Porting & Polishing the XS650 Head

Porting and Polishing the XS650 Head

The stock Yamaha XS650 head has its good and bad points. Good points are the hemispherical combustion chamber and valve pockets, the latter which act to unshroud the valves at low to mid lifts, increasing the overall breathing efficiency. One of the bad points is the degree of finish the factory performed to the head. Mine was in dire need of improvement. The photo to the right shows the head after my light P&P job.

Tools and Resources:

For tools, I used a Dremel tool with flex shaft mostly. A long-shaft die grinder is what professionals usually use, but the Dremel was adequate. I also used a set of riffling files; I found the curved ones to be quite useful getting to the radii in the runners.

For materials, I used a selection of stone bits and metal bits, cone-shaped sandpaper bits, and grit-impregnated polymer bits. I also used emory (aka crocus) cloth in coarse and fine grits.

Resources to learn more about porting and polishing are scattered over the Internet, but I recommend that you start here because it's the most comprehensive source I've found:

Standard Abrasive's P&P DIY

A good source of P&P information specific to the Yamaha XS650 can be found at the XS650 Garage. Here are a few threads:

Working the Stock Exhaust Guide and Boss Area
Porting Your Own Heads
Porting for XStreme Power

Click on the small photos below to see them full size.

This next pair of photo shows about the way the XS650 head looked after having the machine shop do a 3-angle valve job. (Click on either photo for an 800x600 view). I had already begun working on the intake bowl on the left before I remembered to take this picture. If you look closely at the intake bowl on the right, you can see a pronounced ridge about a third of the way between the seat and the valve guide. I had already ground most of this ridge away on the left side. The ridges are left over from the machining process and can be cleaned up quite easily. These marks can be seen a little easier in the second image.

Here is a look at the two intake ports. Even though I've been working on the right one, you can see the lumpiness of the wall. Some evidence of this is visible on the left side port as well.

This image illustrates a lip found inside the exhaust runner. It is there because the factory pipes fit up into the head this far (although aftermarkets like MACs don't), and fit flush against it. Removing this lip is a mistake, according to Michael "Mercury" Morse, a highly respected XS650 tuner, and owner of 650 Central. Michael races these bikes and has a respectable win record in AHRMA racing. The exhaust port diameter needs to be retained to maximize flow, Michael says, and removing this lip, thus widening the port, will reduce the flow (it is well known that if a given amount of a gas is passed through a narrower passage than a wider one, it will flow at a higher velocity through the narrower one, albeit at a lower pressure). With the XS650's exhaust ports in particular, velocity needs to remain high. Michael sells "exhaust port optimizers" that fit into this spot, and which are optimized to an engine's particular needs. They are spendy, but supposed to work very well. A set of poor man's EPOs can be made by cutting off the ends of a set of stock exhaust headers and removing the outer pipe wall. Obviously, the only sort of optimization they have is that which Yamaha designed in at the factory. But they are certainly better than leaving a gap there between the lip and aftermarket headers, which will increase turbulence and decrease flow. They will fit inside the MAC aftermarket headers. Since a set of funky OEM headers came with this bike, I decided to cut out the port ends and go this route. I can always spring for a set of Michael's EPOs later.

Here is a better view of one of the intake ports. It shows a bit more clearly the lumpiness I was describing above. I have already done a fair amount of work to the runner by this point, but as you can see, it still needs more.

The first image is a close-up of one side's intake and exhast bowl areas after working on them for a while. They're starting to look better. The second image shows an even closer look at the exhaust bowl, with a partial view of the side of the intake's. The third photo shows one cylinder's ports after I had just started the smoothing process using special grit-impregnated polymer bits and emory cloth with lots of elbow grease.

The following photos are an assortment of shots taken after I finished. I have learned since completing this project that current P&P theory suggests that intake ports should be left with a somewhat coarse finish while exhaust ports should be smooth. The theory goes that, if an intake port is too smooth, there is a greater likelihood of the fuel condensing on the intake runner walls, which is not good. The fuel-air mixture should remain well atomized when it reaches the combustion chamber. So I'll be going back over the intake ports with some coarser grit emory cloth to roughen things up some. Exhaust ports should be as smooth as possible, so the theory says, to maintain maximum flow.

You'll note that, in the following photos, the valve guides have been removed. I found it very difficult working around them and trying not to nick them as I was grinding away at the rough spots, so I had a local machine shop remove them for me. It is possible to remove them yourself (Mikes XS sells a tool to remove and install them) although I have never tried it. After all this work I've decided to buy new guides anyway, so it's just as well.

That's it for now. I'll post more pics when I have more to post.