using oversized gs400 in gs450

[Frank88]

Hello all,

Currently I am busy restoring a 1989 GS450e. I decided to inspect the whole top-end as it made a rattling sound and had low compression (turned out, the auto tensioner was also jammed). After measuring the cylinders and pistons I was adviced to rebore the cylinders with new oversized pistons of atleast .025mm. I was looking at https://www.cruzinimage.net/ and saw they used to sell oversized 450 pistons but these seem to be out of stock. I did find oversized 400 pistons on the same website which have an diameter of 69mm. As the 450 has a standard piston size of 66.96 mm I thought I might settle for these. But, because I have no experience with this, maybe someone can tell me if this makes sense. I am not looking for max increase in power, I just want to rebore my cylinders and have solid pistons that fit. Thanks in advance.

Gr,

Frank

[Steve]

My records don't go as far as the 1989 model year, but the earlier 450s had a bore of 71.0mm.

.

[Frank88]

You are correct. After your post I started to look more closely to the cylinder and I discovered a stamp stating 399cc! Someone has changed the cylinder at some point as the motorcycle is officially registrated as a gs450! Luckily I bought it at a bargain price. I guess i can continue with the idea of buying the oversized gs400 pistons then...

[Guest]

Being in Europe, are you sure you have a GS and not a GSX? There is the GS450, and then there is the GSX400. The GSX400 is the four valve per cylinder motor with screw and locknut adjusters whereas the GS450 is the two valve per cylinder with shim over bucket for valve clearance adjustment instead. The GSX400 has rectangular cam cover end caps and the GS450 has round cam cover end caps.

[Frank88]

I have 4 valves per cylinder with a locknut mechanism for adjustment. So according to you this would be the gsx400. Still weird that it is officially registrated as a gs450.

[Guest]

While I've not dealt with the internals of a GSX400 I do know they are literally a bolt in replacement to the GS450 motor, so it could be someone has put a GSX400 motor in a GS450 frame. I assume over there it's the same as here where registration details are associated with a frame number or VIN as opposed to the engine number. Of course someone may have just gotten it wrong too given the bikes are so similar

[Frank88]

Thx to all who have helped me so quickly solving this mystery.

Since I am dealing with gsx400 and not a gs450, for what I can gather the gsx400 and gs400 are more or less the same with cylinder/piston sizes. Can I assume that an oversize gs400 piston will be the best option for me?

EDIT:

To be clear, I mean these pistons: https://www.cruzinimage.net/2018/12/...t-69mm-piston/

[1948man]

How much oversize are the GSX 400 pistons? At least .025mm oversize? I assume damage or wear requires at least .025.

[Frank88]

Standard size should be 66.96 mm apparently. The oversized pistons I mentioned are 69mm, so yea the increase is more than .025. but I doubt I can find pistons that are just .025 mm oversized. The oversized pistons are for '77-79 gs400 however...

[GregT]

Somewhere here you'll find a thread of mine about using GSX750 pistons in the GSX 400 engine.

The oversize ones you're looking at are for the 2 valve engine. I went through the same exercise for a customer bike and finished up using the 750 pistons with a 1.2mm spacer under the barrel.

It worked well.

[Frank88]

Thx for the info. What do you exactly mean with 'a spacer under the barrel'?

[GregT]

Thx for the info. What do you exactly mean with 'a spacer under the barrel'?

The 750 pistons are slightly taller than the 400 ones. I made a spacer from sheet aluminium to go under the cylinder block - the barrel.'
1.2mm is 18 gauge in the old terms.

The additional height isn't enough to upset the camchain tensioner or guides - and the std camchain is perfectly able to accomodate it.

[Frank88]

Ah ok, thanks for the explanation.

[Guest]

Another option is to replace the barrels, pistons and heads with used 450 parts. It's a bolt on, and what I've been running for a few years now. Parts are much easier to source than GSX 400 parts. Your carbs will work as will the exhaust. You'd need the GS450 front cam chain guide - not hard to find. There's a fair bit more low end torque and about the same top end. Can't think of anything else you'd need other than new plug caps for the bigger plugs.

There's a thread about it maybe 3-4 years back.

[Frank88]

A side question, does anyone know if the website https://www.cruzinimage.net/ is still active? I tried contacting them since last week but no response so far. Thx