# XR750



## cyclerider57 (Aug 28, 2011)

I've been watching and reading for a little while on the site now and have finally started my first scale project. I'm an engine development engineer and race engine builder so I'm not new to machining but this will be my first micro build. I started yesterday by getting together some parts and laying them up in CATIA to 1/4 scale. I'll try to keep the build updated as it progresses because those are the threads that have inspired me to do a build (such as stevehuckss396 v8).

My build will be a 1/4 scale Harley-Davidson XR750 motorcycle engine. The XR750 is still the dominate engine in grand national dirt track racing and where my roots are so it was an obvious choice.

Regards.
-Jordan


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## kustomkb (Aug 28, 2011)

Welcome to the forum Jordan.

Nice choice of engine.

All the best with your design and build!


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## seagar (Aug 28, 2011)

wEc1 Jorden, I am looking forward to following this build.

Regards,Ian (seagar)
Coffs Harbour,
Australia.


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## metalmad (Aug 28, 2011)

This ones gunna be great ;D
will be watching 
Pete


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## cyclerider57 (Aug 31, 2011)

I've got most of the cam cover setup in CATIA. I'd like to do the cases next so I can get started on machining to help break up the measurement time.


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## stevehuckss396 (Aug 31, 2011)

cyclerider57  said:
			
		

> I'll try to keep the build updated as it progresses because those are the threads that have inspired me to do a build (such as stevehuckss396 v8).



So your blaming me for this one. Well OK! Best of luck and if you need anything, were all here!!


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## cyclerider57 (Nov 21, 2011)

Sorry for no updates. I promise this isn't a lost project, the Masters degree takes up a lot of time once classes start back up. I'm finishing up the right case along with the cylinder heads. I'll have to get more images at a future date.
-Jordan


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## cyclerider57 (Dec 11, 2011)

Attached are a few more pictures of my CATIA files. The left and right cases are pretty much buttoned up as well as both cylinders. I am working on the heads during lunch breaks and started the rocker boxes tonight. I plan to start the machining process as soon as I complete the drawings for the cases.

I've also attached a couple of pictures of one of our race bikes for those that haven't seen a Harley-Davidson XR-750.


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## JorgensenSteam (Dec 11, 2011)

Hey now there is a bike I could really get into.

Very nice.

Pat J


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## Foozer (Dec 11, 2011)

Now thats a Putt Putt

Robert


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## Bob Farr (Dec 12, 2011)

Schwing! Coolest project EVER!


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## cyclerider57 (Dec 12, 2011)

Thanks for the comments everyone.

I see we have a dirt track fan! It was definitely the sport of the past but they are trying to get more interest. Once the races get going again I'll get a schedule up for anyone interested in going to a race.


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## cyclerider57 (Dec 19, 2011)

It is coming closer. The models for the heads and rocker boxes are nearly finished. I have next week off so with any luck, I will be able to start making some chips over the holiday.


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## stevehuckss396 (Dec 19, 2011)

cyclerider57  said:
			
		

> I have next week off so with any luck, I will be able to start making some chips over the holiday.



Sounds good, what time do you want us to be there!! :big: :big: :big:


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## cyclerider57 (Dec 20, 2011)

Lets do it! It'd be cool to meet some of the scaled engine guys in Michigan. I'll put all of you too work on some of my other projects. ;D


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## cyclerider57 (Aug 7, 2016)

After many years and moves causing files to be lost, I have caught back up in replacing the loss. The only  parts previously causing chips were cylinders so I decided to print the parts. Now holding the scaled plastic model I can only debate keeping it 1/4 scale or going to 1/3 scale for the real deal.


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## metalmad (Aug 7, 2016)

Hi Jordan
Glad to see you back on this.
Life just gets in the way sometimes.
Pete


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## cyclerider57 (Oct 3, 2016)

After looking deeper at both sizes in the plastic models I have determined I should go for 1/3 scale. I question my ability to make a spark plug small enough for 1/4 scale and keep the plug from looking out of place. With the decision of scale made, I was able to get on the mill and start cutting. I have started with the primary cover this time to get an understanding of body of work and complexity to determine if I am skilled enough to go for the crank cases next. Without them the rest of the project won't go anywhere and they are definitely the most complex pieces I've ever machined. First thing first though, I need to do the outside geometry of the primary cover next.


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## stevehuckss396 (Oct 3, 2016)

Looking forward to this one!


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## cyclerider57 (Oct 12, 2016)

Never cut corners. Pun intended. For full disclosure I wanted to show a mistake last night. I broke a drill bit in one of my holes and couldn't get it out blind. I decided to move on with machining the top of the primary cover and when I broke through into the the hole I would be able to dig the broken pieces of the bit out. I was trying to avoid taking the part off the mill and having to set it back up again. Well that laziness cost me. I wasn't able to get the bit and decided to make "one last pass" and if I still couldn't get it I would take the part off the mill and just punch the bit pieces out. In my haste, I didn't make sure I was clear to go. Well I went with it and it turns out that the rubber cover keeping chips from getting in the base of the mill got bound up up under the table during the pass. The stock is 3.5" wide and pushes the clearance limits of a desktop mill. I've come to the conclusion that the whole table shifted in the +y axis. I am guessing at this point that the motor slipped in the collar connecting it to the ball screw and threw the y-axis off. Since it is a home grown cnc it doesn't have a feedback mechanism telling the computer the table didn't move even if the motor did. This happened at a very inconvenient time as this was the first pass going along what will become the sides of the part. You can see that the holes are not aligned with their respective cutouts in the picture. Since I wasn't using a flat bottom end mill I may still be able to save the part even if it takes some finessing at the end with a file. I have a lot of time in this one piece and I'd hate to have to start it over. It is luckily shifted so the bottom of the part would be the goofed and wouldn't be as visible so I think I'll be ok. At the end of the fiasco, I still had to take the part off the mill to get the bit out so I didn't end up saving any setup time anyway. The lesson, don't be lazy or try to go to fast. Always do it the right way even if it takes extra time. The good news is that I learn (or at least relearn) something every time I go back out to the shop. Because of work and my carbon fiber work, it has been a quite some time since I machined anything of any significant complexity so even with this screw up I still enjoy it.


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## Herbiev (Oct 12, 2016)

Some good advice there. I was always guilty of taking shortcuts to speed things up. Then I realised " What's the hurry. I'm retired"


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## cyclerider57 (Nov 29, 2016)

I was able to save the part. The bottom has some damage due to the miss cut but I'll be able to smooth it out to the point it won't be noticeable unless you look specifically for it. I finished the outside machining. I only need to do a little cleanup along the edge. I was debating leaving it as machined but to help with the look of an actual scale of the real thing I will likely give it a bead blasted surface. Right now I'm trying to determine the best order of machining for the cases. I'll probably post a few more pictures of the plastic models and ask for advice on ways to do the work holding of the pieces since they lose most flat surfaces through the machining process and I'll need to make sure the two case halves align properly with pins before the final hole dimensions are bored.


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## stevehuckss396 (Nov 29, 2016)

looks good so far.


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## cyclerider57 (Nov 30, 2016)

Here are some pictures of the crankcases. If anyone has a best practice on the best method for order of machining or how to best hold the parts I am open to suggestions.


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## stevehuckss396 (Nov 30, 2016)

I would try to machine the top photo first. To do the second side I would use the bolt pattern in the big oval to mount it to a plate and them machine the second side. Same with the second piece. Do the bottom photo side first and again mount it to a plate. If the holes in the bosses are through holes for bolts drill under sized and tap them for mounting. After your done drill out the threads to the required size. I have done that a few times.


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## cyclerider57 (Nov 30, 2016)

Thanks Steve that's what my initial thoughts were. There are a few surface such as the cylinder mounting locations that I'll need to leave the final cut until they are bolted together but I don't see that being an issue. My real challenge will be at the bottom of the  right case where the oil passages are below the cam area. They have cast in valleys in the original case so I'm not sure how to handle that yet. The size and location mean that the standard practice of an angled drilled hole won't cut it. Sorry for the poor picture quality.


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