Honda CT90 Engine Reassembly

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I have one 1971 CT90 where I was going to rebuild the clutch because the kickstarter just wasn't turning over the engine like it used too,  and I also had a Big Bore kit from DrATV that I wanted to install. My intent was to just do the top end and rebuild the clutch with new disks, but when I opened up the cases to rebuild the clutch pack I found a slick black slime in the bottom of the case.  I didn't like the idea that there could be black slime in the rest of the engine, so I decided to disassemble the entire engine to make sure I cleaned everything out.  Since I was tearing everything apart I thought I would document step by step how everything goes back together in this post in case others were interested in knowing what the inner workings of a CT90 engine look like. Links to Related Posts: My Honda CT90 Clutch and Headset Nut Tool Cut Away of a CT90 Engine - CT90 Engine Exposed! Repairing Damaged CT90 Spark Plug Threads Using a Time-Sert Thread Insert CT90 Cl...
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A 3D Printed Functional Front Air Filter Housing for use on a Honda CT90 K2 and Later

One thing I have had on my list of things to do is to design a functional front air filter housing that can be 3D printed to go along with the rear air filter housing I designed several years ago and that I made a post on here at this link.  

Related Posts: CT90 K2 and Later Air Filter Housing 3D Printed Seal 

After some effort I was able to create a functional 3D printed design of the front portion of the air filter housings used on K2 and later CT90's.  The picture below is of the final configuration that I designed and then printed (in orange) and installed on one of my bikes along with my previous rear air filter design (in grey).

I tried to make the design to be very similar to the original stock Honda air filter assembly and I think I came pretty close.

The design uses all the stock metal components from the original Honda air filter assembly and has the same mounting interface.


Where the design differers from the stock Honda design is that it doesn't use the large rubber seal that goes at the front end of the internal metal components as it is not needed as there is now a surface that the foam air filter element bumps up against in my 3D printed design.

I also didn't incorporate a groove for a seal in-between the front and rear parts of the air filter housing as it really isn't needed.  The fit between the two housings is very snug and the seal is really only relevant if you are crossing a stream and the housing would get submerged, which I figure 99.99% of the CT90 owners would never be doing.  Even without the seal I doubt much if any water would get past the joint if it was submerged for a short period of time.

The front air filter design is actually an assembly composed of three components: a main housing and then a front and rear mount as shown in the photo below.


I designed the housing to be an assembly because most of the time what happens is either the front or rear attach point breaks off and you have to toss the whole housing.  With my design if one of the attach points gets broken due to over tightening a bolt you just print a new one and replace it.  It also allows you to print the attach points out of a different material that may be tougher or more flexible like TPU.

The front and rear mounts assemble by sliding them into grooves on the backside and integral to the main housing


Here are a couple of pictures with the front and rear mounts partially installed. The fit is very snug and once installed they are difficult to remove, but are removable.


I printed the housing on my Prusa Core One+ which it easily fit within the print volume and I think it should be able to be printed on most current 3D printers that have a similar bed size and print volume.


I actually incorporated a removable "fin" support into the design so that minimal support material is required with the housing is printed.

I did use supports around the overhanging edges at the rear opening of the air filter housing as shown in the pictures below.


All in all I'm real happy with how the front filter housing came out and think it is a good functional alternative if you have a CT90 missing the front air filter housing.

If someone would like to print this housing, you can download the .stl file at Pursaprinters here at this link. 


Helpful Links (Shop Manuals, Wire Diagram, Model Information, etc.)


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