The DR900 - DR650 Big Bore Stroker buildup

There's a couple more places that will have to be modified for clearance.
I temporarily installed a used 780 piston and cylinder.


At bottom dead center there's only about .018" clearance between the piston and the crank wheels. Not quite enough for comfort.
This will be easily be taken care of by milling a little material off of the piston in the appropriate place.

Here's the nozzle oil jet that does such a good job of keeping the piston cool.
This nozzle just squirts a stream of oil at the bottom of the piston.

Unfortunately for me, the piston skirt just barely touches the nozzle at bottom dead center. I'll have to make a little notch in the piston skirt for clearance.


The next thing to tackle is modifying the piston. Here I am relieving the bottom of the wrist pin boss for crankshaft clearance.

The right side (left in photo) didn't need as wide a relief milled into it because the right crank wheel has a relief cut into it for clearance with the 5th driven gear on the mainshaft. Note the notch in the piston skirt to clear the oil squirter.

The 790 piston has a rounded square dish in the top of the piston to keep the compression ratio at a manageable 9.7:1
Un-modified this piston would give a compression ratio of about 11:1 with the longer stroke. That's too much for an air cooled single with a 110mm bore. Oh yeah, I might run real strong for a little while but I'd rather try to build something that will stay together for long enough to get some real enjoyment out of it.

The combustion chamber in the head is more of a 'bathtub shape. Rounded on the ends.

With the piston clamped to the rotary I used a ball nose end mill to make the dish in the piston match the head.

The modified piston crown.
Less compression but still takes full advantage of all the available squish area. Lots of squish area and a tight squish clearance is very important to fight detonation.
This will put the compression ratio at just a bit more than 10:1 and should happily rum on premium pump gas.
As a side benefit, removing the material from the piston has lightened it by 16 grams. That should help make the motor smoother.



Finally putting the cases together.
It doesn't show up well in the photo but there's a paper thin layer of Threebond 1104 on the case mating surface.
As thin as you can put it on is more than plenty.

Cases are together and there is only a minimal amount of sealer squeezed out of the joint.
That will peel off easily as soon as it sets up.

I put the starter drive gears and flywheel back on the motor.

And buttoned up the left side case. I added the new seal retainer just to be on the safe side.


It's super important to check the ring gap and adjust if necessary.
In this case all the rings needed to have the ends ground.

Rings installed and nearly ready for the cylinder.
I rigged up a ring compressor. This is a 5" hose clamp and a strip of 1/16" ABS plastic between it and the piston.
I made a plate to use as a piston holder. You've got to have something to hold the piston firmly while installing the cylinder.


With the cylinder on it's starting to look like a motor!

I got the cylinder head installed along with a 190 cam.
All that's left is to get the clutch together.
Oh yeah, and find a bike to put it into...

OK, the clutch mod turned out to be too easy. :yahoo:
I modified the clutch hub to allow one more friction plate in the clutch pack. Here's the new 9-plate clutch.

The first drive plate (steel) on the hub is held in place by a wire retainer.

If you remove the wire you find a conical spring washer and a sheet metal tray the the washer fits into.
I believe these are intended as an 'anti-judder' device to help keep clutch engagement smooth.

With these pieces removed you can see a 2.5mm deep pocket where the spring rides.
I used a leftover mainshaft as a holding fixture and chucked the hub up in the lathe and machined the pocket flat. (sorry no pic)
That made just enough space to replace the washer/spring arrangement with an additional friction plate.


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