Topic: "Gentleman's Express" 550 engine mods...anybody done it?

[moham]

I have read a lot of threads lately on the possible 500/550 mods out there. "The Gentleman's Express" has come up as well as a host of others. So, I am curious about the G.E. article: has anybody here done the 750 piston modification and the 61mm bore? If so, can you comment on the process?

[scunny]

check out paulages 718cc
http://forums.sohc4.net/index.php?topic=40414.0

[paulages]

the piston mod i had to do there was essentially the same thing, though i didn't have the benefit of someone having already figured how much material to remove from the pistons shoulder. my pistons were 64mm, but basically the same.

[moham]

Yeah, I've read that a couple times but it's not the same as the  mods in the article (tho it is quite incredible). He (paulages) used bigger 64mm pistons and 650 head (if I follow the build right). I was curious about the 61mm overbore using modified (what I understand to be standard) 750 pistons and stock rings.

[moham]

the piston mod i had to do there was essentially the same thing, though i didn't have the benefit of someone having already figured how much material to remove from the pistons shoulder. my pistons were 64mm, but basically the same.

I have been following a bit of your progress with that build. As a result, I figured out pretty quick that I couldn't go to the extreme (for lack of better word) that you did, and reading about the old overbore kits (555cc?) that used to be available and trying to find some middle ground.

[paulages]

if i remember right, the info in that article about how much material to remove was pretty decent. mockup and measuring things yourself free. if you can figure out a good ballpark to start from, i think it could be a fairly cheap mod. go back and look at my original 550/650 hybrid thread where i think i talked a little bit about how i dialed in the clearances and squish.

basically: assembly the bottom end and cylinders (use an old squashed base gasket). the shoulder height of the piston will be too tall and stand proud of the top of the cylinder deck. measure this, and subtract the head gasket thickness (cometic can give you a pretty accurate compressed thickness spec) to tell you how much material you need to remove from the shoulder. if i remember right, that article told how far in to come and what angle to cut the crown at. be conservative, and just keep turning the edge down until you have the squish and the valve clearances you want. it'll take several mockups and claying of the cylinder. you'll also have to cc the combustion chamber to fine tune your compression ratio.

add a 650 cam, and you'd have a pretty hot engine for relatively cheap i think.

[crazypj]

I did the CB750 piston mod but it was 1978 or 79 so I can't remember the exact numbers.
I remember I just machined the tops with angle to match combustion chamber.
never had any valve clearance problems, even at 13,500rpm with Piper 'fast road' cam (tad over 82mph in second) I have the specs for that somewhere though.
 You really needed K0~K1 pistons as the tops are thicker ( don't remember exact details of that either )
PJ

[Soos]

Doesn't really apply to a 550....

But I have done the 61mm bore with 750 pistons, but in a 650.
Nice drop in bore job pretty much... and the stocker head gasket will work up to 61mm.(62 mabey... but thats pushing it.)

I believe the 550 can use the same headgasket as well, so theres a few bucks saved.


Dunno on how much you feel like being a guinea pig.... but I got a set of 60mm pistons that would work as a overbore for a 550.
They MIGHT need machining for a 2 valve per cyl motor, as they were stock to a 4 valve per cyl motor.
I will offer the machining of these to match a set of stocker 550 pistons if you are interested. Nothing major needed, just a valve relief pocket.

They are from a kawa GPZ600(ninja)
The deck height is nearly identical to the 550's pistons I got from paulages. Thats just eyeballing them with the wrist pin shoved through both pistons.
Have not actually measured the deck height.


l8r

[moham]

Dunno on how much you feel like being a guinea pig.... but I got a set of 60mm pistons that would work as a overbore for a 550.
They MIGHT need machining for a 2 valve per cyl motor, as they were stock to a 4 valve per cyl motor.
I will offer the machining of these to match a set of stocker 550 pistons if you are interested. Nothing major needed, just a valve relief pocket.

They are from a kawa GPZ600(ninja)
The deck height is nearly identical to the 550's pistons I got from paulages. Thats just eyeballing them with the wrist pin shoved through both pistons.
Have not actually measured the deck height.

I am listening and interested. Do pin dimensions vary and if so does this present problems for the 550? If only simple machining is necessary, this is the kind of mod I'm looking for as I want to spend some dough on porting as well...With improved compression ratio from the alternate pistons, do I need to think about other hardware for the top end (other than the 650 cam)?

[Soos]

No, the wrist pin diameter is the same on both the CB550 and the GPZ600 pistons.
Both 15mm pins.
The only differences I see are the existing valve pockets, and diameter.(possibly weight as well)


... I dunno what else you would need.
Mabey some heavy duty valve springs to help with the 650 cam?
(or springs from a 650 head?) But i'm not 100% sure you would NEED the new(or 650) springs.
I think the 650 cam only has like 1 or 2 mm more lift than the stocker 550 cam.
Perhaps someone more familiar with hopping a 550 could chime in and let ya know.


I will take those Kawasaki GPZ600 pistons to work, and the 550 pistons I have, and get some hard measurements before machining them.
I'm not the fastest person in the world, so it might be a week or so before those are in shipping condition, depends on how busy i get in the next week.


l8r