GS500 motor with GSX400 head?

[donimo]

I was thinking of a possible projust involving a newer gs500 motor. I know the exhaust prevents a direct swap so I thought "why not just swap my head onto it" solving the exhaust port issue and making a cool 8 valve 500cc frankentwin.

Looking at the 2 fiches
gsx400
, gs500
I can't see any reason why it wouldn't work.

The bore on my 400 is a tiny bit bigger than the 500 (60 vs 56.6) would that pose any issues?

my bike seems to make the same (or more depending on sources) as the 500, and a lot of that might be the 8v head so it might be fun to see what you get with an extra 25% displacement.

[donimo]

gsx400 head






gs500

[donimo]

seems the 500 head gasket has some extrat holes around the outside edge (which dont seem to have a home on the cyls?), which most likely could just be skipped? There are also 4 o-rings on the outside holes on the 500 that arent on mine...

[GregT]

The basic engineering questions are --
Are the head studs on the same pattern/spacing - laying an 8 valve head gasket over a 500 one will answer this one.
Cam height relative to the head/barrel joint. If they're the same,easy peasy but if different then you have camchain length and tensioner problems
The stroke of both motors is the same at 56.6mm
The 400 bore is 67mm, the 500 bore is 74mm
The shape of the piston crowns is totally different - 2 valve vs 4 valve so suitable 74mm pistons would have to go in.
As it happens 250 Honda single pistons are 74mm..... and I think the pins are the same dia too.
IMO you want flat top pistons with 4 cutouts as the smaller chamber of the 400 head is going to give quite a high compression.

So - if the first two questions are OK then yes, it's doable.

But - and it's a biggie - the 400's crack their heads quite easily if overheated - and this combo will definitely generate heat.

Up to you....

Greg T

[Suzuki_Don]

The shape of the piston crowns is totally different - 2 valve vs 4 valve so suitable 74mm pistons would have to go in.
As it happens 250 Honda single pistons are 74mm..... and I think the pins are the same dia too.

If you replace the pistons with something else not only do they have to be the same diameter and piston pin size (gudgeon), but you will need to check the deck height as well.

[BentRod]

oh the fun fun problems, but I will start with the good news

if you indeed have a gsx400 head, then it is 8 valves.

Second, if you fit a gs500 engine into a gs(x)400 frame, it will fit, and custom pipes go about 250$ in mandrel bends.

The bad news.

With a gsx400 head you are going to be limited to a 36mm carb. the carbs for the gs500 are bigger. if you run a smaller carb you can look forward to all kinds of interesting tuning.

you will have to use the gs500 gasket. the gsx400 gasket will not work.
if you use the gs500 gasket, you will leave the edges around the bore diameter of the gsx400 head exposed. you will need to grind these down, other wise they will heat up and MAY cause pre-ignition.

ultimately it is likely cheaper and easier to get the pipe custom made and fit the gs500 engine.

I have been inside both of these engines, and and quite familiar with rebuilding them.

I suppose what you want to achieve is this:

[GregT]

Pass me the gas torch and grinder, 20 minutes and I'd have a much better pipe run.
Seriously, these frames are only mild steel and I'd have no hesitation in revising the run of the downtubes to give exhaust clearance.
And it's going to be easier - but less interesting - than building an 8 valve 500.

Greg T

[donimo]

Everything I have seen states teh 500 uses 33mm BS mikuni's, which is smaller than the 34mm ones I have on there now, so I was hoping at most a jetting change.

And the head swap solves the exhaust issue anyways. I was planning on getting a spare 8v head and doing a cleanup on it anyways so a mild grind of the edges shouldnt be too crazy.

The pistons seem to be the only thing I didn't plan for (duh) and may be the only deal breaker.

8v 500 "cool" factor is 99% of my reasoning.

[GregT]

The Honda singles are not the only possible piston - EX500 Kawasaki is 74mm also, gudgeon dia may also be the same.
You need to physically compare the GS500 piston to any possible substitutes, looking particularly at the deck height - the distance from pin centerline to the top edge of the piston. It is usually possible to adjust this by machining - but within safe limits - around 1mm off is usually OK.

Greg T

[BentRod]

I know it's not good to argue, but I'm going to motion against chopping into the down tubes. I'm not saying it cannot be done, but you are going to need a pro welder, and a good size steel section to retain any rigidity.

Ultimately, though, I'd say that replacing the down tubes is probably more likely to work out than messing about with pistons and wrist pins from other bikes.
there is no standardization there, and forces, clearances, and tolerances would all be different. you would have to hope that the height from the wrist pin to the top of the piston was equal or less on the replacements.


For the record, the builder of the above bike was OldSchoolOrange. The bike was lovingly called the 'weapon', or 'that P.O.S that almost killed me!" by most people who tried riding it. the bike had about 50 lbs stripped from it, and did acceleration wheelies in third gear.

If you put in the gs500 engine you are going to have to grind your steering stops down at least an 1/8th of an inch, BTW.