Flowing Your Crank Case

Luders · Post by **Luders** » Sat Jul 16, 2011 7:11 pm

Thought this might be useful to a few of the newer members, who are thinking of running their bikes for track/race use.

I'll be taking some pictures of the areas to fill on the crank cases and I've started today on the upper inlet section.

Once you've stripped your crankcases down, you'll just need to sand the key areas you'll be working on with a coarse grit, to take the smoothness of the surface off and give these areas a good clean and degrease.

Next you'll need some metal filler. I went into Halfrauds this afternoon and bought some of this evo-stik metal putty.
It comes in a stick with two compounds. Simply cut off the amount you require and using your hands, mix the two together so the colour is all the same.

Work it into the areas and go quickly as it doesn't take long to go off and after 10 minutes is too solid to manipulate. Whilst it's still maluable, you can smooth it over with a wet cloth to get a nice finish.

In these pictures, I have done the left hand side inlet only, so you can compare the two and see what they are like before and after.

Non filled right side

Filled left side

Looking at the areas from the inside of the crankcase, notice the difference between the two side

Notice in this next picture, I've tried to follow the contour of the crank when shaping the infills

Finally, (you can feel this ridge if you remove your carbs and reed valves and place your finger on the upper side of the inlet) I have ground down the casing and put a radius on that edge in the pictures below.

Standard, notice how sharp the edge is.

Flowed, notice the radius

If you comapare the two sides, you can see how the left side has been flowed nicely, allowing the air/fuel mixture to flow through the crankcase and up the transfer ports on the barrels.

non-flowed

Flowed

This is the view from the inlet side on the finished case

On the bottom half of the crank case, there are 4 areas which require filling. Note the holes, these are to allow the oil to pass through, so after preparing the surface, I've inserted drinking straws so that the putty can be applied, without causing blockage and having to drill them afterwards.

Once you have filled in and let the putty go off, remove the straws

Let the putty set and dress later, once you've done all that, you're done and can start to reassemble.

blanc · Post by **blanc** » Sat Jul 16, 2011 10:01 pm

looks great keep it coming.

ScottaKR · Post by **ScottaKR** » Sat Jul 16, 2011 11:41 pm

Luders, you didn't mention anywhere about keying up the surface your applying the putty to so it grips the surface properly. If you apply that stuff to a smooth surface it's likely to come off after a while due to vibration from the engine itself.

JanBros · Post by **JanBros** » Sun Jul 17, 2011 12:45 am

I just did a set myself 2 weeks ago, will take pictures tomorrow

Luders · Post by **Luders** » Sun Jul 17, 2011 8:37 am

ScottaKR wrote:Luders, you didn't mention anywhere about keying up the surface your applying the putty to so it grips the surface properly. If you apply that stuff to a smooth surface it's likely to come off after a while due to vibration from the engine itself.

Good point, I'll add that in as an edit.

JanBros · Post by **JanBros** » Sun Jul 17, 2011 3:57 pm

I used JB weld.
for the oil holes : I used a drinking straw. it's a bit to thick, so made a 0.5cm slice in it so it went in. benefit is also that the straws bends a bit because off this, so you can place it in such a way, the straw bends to the inside giving more room to make sure you can fill the tiniest spot. biggest advantage off this mthode I only found out afterwards : JB weld doesn't stick to it, so I could just pull it out leaving a nice perfect hole and no need for re-drilling

ben : the part I marqued in green : you can grind it all down, so that you have a nice flow from the reeds directly to the bottom transfers . look at my cases

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edit : just had another peep at MJ's cases, and seems even I can grind off a lot more

Luders · Post by **Luders** » Sun Jul 17, 2011 9:21 pm

Thanks for the pictures, they are useful to me, but will also be useful to others.

What I am doing here, follows more or less what MJ showed me.

Love the idea of the drinking straw, it's a simple, ingeneous way around the oil hole problem.

JanBros · Post by **JanBros** » Sun Jul 17, 2011 11:44 pm

Ben, I've added some red lines on your picture ; they should all be rounded off to give better flow

KR-1R · Post by **KR-1R** » Mon Jul 18, 2011 12:53 am

HARD & FAST - if it's like our version (called KNEAD-IT)... it becomes much more workable (softer like chewing gum) if you mix it with a few drops of water

http://kr-1s.co.uk/forum/viewtopic.php?t=1418
viewtopic.php?t=5001

It has occurred to some people that the 'vestigial' slots in the crankcase below the reed housings surely had a purpose.
Perhaps a way to locate an INSERT or RAMP over the crankwebs to reduce turbulence ("windage") to the incoming charge flow?

mj43 · Post by **mj43** » Mon Jul 18, 2011 12:47 pm

JanBros · Post by **JanBros** » Mon Jul 18, 2011 12:51 pm

mj43 wrote:

meaning ....

maccas · Post by **maccas** » Mon Jul 18, 2011 1:06 pm

Ben, be careful how close to the reeds you fill with the epoxy, put one of you reed blocks in and check that the reed tips don't foul on anything when you open them with your finger.

When i filled my cases 2 of the petals in each cage fouled on where i had filled so i had to take it back with the dremel.

Janbros is right with filing that bit out too, you can remove quite alot of metal which makes a lovely smooth passage to the transfer tunnels.

Maccas

Luders · Post by **Luders** » Mon Jul 18, 2011 1:43 pm

I'll be removing more material once I finish work

mj43 · Post by **mj43** » Mon Jul 18, 2011 6:41 pm

JanBros wrote:
mj43 wrote:
meaning ....

Yes but much more

TwoStroke Institute · Post by **TwoStroke Institute** » Wed Jul 20, 2011 3:23 pm

One would need a big tube of JB Weld to fill all the holes in these A Kit RS 250 NXA cases.