Gsxr750 head on a GS500e

Just out of curiosity, back in the day, when this used to be a more well known modification, how was the camshaft situation sorted? Since the inner two pistons are phased differently as the GS500 twin piston. You cannot simple use the gsxr750 camshafts.

Was it possible to modify the 750 head and use the gs500 camshafts (with custom lobes perhaps) or was an entirely custom solution needed?

I was hoping some of the oldtimers could shed some light on this.

It may work, but would sure look odd with the 2 outer 750 combustion chambers sticking out either side with no cylinders nor pistons under them... I don't remember reading where anybody has tried that upgrade "yet"

No such mod was ever well known "back in the day" or even at the present..

rphillips said:

It may work, but would sure look odd with the 2 outer 750 combustion chambers sticking out either side with no cylinders nor pistons under them... I don't remember reading where anybody has tried that upgrade "yet"

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You would, ofcourse, cut the outer 2 off and create end-caps for the head.

zed1015 said:

No such mod was ever well known "back in the day" or even at the present..

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Maybe not well known, but definitely done by multiple people, Bob Broussard was the most well known.

In an effort to find sources on this to show you, I found a post by Bob himself detailing a lot of what he did on GSTwins, I'll link it but will also copy the information here, all this stuff is slowly disappearing off of the internet and knowledge is lost...

He Wrote:
"The head I have is an 89 gsxr 750. There was one on ebay last week, but I decided pass on it.
Here's the deal. The 4 inner stud holes lineup perfect. The outer 4 need to be relocated outward a bit. The GS head gasket works fine. The cam chain slot on the gsxr head is longer than the cylinders. So you need to fill it in at the rear. The GS cam sprockets will fit on the gsxr cams, so you can use the GS chain and sprockets. The bolt holes need be redrilled 1mm bigger (better yet slot them for degreeing).
The cam lobes need to be relocated for a 180 degree crank (Megacycle cams can do this). Might as well get higher lift at the same time. You'll need hardened rocker arms (Megacycle again) Valve spring set.
The inner cooling chambers normally drain down the stud into the lowerend on the gsxr. The GS studs are open in front (major oily mess :lol: ).
Fill in the drain holes and drill other holes so oil drains into cam chain area.
Then you need to make end plates to seal the the sides of the head.
These need oil passages to oil the cams and rockers through the rocker shafts. And you need to make drain lines to drain the cooling oil.
I went the extra step of installing a second oil pump in the sump, driven off the transmission. The oil goes from the pump to the oil cooler (B&M Supercooler) then to the valve cover. It fills the cooling chambers in the head and then drains back down into the motor.
The original pump does the lubrication. I tapped into the plug just below the igniton cover. It goes into the side plates and into the rocker shafts.
I'm running Carrillo rods and JE pistons. I had the pistons designed for the gsxr head. The exhaust cutouts needed to be cut forward a bit more. That's why I broke the first head :roll:
The weak link in the crank seems to be the right side rod journal where it connects to the outer weight. The drive gear is helical. This flexes the crank and snaps it eventually.
I've milled a slot for another thrust bearing into the cases in an effort to limit the movement in this area. The bearing shows definate contact, so it's doing something (time will tell :mrgreen: )
I have pictures of all the mods. Was planning on putting them on my website www.gszilla.com but i've been too lazy to complete the site.
Email me for the pics and more details.
There's other things, like gsxr cylinder studs (APE). Stronger and equal length. GS studs are different lengths.
By the way how rich are you? :lol: :lol: :lol: "

So, in the end, my question is answered; he had MegaCycle changed the camlobes to a 180 degree twin setup.

I've utilized AI to collect every single post from Bob Broussard, and have an archive of what was written on the net, below is an overview of what I now know:

Based on Bob Broussard’s old GSTwins posts, the conversion was roughly done as follows:

1. Start with a 1989 GSX-R750 head.
   Cut off the two outer cylinder sections with a bandsaw, then machine both ends flat.
2. Adapt the head stud pattern.
   The four inner stud holes line up with the GS. The four outer holes need to be relocated and should be machined with an end mill rather than drilled. Bob’s old posts are contradictory about whether these move inward or outward, so this needs to be measured directly.
3. Close off the cut ends.
   Machine aluminum end plates to seal both sides of the head. These plates also need oil passages for the cam/rocker lubrication.
4. Modify the cam-chain opening.
   The GSX-R cam-chain tunnel is longer than the GS cylinder opening, so the rear section must be filled/closed.
5. Use GS cam sprockets and chain.
   GS sprockets fit the GSX-R cams. Bob enlarged/slotted the sprocket bolt holes, preferably allowing adjustable cam timing.
6. Re-phase the camshafts for the 180° GS twin crank.
   The GSX-R cam lobes must be relocated. Bob used Megacycle and recommended higher lift, hardened rocker arms and stronger valve springs at the same time.
7. Use suitable custom pistons.
   Bob used custom JE pistons designed for the GSX-R combustion chamber. Correct exhaust-valve pocket position is critical; his first version suffered valve-to-piston contact and destroyed a head/cylinder.
8. Rework the oiling system.
   The original GSX-R head oil feed through the outer studs can no longer be used. Bob plugged the original feed and supplied lubrication externally from the GS oil-pressure test port to the end plates and rocker shafts.
9. Provide a separate head-cooling circuit.
   Bob was unsure whether the stock GS oil pump had enough capacity for the oil-cooled GSX-R head, so he added a second oil pump driven from the transmission:
   second pump → B&M oil cooler → valve cover/head cooling chambers → return to engine.
10. Modify the oil returns.
    Original GSX-R drain paths through the studs must be blocked/re-routed. Bob redirected oil toward the cam-chain area and added external drain provisions.
11. Seal the valve cover against the new end plates.
    The ends of the GSX-R valve cover were machined flat. Bob used “The Right Stuff” sealant between the cover and the new side plates.
12. Strengthen the bottom end.
    Bob used Carrillo rods, custom JE pistons, a balanced crank and APE GSX-R studs. He also added an extra thrust bearing in the crankcase in an attempt to reduce crank flex from the helical primary drive.
13. Expect the crankshaft to be a limiting factor.
    Bob broke several cranks, including one while racing the GSX-R-head motor. Cedrick Smith’s 650 cc version reportedly broke enough cranks that he eventually used a custom billet crank.

Bob’s own GSX-R-head engine was 513 cc, used FCR35 and later FCR39 carbs, and he estimated it somewhere around the 70–75 hp range, but he never published a confirmed dyno figure.


The major missing details are the exact stud-hole coordinates, cam phasing, oil-line sizes, second-pump design, piston drawings, compression ratio and thrust-bearing dimensions.

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Now I am trying to find as much as I can on Cedric Smith's bike, he had a 650cc GS twin with a billet crank.

Clearly this conversion was of interest to a very small circle of individuals and not a well know mod to the wider community.
Unless there happens to be a member here who shared the same interest you would probably be more successful trying to contact Bob Broussard or Cedric Smith etc direct for further information not available online
As for the camshaft phasing .
If you have the knowhow and access to a machine shop etc it wouldn't be a complicated job to cut and shut the original gixxer cam and phase it to your requirements.

zed1015 said:

Clearly this conversion was of interest to a very small circle of individuals and not a well know mod to the wider community.
Unless there happens to be a member here who shared the same interest you would probably be more successful trying to contact Bob Broussard or Cedric Smith etc direct for further information not available online
As for the camshaft phasing .
If you have the knowhow and access to a machine shop etc it wouldn't be a complicated job to cut and shut the original gixxer cam and phase it to your requirements.

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I'm not debating whether it is a well known mod or not.

I'm trying to contact Bob directly but the email adres I found is dead, he last logged in somewhere around 2014 I believe...so easier said than done.

Just hoping some people from back then are still active and can comment on this.

Hope you find some pictures, I'd love to see hose 2 GSXR 750 combustion chambers sticking out each side of the eng with no cyl nor pistons underneath them. Seems would look stranger than a BMW.

rphillips said:

Hope you find some pictures, I'd love to see hose 2 GSXR 750 combustion chambers sticking out each side of the eng with no cyl nor pistons underneath them. Seems would look stranger than a BMW.

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Not sure if you are joking, but here you go:


And the article:

Well, I'd wondered if you were joking or not. A four cyl head on a 2 cyl eng sure had the look of a joke... But we never know. I remember, back in the day, somebody was taking 2 GS1100 engines and making V8's out of them and once read of 2 KZ1300 6cyl. engines being put together to make a V12... So who knows if joke or not??? I would like to see a picture of the GS500 "ENG.", not the bike, with the 4 cyl. head on the 2 cyl. engine.

rphillips said:

Well, I'd wondered if you were joking or not. A four cyl head on a 2 cyl eng sure had the look of a joke... But we never know. I remember, back in the day, somebody was taking 2 GS1100 engines and making V8's out of them and once read of 2 KZ1300 6cyl. engines being put together to make a V12... So who knows if joke or not??? I would like to see a picture of the GS500 "ENG.", not the bike, with the 4 cyl. head on the 2 cyl. engine.

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Ik wish I had photos, would love to show here. I'm working on it!

I'm collecting as much knowledge as I can for when I have finally finished all distraction from starting on my dream project.

Currently thinking about how to build a 725cc gs400, I think I have found a way...