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Discussion Starter #43
Updates have been slow , work has been busy and theres been some other builds taking precedence over this one but once the engine comes hope the chasis will be going together



 

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Man, you always are building some cool shit.
 

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Do XR aluminum cylinders, which obviously look so much like traditional ironhead cylinders, match a 900 ironhead base bolt pattern? The stroke is less so I also wonder about deckheight for the cylinders...

I'd probably sell my soul to get my grimey hands on an XR750. If only souls were worth any monetary value.
 

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What kinda magic is going on with these things that they are making that kinda power? Big valves, high compression, heads flowing perfectly. Is the stroke the same as a 900 but with smaller bore or is it the other way around?
What is interesting is that back in 1971 when O'brien and HD developed the alloy XR750, they realized that to make power it had to be oversquare. It ended up with a stroke of 2.15 inch in order to rev (compared to 3 13/16 on the Sportster). They also needed a large diameter crank pin to live at high revs. That left no room for tapered crank pins and nuts, so they developed the pressed crank with a straight crank pin.

Twenty-eight years later, in designing the Twin Cam, H-D needed a much bigger crank pin to survive with the big displacements they were planning for. So they used the same trick they used on the XR750: pressed crank with no tapers or nuts.
 

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Discussion Starter #47
Thanks Govmule, The bolt pattern is not the same as ironhead not to mention the motors are stud motors as well , it could be done but wouldnt be easy to do on a set of ironhead cases
 

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Hi,

Shupe: That left no room for tapered crank pins and nuts, so they developed the pressed crank with a straight crank pin.
Small memories even at H-D!!!!

1st done on the K model in 1952...

Then redone in 1960, all the 250cc CRTT are built like that...



There is a lot of CRTT Italian flair/thinking inside any XR750...

Patrick
 
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