Piston Replacement

[thumper135]

I am working on replacing the Pistons on a old 1983 Yamaha XJ400, 4 Stroke 400CC Bike engine, the previous pistons are well over the wear mark. Upon looking up for replacement pistons sadly I came to know none of the manufacturers make the pistons I'm looking for anymore, wiseco included etc.

So now the best replacement pistons I have found from a different bike engine look to be identical EXCEPT the pin to deck height which is 1mm to 1.5mm shorter than the original stock pistons. I can make up for the difference of height by shaving either the bottom base of the cylinder or the head gasket surface area. Now I'm just unsure if it will cause any issues in the stroke or performance. Also if I can go ahead with the shave idea, what would you recommended to shave, the base of the cylinder or the top?

 

[whenfoxforks-ruled]

A shave will change the timing and cam chain tension? Its an over head cam? Vintage Bob

 

[mideastrider]

Timing chain is the issue that comes to my mind. I don't think you will be able to take up that much slack. Also make sure the skirt will not hit anything down in the crankcase at the bottom of the stroke.

 

[thumper135]

A shave will change the timing and cam chain tension? Its an over head cam? Vintage Bob

Yes, I believe the timing will be retarded slightly which can be dealt with, same goes for the timing chain slack as there is quiet a bit of travel in the cam chain tensioner left.

Yes, Its a Dual over head cam setup.

I have also been told that if no corrections are made to the Cylinder the engine will run fine, at most It will lose an hp or 2 but apart from that no other issues. I.e: Octane or detonation etc.

Your help in this case will be precious.

Thanks.

 

[thumper135]

Timing chain is the issue that comes to my mind. I don't think you will be able to take up that much slack. Also make sure the skirt will not hit anything down in the crankcase at the bottom of the stroke.

The Cam chain tensioner has enough travel left on it to take care of the extra slack, and yes If i go the shaving cylinder route I will make sure the sleeve bore does not hit the crank down below. However on a more closer inspection it will be alot easier to shave the top of the cylinder than it will be to shave the bottom base of the cylinder.

 

[whenfoxforks-ruled]

I would be taking as much as possible off the head first. .039" looks better even if you can split the cuts. Take 20 off the head, and 20 off the top of cylinder if you need it.

 

[thumper135]

I would be taking as much as possible off the head first. .039" looks better even if you can split the cuts. Take 20 off the head, and 20 off the top of cylinder if you need it.

Sorry for the late reply. What I did was use a .5mm head gasket and use a slightly thinner base gasket than stock. I've squeezed out almost 1mm by using thinner base and head gaskets, so I believe that makes even for the piston to deck height that was previously shorter.

The new pistons are however without a dome, whereas the old pistons were high compression with a dome of atleast 1.5mm. Would there be a major difference if I used the short domed pistons? what performance decrease or gains would I be dealing with.

 

[whenfoxforks-ruled]

Let me guess, the stock piston was domed, and pocketed for the valve clearance? If I remember that bike has a ton of compression to begin with. If it will start without needing to be pulled by a quad, success. I suppose reworking the head is out of the question. Getting companies to make a run on an obsolete part, VERY EXPENSIVE. Compared to back in the day, not impossible. The no longer made rod for my 370, can be had with a minimum 10 order of rods at around 200 dollars a piece. I do not suppose there are web sites dedicated to this model?

 

[thumper135]

Let me guess, the stock piston was domed, and pocketed for the valve clearance? If I remember that bike has a ton of compression to begin with. If it will start without needing to be pulled by a quad, success. I suppose reworking the head is out of the question. Getting companies to make a run on an obsolete part, VERY EXPENSIVE. Compared to back in the day, not impossible. The no longer made rod for my 370, can be had with a minimum 10 order of rods at around 200 dollars a piece. I do not suppose there are web sites dedicated to this model?

yes, the stock piston was domed while the new is one fourth domed than stock, I think by using a thinner head gasket I can get away with it.

Now there is another issues I'm dealing with while installing the pistons in the cylinders, I measured the ring end gap in the bores, on top of the bore the ring end gap is around .13mm while in the bottom of the bore the ring end gap is approx .15mm. I believe the bore is slightly out of round but I have been told not to worry about it and that the rings will seat inside the bore accordingly once the engine is run.

Your suggestions?

 

[whenfoxforks-ruled]

If it is near or more than .003" of taper/runout, it will never work properly.

 

[thumper135]

there is almost .002" of out of roundness in the bore, which is below the .003" you mentioned, therefore I believe it will be fine to run the cylinder as it is? any compression or any other problems I might face?

 

[whenfoxforks-ruled]

Low compression means it will be hard to start.