Topic: 592 overbore. What do I need to tell the machinist?

[Terry in Australia]

I'm doing a "605 kit" (actually just 592cc) for Al Harper, it's just a Henry Abe kit, nothing really special, with light cast flat top 61mm pistons.

I haven't closely compared them to OEM CB750 pistons, what's the difference? I've got some CB750 pistons here I could machine on my lathe or my mill if I knew what mods are required? Cheers, Terry.

[Shenanigans]

I'm doing a "605 kit" (actually just 592cc) for Al Harper, it's just a Henry Abe kit, nothing really special, with light cast flat top 61mm pistons.

I haven't closely compared them to OEM CB750 pistons, what's the difference? I've got some CB750 pistons here I could machine on my lathe or my mill if I knew what mods are required? Cheers, Terry.

I know the skirt has to be shorten, but what I am unclear about since I dont have a set of 550/750 pistons to compare right now is if the distance from the wrist pin to the top of the pistons is the same for both 550/750. If it was I think it would be a simple job to just mimic the valve reliefs onto the 750 piston but I think the 750 piston is taller    Hence my problem of figuring out what exactly to do.


Edit: Wrist pin center to top surface on a CB550 piston is 23mm, anyone know what the K model 750 pistons are?

Double edit: 750 pistons are way more than 23, 25mm I think. 

[DsKing416]

How is this going I'm on the same problem.

[Terry in Australia]

How is this going I'm on the same problem.

Sorry guys, I've been away for awhile looking for info on my new Triumph Rocket 3, anyway, I had a quick look at some CB750 pistons compared to some 592cc "Henry Abe" pistons, the 592 piston skirts are much shorter than the 750 piston skirts, and depending on which CB750 pistons I look at, (K2, K7 and F2) the 750 piston's domes are between approx 2 and 4 mm taller.

My suggestion would be to mod early K0-K2 "flat top" pistons as they're the closest to the 550/592 pistons dome height, so machining a couple of mm of meat off the top shouldn't affect the integrity of the pistons too much. Before I re-assemble Al Harpers engine I'll machine up one CB750 piston to match his Henry Abe 592cc piston for reference, in case anybody here wants some CB750 pistons modified for their 500/550 engines in future. Cheers, Terry.    

[bistromath]

Terry, I'd appreciate that. The early K0-K2 pistons you're describing: are they the "300" pistons? The ones I have show a slight dome of 1.5mm from edge to center.

[Terry in Australia]

G'Day mate, are the "slightly domed" pistons you're talking about "300"s? The one's I've got here look as flat as a shiit carters hat!

The F2 pistons I've got have a dome similar to the one's you describe, I need a good set of them, if you're not gonna use 'em? Maybe we can do a trade? I'd take them as a "trade in" on a set of CB750/592 modded slugs? Cheers, Terry.

[bistromath]

What freaky mutant piston do I have, then? Here's a photo:



Best I could do to get it edge on. Like I said, they're about 1.5mm higher in the middle than at the edge. Not much of a dome, but not flat, either. If you've got different ones I'd be interested in figuring out which godd%^& pistons the Gentlemen of the aforementioned article actually used!

[Terry in Australia]

Oh ok mate, they definitely aren't F2 pistons, I'll go check those "flat" pistons again, it might be I need new glasses!

[bistromath]

Don't trust your eyes, then! Just stick 'em upside down on a table. 

[Terry in Australia]

Don't trust your eyes, then! Just stick 'em upside down on a table. 

Nah, no good mate, my house is haunted, the table is floating 2 feet above the floor as I type this, something keeps poking me in the back of the head, damn this cheap "house and land" package I bought here in "Hades Vistas" estate, which apparently was built on the old cemetry land, I'd sell if it wasn't for the soothing music eminating from the attic, and that lady in white who stands at the top of the staircase is a real babe! "Hey ghostly chick, show me those transluscent tata's"! Ha ha, cheers, Terry.    

[Tretnine]

I'm glad this came up. I've never been to a machine shop, but then again I just started wrenching bikes seriously about a year ago. Now I'm also in a position like the OP and need to know everything there is to know before deciding to go and where to go, how to decide where to go, etc.

[bistromath]

Terry dude, love you man, but lay off the sauce! 

[Terry in Australia]

Okey Dokey, lay off the sauce, I'll do my best! Now I went out to the garage and measured some pistons, it was my bad, the 550 pistons were dead flat, (about 15.10mm from the top of the wrist pin to the piston "crown".

The others are as follows:

"Henry Abe" 592cc pistons - 15.50mm
CB750K2 (marked "300") - 18.15mm
CB750K7 (marked "392") - 18.90mm, and
CB750F2 (marked "410") - 22.75mm.

Looking at that info, I'm guessing the CB750K2 or K7 (same as CB750F and F1) pistons are the ones to modify, take around 2.45mm off the crowns, lop another 13.5mm off the skirts, and you've got some 500/550/592 pistons. Cheers, Terry.

[DsKing416]

If we are all going through the same problem and there are still a million of these bike out there then why doesn't someone with a few grand extra get Wiseco to make 20 sets as a test batch. I talked to them about making some for me but they want 8 pieces as a minimum order, totaling around $800 dollars. I only need 4 pieces and we all only need 4 pieces. If a larger batch is made then the price goes down. I'm not saying we should all pool our money and get a bunch made. They should already be available. JE pistons, Wiseco, or countless other piston companies could make a set for around $40 and sell them for $300. Probably moving 500 sets a year. I "think" these sound like good numbers. If anyone works at a machine shop or owns one please just start making performance 550 pistons.

[Terry in Australia]

Well as much as I sympathise with you mate, I doubt seriously that you'd sell 500 sets in the next 10 years, 500/550's are still around, and there are folks like a few guys here who will try to hot up anything with an engine in it, but low number production for "vintage" motorcycles, is of little interest to manufacturers, when they can make and sell tens of thousands of pistons for modern bikes annually.

Seriously, for less money than Wiseco want, you can buy a good set of used CB750 pistons off EBay and get them cut down to suit, and you'll have original quality (better than Wiseco) for less money. Cheers, Terry.