't hard to do but the engines only last a few hundred miles per build. Not very cost effective for a 35+ year old bike you plan on riding much.
Leo
 
XSLeo said:
Highside, if you want a 75+ hp engine get something that has it to start. Building the XS to those levels isn't hard to do but the engines only last a few hundred miles per build. Not very cost effective for a 35+ year old bike you plan on riding much.
Leo
Click to expand...

That's the answer I was dreading to hear. I was looking to build a cheaper alternative to a Norton 850 Commando.

When you build to these power levels, what are the first things to fail??
 
Cases mostly. To get the larger bores you need to open up the hole the sleeves pass through. This makes the case weaker.
In a race bike where the engines get torn down and inspected frequently the weak points are found and fixed.
I just read your quote, I agree 100%.
Leo
 
Hillside, I don't know how much history of this bike you know.
It started as a 500 cc German bike. That company folded and after the engine passed through a few hands Yamaha got it. Each company made a few changes, Yamaha added their tweaks and built it as the XS650.
The engine works very well as a 650 to 750 with out cranking the power up too much. The crank even with four bearings still is built up out of many parts. This can lead to crank failures at higher horsepower.
The 750 kits can with some head work, bigger cam and welding the crank, rephasing can get a usable 70+ hp and not hurt the reliability to much.
I found with my 75 adding just the 750 kit boosted the hp to somewhere around 55 to 60 hp using the stock cam and some mild head clean up, just remove the casting marks and smooth things.
One thing I found was the vibes above 4000 rpms was greatly reduced.
Between the extra hp and vibe reduction the highway cruising was much better. Dropping a some teeth on the rear sprocket helps too. I have ran the stock 17/34, the 17/32 and 17/30.
The stock sprockets are great around town, quick acceleration. The 17/30 is great if you just run the highway. Lower cruise rpms, less vibes, smoother. Sucks off the line.
The 17/32 is a good all around ratio. Not bad off the line and not bad on the highway.
Leo
 
XSLeo said:
Cases mostly. To get the larger bores you need to open up the hole the sleeves pass through. This makes the case weaker.
In a race bike where the engines get torn down and inspected frequently the weak points are found and fixed.
I just read your quote, I agree 100%.
Leo
Click to expand...

Is it possible to weld some braces into the case to beef it up??
 
XSLeo said:
It started as a 500 cc German bike. That company folded and after the engine passed through a few hands Yamaha got it. Each company made a few changes, Yamaha added their tweaks and built it as the XS650.
The engine works very well as a 650 to 750 with out cranking the power up too much. The crank even with four bearings still is built up out of many parts. This can lead to crank failures at higher horsepower.
The 750 kits can with some head work, bigger cam and welding the crank, rephasing can get a usable 70+ hp and not hurt the reliability to much.
I found with my 75 adding just the 750 kit boosted the hp to somewhere around 55 to 60 hp using the stock cam and some mild head clean up, just remove the casting marks and smooth things.
One thing I found was the vibes above 4000 rpms was greatly reduced.
Click to expand...

They claim that the 80mm pistons weigh 20% less that the stock pistons, that would help reduce vibrations alone. Re-phasing reduces vibration the most though, that's about all it seems to do.
 
jay760 said:
According to this you use XT500 pistons with the 447 motor .http://www.oldsidecarcross.com/phomot/index.html
Click to expand...

From the looks of it, the problem there is that you end up with a low CR.

That seems to be an issue with the engine in the first place, the combustion chambers are really too big for a high performance engine.

A chamber that's small enough to achieve a 10:1 CR with a flat top piston would be ideal.
 

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