Pistons

[Mekanix]

Where can I find pistons that will fit an 81gsx400l ?


I've noticed that the 750 4 valve has the same pistons but I can't buy them anywhere.

Where would you get them from ?

thanks

 

[rapidray]

I can get them made for you if you want some really trick ones! What is the stock bore size? Is it 67mm? Ray.

 

[gearhead13]

I have a set of good '82 750 pistons with one set of stock rings if you are interested.

 

[Mekanix]

Ray,

Stock piston is 67mm, (66.945-66960mm)
Wrist pin 18mm exactly
height is 54mm exactly
center of pin to top of piston is 26mm
from top of pin to top of piston 17mm exactly
from bottom of pin to bottom of piston skirt is 19mm exactly
center of pin to bottom of skirt is 28mm

One cylinder is at 67.17mm and the limit is 67.08mm causing a tick on one cylinder

Thanks gearhead13. Unfortunately I have to bore larger than stock.

 

[gearhead13]

Ray,

Stock piston is 67mm, (66.945-66960mm)
Wrist pin 18mm exactly
height is 54mm exactly
center of pin to top of piston is 26mm
from top of pin to top of piston 17mm exactly
from bottom of pin to bottom of piston skirt is 19mm exactly
center of pin to bottom of skirt is 28mm

One cylinder is at 67.17mm and the limit is 67.08mm causing a tick on one cylinder

Thanks gearhead13. Unfortunately I have to bore larger than stock.

You are worried about less than a tenth of a mm??

 

[Mekanix]

The cylinder is at .1 mm larger than that stock service limit but the problem is the pistons I have "seem" to be worn below the service limit. It depends where I measure them.

I might be worried for nothing though.

here are the measurements I took.

the bottom of the piston skirts are 66.96mm and 66.97m

the bore measures front to back 67.17 and 67.19 in several places on both cylinders. service limit according to the book is 67.080mm.

Should I be worried about 1 10th of a mm ?

 

[tkent02]

What are your plans for this bike?
Need it to run perfectly forever?

 

[Mekanix]

Nah, just for once would be nice Especially after rebuilding this bike for the past two months. I only ever get this deep into a project like this once every couple of years.

This is all because of a tick that I hear on the left cylinder that doesn't go away and gets worse when it warms up.

But when I took the head apart everything seemed good except that the pistons have some movement and make the same ticking noise.

I'm assuming piston slap because I can't find anything else that would cause this tick.

I took this late last night before disassembling the top end

Here is the link : http://www.youtube.com/watch?v=geQFNM6ON-M

thanks

 

[gearhead13]

Any piston slap noise would go away or lessen as the engine warms up as the pistons expand and the tolerances get tighter.

 

[Mekanix]

What am I left with ? Maybe something loosing oil coverage ?

After watching a few videos of last night i did notice that

1 it got worse as it warmed up and
2 made the noise even on deceleration with the ignition off
3 mainly at 2-3000 but always there
4 is loud after running but gets quiet after a short stop and restart.
5 louder under load


After I took the head off I noticed

1 the combustion chamber and pistons were very black with carbon, which could be due to idling for a while or a carb issue i haven't found yet.
2 intake runners had carbon on them
3 exhaust was covered in carbon,
4 with the carbs and exhaust off of the engine I put my hand on one intake and then the other while cranking and I noticed that the left would hold suction well and the right would not hold long and I felt some air coming back into the intake when the piston was passing bdc.


Fresh gas only, fresh oil only, new gaskets, valve seats cleaned,

 

[gearhead13]

could be the timing chain adjuster, have you had a look at it? does it work properly?

 

[tkent02]

What was the compression before you took it apart?
You did adjust the valves correctly, right?

 

[Mekanix]

Completely rebuilt everything in the motor including the timing chain adjuster and its working fine and does adjust the chain properly with out moving or ticking or binding..


Compression was 145 L 147 R and after rebuild it was 143 L 140 R

adjusted the valves cold to .004" on all 8 of them just because that was between the .003 -.005 spec in the manual.

And after I pulled the head off last night they were all still at the same spec.

both cylinders were identical but the right side was quiet and the left was ticking in the area of the combustion chamber and the intake cam area.

My stethoscope only narrowed it down to these two areas.


Also I just remembered checking the timing with a light and it was right on the marks

 

[GregT]

Have a look at the fit of the shims in the top of the buckets.
It's not impossible that you have a Kawasaki shim fitted - which is of course smaller OD and would probably make a tick moving around in the bucket.

Greg T

 

[Mekanix]

Thanks Greg T but unfortunately I don't have shims or buckets.

The gs 400's were made with a 4 valve per cylinder head and I had to adjust 8 valve tapers.