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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Honda CT90 Engine Fasteners

I put this post together as reference for the length of the fasteners used in a 1972 Honda CT90 for assembling the engine cases and covers, but also for some of the fasteners used internally within the engine.

Screw size convention
Example: M6 x 1.00 x 20
M6 - is the diameter of the screw, which in this case is 6mm
1.00 - is the thread pitch of the screw in threads per millimeter
20 - is the length of the screw, which in this case is 20mm

On a few items like pins and a washer I provide dimension in inches instead of mm.


Right Side Clutch Side Cover Screws
Two  M6 x 1.00 x 40
Seven  M6 x 1.00 x 20



Oil Pump Fasteners
One Bolt  M8 x 1.25 x 20 with a 0.118 inch diameter hole down the center that intersects a 0.040 inch diameter hole drilled radially in from the side.
Two  M6 x 1.00 x 25
One  M6 x 1.00 x 15
Two  M5 x 0.8 0 x 10




Shift Drom Stopper Plate Screw and Pins
One  M6 x 1.00 x 15 Counter Sunk Oval Head Screw
Four Pins (not shown) 0.156 inch in diameter and 0.513 inches long



Center Engine Case Screws
Nine  M6 x 1.00 x 40


Left Side Case Generator Cover
One   M6 x 1.00 x 45
Two  M6 x 1.00 x 40
One   M6 x 1.00 x 35
One   M6 x 1.00 x 30
Five   M6 x 1.00 x 25
One clamp for over flow line from carb



Timing Cover Screws
Three  M6 x 1.00 x 12 Counter Sunk Oval Head Screws

I have seen these screws vary in length on different bikes I have worked on and while I have never seen a screw shorter then 12mm, I have seen a number of 15mm and even an 18mm long screw.  There is not really much for the end of the screw to interfere withs whatever is on your bike is probably just fine.  Do remember to use the small o-ring on the screw that acts as an oil seal.



Sub Transmission Cover
One   M6 x 1.00 x 40
Three  M6 x 1.00 x 30

If you have a later model where the cover has two guide bushings then you would need:
Two   M6 x 1.00 x 40
Two  M6 x 1.00 x 30


Timing Gear Cover
Three   M6 x 1.00 x 12



Spark Advancer Retention Bolt and Washer
One M6 x 1.0 x 16
6mm Washer that is 0.665 inch in diameter and 0.078 inches thick




Points Plate Hold Down Screws
Two   M5 x 0.8 0 x 10


Screws to hold Points Assembly to Points Plate
Two  M4 x 0.70 x 5 (not shown)

Points Cover Screws
Two   M5 x 0.8 0 x 18 Counter Sunk Oval Head Screws




Rocker Arm shaft Access Cover
Three   M6 x 1.00 x 20




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

 Link to page with listing of CT90 parts available on Amazon



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