Topic: Honda GY6 pistons for 500/550 big bore

[bistromath]

Hi guys,

I got a little carried away since I had to drop the motor on my 550 and decided to go the 592 big-bore route, since I'm already doing port work and adding a 650 cam. Might as well go whole-hog, right? 

I found mention of using Honda GY6 pistons for a 550 big bore, and I'm wondering if anyone has details on these guys. I know you can get them in 61mm, and I know they use a 15mm wristpin. They also have two valve reliefs (instead of four like many modern pistons), which is great. They look flat-topped, but I don't know the wristpin to crown height (compression height). Does anyone have one of these motors, or happen to know the details on the piston? They're a much more modern piston design than the 750 pistons, and look to be a LOT lighter. Oh, and they're cheap. Can't beat that. 

[turboguzzi]

went through  similar thoughts early on my tuning journey.

main problem with all the newer pistons that have a 15mm pin is crown height, you need about 23mm and they are all off by a good 2-3mm, i.e. unusable UNLESS you mill big chunks off the block and head.

I went that path since I wanted the lightest possible pistons for my pure race motor but that required some heavy interventions to keep the cam chain tight.

unless you constantly hit redline, I'd say that you have a much easier path by using "heavy", stock 750 / XL185 pistons. did a full season on stock pistons and they held extemely well. the aftermarket I got not so.

In any case, wouldnt touch any Chinese made gy6, would go only with high quality mini moto big bore kits. cheap might mean also an expensive blowup

TG

[bistromath]

Thanks, TG. From what I can see it looks like the GY6 uses a 20mm pin-to-crown height, but I can't seem to find any firm numbers. Not really interested in taking 3mm off, and if anything I'd like to end up with a little MORE compression. Guess I'll stick with the 750 pistons.

[FunJimmy]

This is a very old but relevant topic as so many are looking to find readily available big bore kits for the little 550's. If the GY6 pin to crown height is 3mm shorter than the 550 piston, you could always put in a (stroker) 650 crank.

[DavePhipps]

55.8 stroke cb650
50.6 stroke cb550

5.2mm sifference
I'm still looking into this also. TG mentioned the xl125 piston and I found that some have used FZR600 pistons in thier Xl's as a big bore. Trying to find my FZR600 factory service manual to see if they will work as a cheap big bore.

There is always getting custom pistons made if you have the dough.
My leaning is just to use the modified CB750 pistons as mentioned in the gentalmans express. I think Soos will modify them for a small fee.

[FunJimmy]

55.8 stroke cb650
50.6 stroke cb550

5.2mm sifference
I'm still looking into this also. TG mentioned the xl125 piston and I found that some have used FZR600 pistons in thier Xl's as a big bore. Trying to find my FZR600 factory service manual to see if they will work as a cheap big bore.

There is always getting custom pistons made if you have the dough.
My leaning is just to use the modified CB750 pistons as mentioned in the gentalmans express. I think Soos will modify them for a small fee.

Don't forget that the 5.2mm stroke difference is divided by 2, for a 2.6mm actual.

I've heard that Soos does machining on pistons, but he hasn't responded to my PM's or the post in his "Service Offered" thread.

[DavePhipps]

Don't forget that the 5.2mm stroke difference is divided by 2, for a 2.6mm actual.

I've heard that Soos does machining on pistons, but he hasn't responded to my PM's or the post in his "Service Offered" thread.

Sorry about showing my ignorace here but, why do you divide the difference by 2?

[Tintop]

Don't forget that the 5.2mm stroke difference is divided by 2, for a 2.6mm actual.

I've heard that Soos does machining on pistons, but he hasn't responded to my PM's or the post in his "Service Offered" thread.

Sorry about showing my ignorace here but, why do you divide the difference by 2?

1/2 the stroke is up, the other half down.  Only the up part matters regarding piston to deck height.

[DavePhipps]

At first I was like ??
Now I get it. I think I have a bad case of the dumb tonight.

[FunJimmy]

Doing a little internet research has turned up these 61mm CB pistons.

http://www.petrolscooter.co.uk/piston-kit-61mm-15mm-cb-150cc.html

Has anyone tried these slugs in their 550?

[DavePhipps]

I went to my fave online calculator and got this using the 650 stroke and 61 mm piston.

Bowling's Engine Displacement Calculator
Input Parameters Are the Following:
Number of Cylinders are = 4 to 1
Piston Diameter = 61.000 Millimeters
Piston Oversize = 0.000 Millimeters
Engine Stroke = 55.800 Millimeters
Computation Results:
Computed Engine Displacement (CID) is 40
Computed Engine Displacement (Litres) is 0.7
Computed Engine Displacement (CC) is 652.3

http://www.bgsoflex.com/displacement.html

Also about a 10:1 CR using the 550 head according to this calculator. 15cc chamber flat pistons .5mm gasket
http://www.csgnetwork.com/compcalc.html

[bwaller]

The 550 chamber is 13cc's.

[DavePhipps]

The 550 chamber is 13cc's.

I'm going to have to make a referance sheet jut so I can keep track of all this.
Thanks bwaller.

[turboguzzi]

I went to my fave online calculator and got this using the 650 stroke and 61 mm piston.

Bowling's Engine Displacement Calculator
Input Parameters Are the Following:
Number of Cylinders are = 4 to 1
Piston Diameter = 61.000 Millimeters
Piston Oversize = 0.000 Millimeters
Engine Stroke = 55.800 Millimeters
Computation Results:
Computed Engine Displacement (CID) is 40
Computed Engine Displacement (Litres) is 0.7
Computed Engine Displacement (CC) is 652.3

http://www.bgsoflex.com/displacement.html

Also about a 10:1 CR using the 550 head according to this calculator. 15cc chamber flat pistons .5mm gasket
http://www.csgnetwork.com/compcalc.html

arent you building a 600 for your class? 61mm X 50.6 stock stroke will give you 591cc, which is pretty close to your limit no?

[DavePhipps]

yep, now all I have to do is get a real job so I can afford to build it.