Big bore GS(X) 400

[GregT]

I recently pulled down a customer GSX400 motor for a freshen up. On examining the pistons it was found that the top ring grooves on both pistons were worn beyond reuse.
Costing out the options it was immediately apparent that Wiseco pistons were the cheapest option. Remember, I'm in New Zealand...
A pair of Wiseco big bore GS(X) 750 pistons were ordered - Wiseco part no 4188MO7000 which are for the later 16v GS750. There are at least three ebay suppliers who will sell individual pistons so a set of four is not required. This means the bore goes from 67mm to 70mm, quite a safe overbore in standard liners.

On receipt of them, comparison to OE showed they have 1.00mm more compression height - or deck height if you prefer. Not a problem as base gaskets can be juggled. Valve cutaways matched OE400 nicely, and of course the pin is the same.

Once the barrels were bored and a mockup performed, it was necessary to make up an aluminium base gasket from 1.2mm - or 18G - sheet stock.In conjunction with the gaskets I had here, that gave me a squish clearance (piston to head) of .039in which is right where I wanted it.

The engine is on final assembly now including dialling the cams to allow for the slightly taller barrel. Amusingly, the aftermarket head gasket I'm using (customer supplied) is right on 70mm bore....what were they thinking ?

Mechanix asked me how the piston weights compared, I jokingly answered the wisecos are probably lighter...Ha, yes they are..
Both with rings, pin and one circlip. OE 278 grams Wiseco 262 grams....shouldn't be any balance problems.

Once it's running and carburation sorted, I'll tell how it goes.

[pete]

Ok I need to see how this goes! And we need pic's Greg!

[GregT]

Head's already on sorry - i'm not pulling it down just for pics. Too busy at present anyway, three more race motors on the bench...

Hopefully the only relevant pic is the smiling owner anyway....

[Charlie G]

Sounds interesting Greg. We didn't get the GS(X)400 in the states....darn!

cg

[GregT]

Sounds interesting Greg. We didn't get the GS(X)400 in the states....darn!

cg

Well your Canadian neighbours certainly did. Plenty of threads from them.

Anyway - cams dialled in. The difference from the taller barrel was only 3 - 4 degrees. Frankly if I was doing this on a road bike I wouldn't bother dialling them in.
This one however gets raced - in a gentlemanly fashion...So I moved them to where I wanted them. To complete the story, it's been lightly ported,new stem seals,valves refaced, stripped and cleaned out. Bearings all good, new camchain. New clutch plates. It'd been apart before at least once - i suspect it damaged a gearbox as the bearings are all original codes. The clutch nut was not tight when i stripped it - found out why when i reassembled it - it locked up on the shaft...I'd guess a gearbox swap in the past using the original primary gear/basket. Obviously the shimming was out with a different mainshaft hence the nut not done up tight....All sorted now and nut tight.
The only visible tell of the overbore is the edge of the base gasket

[Ghostgs1]

Maybe I'm missing something, but isn't this the bike called GS400 here? One of the first of the GS series? Or, is this the 4 valve engine?
Ray

[GregT]

It's the 4 valve per cylinder 400 twin...all the 4v motors are called GSX outside the US but were marketed as GS inside the US. Hence the GS(X) in the title of the thread....I don't know if the US got the 4V 400 but Canada certainly did - and they were very popular around the world too.

[Ghostgs1]

Thanks for clearing that up in my fuzzy mind. I don't think we ever had any 4 valve twins available, at least I've never seen any. Ray

[eran_k]

how did this came out? whats the capacity with the new pistons?

[GregT]

i haven't bothered to work out the capacity as it runs in a class where the top limit is 600cc - and it's well under that.

it's turned out very well. The owner is very happy with the performance increase. Carburation took a little while to sort out as he lives some hours away from me. As you might expect with something this age there's also been some electrical problems - which seem to have been a faulty battery.
He's now budgeting for a better set of carbs - probably Keihin CR's. His aim was to do all the South Island street race meetings in one season. He's done Greymouth and Wyndham, both of which had filthy weather. Looking forward to Nelson on New Years day and Methven at Easter.

So yes, worth doing. Not particularly difficult and the parts were cheaper than a rebore with OE pistons - at least here in NZ anyway.

[Chuck78]

Well this thread is very reassuring! I was wanting to eventually build a 2 valve per cylinder gs1000/1100 resleeved big bore gs425 at somewhere around 6-7mm overbore, and was worried about piston weights & finding a competent machine shop that could rebalance a balance shaft engine like the twins. Sounds like you could go several millimeters further in a lightweight aftermarket CNC machined piston before you get to the same weight as stock. I could even possibly specify to Wiseco or JE when ordering the custom pistons, that they keep them within 5% or so of the weight of the stock slugs. There's probably a fair margin of tolerance there, as I bet the engineers designed the balance shaft to deal with future factory +1mm o.s. replacement pistons, which despite a slightly lower dome to keep compression the same with bigger bore, would still likely be heavier due to the added diameter.

[Chuck78]

Greg's build would be a bump from 399cc stock c.r. to 436cc 10.25:1

I'm really hoping to be able to pull off a gs1100 sleeve (72mm stock bore) shortened & bored into a GS425 block. If that's doable, then 516cc's at 74mm would be very doable, & maybe 75mm at 530cc to go all out. 76mm max overbore would probably be pushing it for heat dissipation with minimal cylinder walls and also near the reasonable limit of what even a ported head with oversized valves could support. The GS500 sleeves are 74mm stock, but that would really be pushing the 425's block as far as sleeve boring capacity.

Thanks for sharing your experience on the gsx Greg!

[GregT]

In my experience, a replacement piston weight within 10% of the original will be fine as far as vibration goes. I certainly wouldn't rebalance for anything less than 10% difference.
There are two 450's with 500 pistons local to me and this GSX400. None have been rebalanced. all are quite acceptably smooth.

[MuzzaB]

Hi,

The bike Greg is talking about is mine, I'm a newbie to the GS forum and chuffed to find this thread.

Since the last posts in here the bike has been raced, finally got it to like it's stock CV carbs, crashed it at Nelson, and now have fitted a pair of Mikuni VM34mm Roundslide carbs to it, still trying to get it to like them, It fired straight up when I fitted them but first time I tried it at the track it ran poorly and I put it down to teething problems with the new carbs. At home earlier this week I was having a jetting session and it dropped a cylinder, I found a loose plug in the wiring to a coil, hopefully got that sorted so back to square one with the tuning. If anyone has fitted VM34's and has any tuning tips it would be much appreciated, I'm no carb expert but am learning as I go, and pestering Greg when I see him we live on opposite sides of the South Island and it's a 500km round trip for me race at Ruapuna in Christchurch.


IMGP3556.jpg

Cheers
Muzza

[GregT]

Hi Muz - wondered when you'd find this...post one of the horizontal pics from nelson for a giggle.

as owner/user any comments about vibration ?