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Old 11-10-2012, 06:39 PM   #143
FJ_Kevin OP
Gnarly Adventurer
Joined: Dec 2010
Location: Maryland ==> originally from Long Island
Oddometer: 363
Top end continues...

OK, fair warning, I am really going to beat a dead horse in this section!

------------------ Top end continued

During the time the cases and top end were apart, I made some effort to consider how the Yamaha OW factory porting methods (shown earlier) could be applied to the YZ465 / 490. Of course this would be for future modifications to another motor as the top end of the current motor has already gone back together.

This is interesting to me because many of the methods used in the factory OW’s were closely held secrets back then. So it is fascinating now to be able to look back at what was done and how some of the same methods have turned up in later production race bikes. In fact, I found several photos of big bore MX cylinders and competition snowmobiles that will be included to illustrate this point.

In any case, two questions came to mind after studying the photos of the factory OW cylinder photos and how they may relate to modifications of the 465.

1) Can the two additional intake to transfer ports be added to the 465 cylinder?
- We already know the answer is yes as LOP cylinder shows this. Still I think it is worth taking a closer look especially when comparing these early implementations to those in modern big bore cylinders.

2) Can the OW style case induction port be added?
- To answer this we can look at the cylinder to case interface to consider what is possible.

To refresh, here again is the OW cylinder where the new ports in question have been labeled.

New intake/transfer ports

The next photo better compares the OW cylinder to the LOP 465 cylinder where the top row is the OW cylinder and the bottom is the LOP.

The red arrows show the corresponding intake to transfer ports. The blue arrow points to the case induction port in the OW cylinder and where we would like it to be in the 465 cylinder.

The OW cylinder is thought to be from a late ‘70s (pre-watercooling) 250. The LOP 465 photos (bottom row) show the addition of the intake/transfer ports like the OW. I was later to find out that these are called "Boyesen Ports" after inventor Eyvind Boyesen of dual stage reed valve fame.

It turns out the ports in both these cylinders are relatively small compared to photos of newer model production cylinders like CR500 and others that I have found. Unfortunately, making these ports much larger in the 465, without breaking through the side, would be difficult due to the location of the cylinder studs and the clearance needed for the stud nuts.

Perhaps LOP has defined the limit on this?

Case Induction
Regarding the case induction port question, the first thing I tried to do was find another example of this, especially one with photos showing the engine case side as I do not have a photo of the OW engine case. I was lucky to find this example,

The left photo above shows the induction port on the engine case side. It is milled to some depth below the base gasket surface.

The right photo above shows the base of the cylinder. The induction port is milled from the base right through to the intake port. Notice too how the intake side of the sleeve is cut back near the induction port.

This cylinder also has the LOP/OW style intake to transfer ports (although they appear much larger). I believe this is a good example of how to go about adding the case induction port to the 465 motor.


Now let’s look at the YZ465 case to cylinder interface where the cylinder to case induction port would be. The upper left picture shows the area in question and where any modification would take place.

On the case side, there is danger of milling through the crank case and into the transmission cavity (very bad!). A possible region for milling of the case and cylinder is shown in the lower left photo. This region, in green, leaves just enough surface area in the case for a good seal when the case halves come together.

The above right photo shows how the sleeve might be cut away. Only half the region is colored in green (along the case centerline) so the width of the cutout would twice shown.
Note that the sleeve cut away is in an area corresponding to the skirt cut away in the piston. Cutting the sleeve in this area does not reduce support for the piston skirt.

So, given the above, it does appear that both OW style port modifications could be applied to the 465.

Take a break now with a few photos. Here is Lauren Offner of LOP on left,

My '81 YZ465 Factory Race Replica

My Vintage Husqvarna/Motocross Restoration Extravaganza Thread

FJ_Kevin screwed with this post 12-14-2012 at 02:55 PM
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