GS400 bored to GS448

[droyce]

Hello All,

My 1977 GS400 (2-valves per cylinder) makes a really bad sound, the left side plug is completely black, smoke from exhaust, no power, and playing with the pilot screw on the left side carb made no difference in idle speed. Also the left side compression came in low, if I remember correctly about 20-30psi lower than the right side.

After tearing it down, the cylinder head seemed fine with no visible wear.

However, the pistons are a different story. The left side looked wet and oily on top, and both piston skirts have marks on them.

I was recently made aware that it may be possible to replace the GS400 pistons (65mm) and use GS850 pistons (69mm) to up the displacement from 398cc to 448cc given enough material on the sleeves. (stroke is 60mm on the '77 GS400)

Looks like a good 9mm of sleeve there, can I get away with taking it down 2mm to fit the 69mm piston?

I've taken some measurements of my GS400 pistons and would like to compare them if someone can provide the GS850 measurements.

The pin looks to be 15-16mm

Piston skirt looks to be about 52mm

Also, the piston dome looks to be about 10mm tall at its highest point.

Does anyone have pictures of GS850 pistons? for sale? Mekanix has made me aware that Cometic can make a headgasket in any thickness I need but can the stocker be made to work? Is there anything I'm overlooking here?

Another thought, how tough will tuning the carbs be with the added displacement (I have pods)? Will they be sufficient? I'm assuming a simple jet change will do the trick.

Thanks in advance

 

[droyce]

I guess this is another option...

http://www.wiseco.com/ProductDetail.aspx?AppID=&ItemID=4035M06900&ModelYear=&ModelID=

Wiseco GS750 10.25:1 compression pistons with 69mm bore

 

[gearhead13]

You dont have a 9mm thick sleeve, that is the top called the flange, measure at the bottom. 2mm overbore is doable, even 3mm is usually ok.

 

[Guest]

Going to 69 is ok. You should have 2mm of sleeve on each side to play with.
Tuning should be easy. Might have to go down in jet size and drop needle 1 notch.

Need new head gasket. On the old one the metal sealing ring is too small and would be inside the combustion chamber when you put the bored cylinder on it. Call cometic and send then a scan of your original gasket. They will make one 69 mm for you from scratch. They do have the drawings in cad for it but its better if they have yours to compare.

 

[droyce]

Here you can see here where piston is slapping the sleeve. Any idea what causes this wear?

 

[Nessism]

The pistons don't look that bad. Not great, but not horrible. Maybe it overheated or someone ran it with a poor air filter (like crappy pods)? Of greater concern to me is the corrosion in that cylinder, up near the top. Looks like the bike sat a long time and there is corrosion up where the rings sat. The piston scuffing could have even occurred after it sat for a long time and someone tried to start it.

 

[7981GS]

I could probably sell you a couple of stock 850 pistons for cheap.
The piston pins are 16mm.
As stated earlier, you measure the wall thickness at the sleeve.
2mm over is fine.
Cometic is the place to get your custom head gasket.

Daniel

 

[scottmc]

This is interesting...

Still mark it down as a 400 on the insurance?

 

[droyce]

I could probably sell you a couple of stock 850 pistons for cheap.
The piston pins are 16mm.
As stated earlier, you measure the wall thickness at the sleeve.
2mm over is fine.
Cometic is the place to get your custom head gasket.

Daniel

Ahh where were you a week ago? I just ordered new!



Scott, I'll have to cross that bridge when I get to it.

 

[droyce]

Just for future reference, old '78 GS400 (65mm) on the left and new '79 GS850 (69mm) on the right.

 

[droyce]

Unfortunately I received everything but the other piston, its on back order for another week. Just have to get the sleeves bored and slap it back together.

 

[gearhead13]

Ahhh! Bigger is ALWAYS better and prettier to

 

[Guest]

So.. whats the compression ratio going to be ?

 

[TeamDar]

I would say real close to stock. See if you can follow me on this. A GS400 and GS750 use the same piston, valves, ect. In 1979 when the GS850 was introduced it had the same cylinder head as the GS750. If you look at a GS850 piston next to a GS750/400 piston you will see the dome is smaller to make up for the larger cylinder displacement thus keeping the compression ratio about the same.

 

[Guest]

Yeah i see that in the picture now, its going to be a nice and easy mod

 

[droyce]

Okay gentlemen I need some advice here.

Top ID of the sleeve is: 64.95mm
Bottom ID of the sleeve is: 64.67mm
Bottom OD of the sleeve is: 72.50mm

If I bore out the sleeves to fit the 69mm pistons, this leaves me with 1.765mm of sleeve at the bottom.

Is there any chance this is enough material to hold up? If not I have a pair of GS850 pistons w/ rings for sale haha.

 

[7981GS]

Typically 2.0 mm is considered safe. I would go for it myself.
If it worries you, run synthetic oil and an oil cooler after it is broken in.

Daniel

 

[Guest]

It might be more than you think. The bottom is tapered but the middle of the sleeve is a little thicker. you would have to take the sleeve out to see what i mean but I'm sure i have a picture around here to show you.

I would go for it.

See how the bottom of the sleeves are shaded? They are tapered a bit. Mine are a full 0.55mm smaller in the bottom than just above the taper. And then its consistent all the way up to the top.


So for you that means just above where you measured you have a little more meat.

OD would be 72.5mm + 0.55mm = 73mm

(73mm-69mm) [SIZE=+1]? 2 [/SIZE]= 2mm sidewall where it counts, and .25mm less on the bottom.

 

[droyce]

Second shop said its pushing it, but should be okay. One week turn around. Now things are gonna get interesting

 

[Guest]

I don't know why I haven't seen this before, but I need to see how this goes...