Topic: How to make a cb550 engine to a 500 ?

[Sam Green Racing]

I still don't see the 250 racing pistons.. 

Mark, it shows a picture and says CB250/CB350.

Sam.

[DresdaHonda]

To TG
It's ported, OK job, I think
I don't know if it's possible to weld it up, and then use copper gasket.
Or make a new ported head -now I have one to look at.
Peter

[mlinder]

I still don't see the 250 racing pistons.. 

Mark, it shows a picture and says CB250/CB350.

Sam.

Yah, I see that, but it also says "Omega piston kit 66mm bore (pair)"

66 is 2mm over stock 350 size, and 12mm over stock 250 size.

Maybe they have a 56mm 250 racing piston that they aren't listing, or somesuch.

[DresdaHonda]

Is`t possible to make the high compression (12,5:1)with the standart CB500 piston and then weld in the head (smaller dome)
Peter
PS. what is the weight of a standart CB500 (57mm) piston ? 

[mlinder]

Dunno what stock pistons weigh, I could measure one tonight, though. Stock is 56mm.

Stock 500 pistons are too flat. You'd just screw the piston up trying to weld that much material onto them.

Again, xl125 wiseco pistons are probably the place to start with this.

Also, NOS cb250 pistons have quite a bit of dome up top. The wristpin is correct, but you'd probably have to do quite a bit of machining to get the shoulder height correct, and valve pockets. Dunno if there is enough meat up top there to do too much machining, though. I have some 1st over cb250 pistons at the shop, I can take a pic of them if anyone wants to see em. Could prolly measure em for ya too.

[turboguzzi]

Is`t possible to make the high compression (12,5:1)with the standart CB500 piston and then weld in the head (smaller dome)
Peter
PS. what is the weight of a standart CB500 (57mm) piston ? 

I run my first season with stock piston and 12:1 CR and bike went quite well. You will need to remove about 1.6mm from the block + use a 0.8 copper gasket. total 2mm lower or so. this is form memory, I can check my notes. but its easy to make the calculation to see how low you have to go to get from the stock 9:1 to 12:1.

no need to weld the head but you will need to machine the piston so it fits into the 15 degree cone in the head and go for 0.8 squish band.

TG

[Sam Green Racing]

I'm not trying to sell pistons here, I just thought that if there was a piston availabe that would save you a lot of work machineing head and blocks to get your desired compression.
If the race pistons this guy sells have to much of a dome, how about a stock CB250 piston, someone must have one kicking around that they don't want for you to try.

Sam.

[mlinder]

I'm not trying to sell pistons here, I just thought that if there was a piston availabe that would save you a lot of work machineing head and blocks to get your desired compression.
If the race pistons this guy sells have to much of a dome, how about a stock CB250 piston, someone must have one kicking around that they don't want for you to try.

Sam.

Yeah, I have some. I can take measurements if people need em.

[Sam Green Racing]

That's what I thought

Sam.

[mlinder]

That's what I thought

Sam.

pthhtlblthppththpthpth!!

 

[mlinder]

Seriously, folks. I'm heading down to the shop in a couple hours, let me know what you would like measured.

[Howell]

Is`t possible to make the high compression (12,5:1)with the standart CB500 piston and then weld in the head (smaller dome)
Peter
PS. what is the weight of a standart CB500 (57mm) piston ?  

I run my first season with stock piston and 12:1 CR and bike went quite well. You will need to remove about 1.6mm from the block + use a 0.8 copper gasket. total 2mm lower or so. this is form memory, I can check my notes. but its easy to make the calculation to see how low you have to go to get from the stock 9:1 to 12:1.

no need to weld the head but you will need to machine the piston so it fits into the 15 degree cone in the head and go for 0.8 squish band.

TG

Why using a copper headgasket , we use the stock one during the whole raceseason , no oil leaking , and also re-used after the valves problems we had.

[mlinder]

Is`t possible to make the high compression (12,5:1)with the standart CB500 piston and then weld in the head (smaller dome)
Peter
PS. what is the weight of a standart CB500 (57mm) piston ?  

I run my first season with stock piston and 12:1 CR and bike went quite well. You will need to remove about 1.6mm from the block + use a 0.8 copper gasket. total 2mm lower or so. this is form memory, I can check my notes. but its easy to make the calculation to see how low you have to go to get from the stock 9:1 to 12:1.

no need to weld the head but you will need to machine the piston so it fits into the 15 degree cone in the head and go for 0.8 squish band.

TG

Why using a copper headgasket , we use the stock one during the whole raceseason , no oil leaking , and also re-used after the valves problems we had.

To increase compression, due to the much thinner gasket. Copper gaskets are also much better at transferring heat from the head to the cooling fins of the jugs.

[turboguzzi]

Is`t possible to make the high compression (12,5:1)with the standart CB500 piston and then weld in the head (smaller dome)
Peter
PS. what is the weight of a standart CB500 (57mm) piston ? 

I run my first season with stock piston and 12:1 CR and bike went quite well. You will need to remove about 1.6mm from the block + use a 0.8 copper gasket. total 2mm lower or so. this is form memory, I can check my notes. but its easy to make the calculation to see how low you have to go to get from the stock 9:1 to 12:1.

no need to weld the head but you will need to machine the piston so it fits into the 15 degree cone in the head and go for 0.8 squish band.

TG

Why using a copper headgasket , we use the stock one during the whole raceseason , no oil leaking , and also re-used after the valves problems we had.

Sure, it's not a must but a copper gasket promotes heat equalization between head and block so in theory less distortion and better thermal load distribution. have no scientific evidence that it lowers your lap times

Mlinder, like i said just above your post, the 500 chambers are so shallow the you can obtain 12:1 with reworked stock pistons, no welding of the pistons is required.  I know I did

Sam, looking at my catalogs, those racing cb500/4 pistons look like XL125S jugs. that's a solution many people use here, but these are full skirt, thick rings, a bit old school really but they sure work. some very fast bikes here use them so I think they are more than fine.

TG

[DresdaHonda]

sorry for this jump
-What about using a CB650 head on a CB500 cylinder.
I've read a old post ,that the CB650 head is a nicer design for flowing
-what is the valves size on a CB650 ?

Peter

MORE piston talk please ! -Sound like lot of CB500 racers have this problem
-is it possible we order a series of special design pistons from wise co, or another fabrication ??

[turboguzzi]

sorry for this jump
-What about using a CB650 head on a CB500 cylinder.
I've read a old post ,that the CB650 head is a nicer design for flowing
-what is the valves size on a CB650 ?

Peter

MORE piston talk please ! -Sound like lot of CB500 racers have this problem
-is it possible we order a series of special design pistons from wise co, or another fabrication ??

member cb500four is using a 650 head in his 600 cc racer with a 60mm bore (i think) the 650 chamber diameter is bigger than the 57mm bore so you will need to weld it to make this work, assuming that your rulebook allows you to use it.

some four years ago, when I started building my bike i proposed here making some piston sets but in reality, not that many out there are racing in 500cc capacity... everybody wanted bigger than 57mm bore for the street... so no go from the quick check i made, even ordering 10 sets in asia, cost will still be around 400-500 euro per set, hard to justify compared to modified XL125 or other honda kit pistons.

but if you want, pistal racing in italy or wossner in germany will be happy to take your money    

TG

 

[DresdaHonda]

I like to use the early CB650 head, and as long I'm at 500cc I don't think it's a problem
(the rulebook say parts up to 1972)
-But is there a big difference of the look at the head on a CB650 ?
What about valves cover CB550 or CB650 ?

About Pistons
Is the XL125 pistons the only opportunity, when you need a piston with some dome ?

Or to make the high compression only in the head, by welding, (like modern engines)
and then just use flat pistons (in 57mm)
Maybe when it's possible to bye some skirt pistons (in 57mm)
Peter

[mlinder]

I like to use the early CB650 head, and as long I'm at 500cc I don't think it's a problem
(the rulebook say parts up to 1972)
-But is there a big difference of the look at the head on a CB650 ?
What about valves cover CB550 or CB650 ?

About Pistons
Is the XL125 pistons the only opportunity, when you need a piston with some dome ?

Or to make the high compression only in the head, by welding, (like modern engines)
and then just use flat pistons (in 57mm)
Maybe when it's possible to bye some skirt pistons (in 57mm)
Peter

It's very different in appearance. They will notice, and your bike will be disqualified.

You will have to weld, then machine the head, so the combustion chamber (stock - about 60mm) matches the bore (57mm) or you will have a not-so-good combustion chamber and squish design.

Actually, not just not-so-good, but actually quite bad.

Welding material onb top of pistons then requires machining. Probably quite expensive, after you've ruined at least one of your pistons by welding material, the cost of the welding, then the cost of the machining, then the cost of fixing the ones that need more welding and more machining.

As turboguzzi said, he reached 12:1 CR with stock pistons by decking, gasket thickness, and a bit of machining on the stock pistons.

[turboguzzi]

http://forums.sohc4.net/index.php?topic=55162.0

[DresdaHonda]

Thanks TG for the link, nice reading

I stay with the 550 head, and try to weld the "Rings gasket" up.
After I get some pistons I will plane the head down (to get a CR of 12 or 12,5)

Peter
PS. were do I get thin cobber head gaskets ?

[mlinder]

http://www.cometic.com/

[mlinder]

Honestly, why not buy a 500 lump?

You can't just weld the combustion chamber edge smaller and call it a day. It has to be machined precisely, with special care taken to the squish-band design, to get good power.

Also, some racing organisations will not allow the 550 bottom end in the '500' class, as it has a different (better) clutch design.

[mlinder]

Or, buy jugs and head. Pop em on.

I see 500 cylinder heads here in the states going for 65 bucks on ebay.
Jugs for less.

You will spend far less time and money buying jugs and head in decent condition and cleaning those up than you will buying new sleeves, and doing all the welding and machine work required for getting the 550 head to work correctly with 500 pistons.

Ask me how I know.

[turboguzzi]

Or, buy jugs and head. Pop em on.

I see 500 cylinder heads here in the states going for 65 bucks on ebay.
Jugs for less.

You will spend far less time and money buying jugs and head in decent condition and cleaning those up than you will buying new sleeves, and doing all the welding and machine work required for getting the 550 head to work correctly with 500 pistons.

Ask me how I know.

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