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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Using a Spark Plug from my Subaru Outback on my CT90!

 I was changing the spark plugs on my 2015 Subaru Outback the other day, which isn't the most enjoyable task, but while cleaning up and putting away my tools, I noticed that the old plugs I pulled out of my Subaru had the same M14x1.25 thread as what is used on the standard D8HA CT90 spark plug.


I really like the technology behind newer spark plug designs like the  NGK Iridium's I use on my Outback and in the back of my mind I was one day going to sit down and search the internet and see if I could find an NGK Iridium in the same heat range and size that would fit a CT90, but have never have gotten around to it and I don't know if one even exists.

Since I had the old plugs from my Subaru, I thought I would do a proof of concept test and design an adapter to allow me to use the plugs on my CT90.  I had some scrap aluminum, so I chucked it up in my lathe and made a simple sleeve that makes it so the NGK LKAR7B1X-11S plug would protrude the same distance into the CT90 head as a stock D8HA plug.


Here is a picture of the finished adapter sleeve next to the two plugs and then slipped on the Subaru plug.


Before installing the Subaru plug on one of my CT90's, I first checked it out on a spare CT90 head I had in the shop and found that it protruded about the same or actually slightly less than a stock D8HA plug, so I was good to go to install it on a bike to test it out.


This is what the Subaru plug and adapter sleeve look like installed.


The next issue I had to address was that the end of the Subaru spark plug wasn't compatible with the cap on the spark plug wire on my CT90.  The end of the Iridium spark plug is typical of standard automotive spark plug, but it is permanent and can't be unscrewed, so I needed a compatible cap to put on the spark plug wire of my CT90.  Luckily I had a LB05EP cap in the shop, so I swapped it out with the one currently installed on  my CT90.


Once I changed out the spark plug wire cap I decided I should check to see if I could get a spark with the Subaru plug before I installed it by grounding it to the head of my CT90.  The first time I tried getting a spark, I only got a weak spark, so I thought it might be due to the stock .040 gap used on the Subaru plug and the CT90 coil just didn't have enough output to produce a decent spark with that gap.  I reduced the gap on the Subaru plug to about .025 and then I got a good looking spark and went ahead and installed the Subaru plug with the adapter on my CT90.

Here is a picture of the Subaru plug and adapter installed on the bike.


With the Subaru plug and adapter installed it does move the plug wire closer to the exhaust, but it was still had acceptable clearance.

For the final test I tried to start the bike and it fired right up without and issues!  I then proceeded to take my CT90 out for a ride in the neighborhood.  The bike ran fine and I couldn't tell any difference compared to running with a stock plug.

So, from a proof of concept perspective I think I can say that yes, you can use a Subaru spark plug on a Honda CT90.

One of the questions I had in the back of my mind was potential leakage from the combustion chamber past the threads of the spark plug and adapter.  I couldn't detect anything while the engine was running, so I don't think it is an issue.  While it may not be apparent in the pictures, I did incorporate a small raised feature with a 45 degree chamfer on the end of the adapter that contacted the cylinder head that matched the chamfer on the spark plug hole in the head to act as a seal, but I don't think it was needed.

From this successful initial test I plan on refining my adapter as I like the idea of using a newer technology NGK Iridium spark plug as they offer greater durability and I think the design may be less prone to fowling.

I hope you enjoyed this post and I will added to it or make a new post when I refine my adapter design and take my CT90 out on longer rides using Subaru/NGK Iridium spark plug.


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