XR100 oil cooler install with pics

So I did quite a bit of reading the last few days here looking for tips on a subject well covered. How to install an oil cooler on a stock cylindered XR100. And I found so much great advice but no pics.. So I did mine today and took plenty of pictures and notes.. So I figured I would give back and post them up. So here ya go.

I guess this is called the OLD Skool method. but basicly its the only way to install a cooler with a stock cylinder and clutch cover on a XR80/100. The Aftermarket cylinders have fittings and a few companies make aftermarket Clutch covers with fittings as well (for the APE100 in Japan). But if you dont have those they you are stuck..

Yes its true most XR100's dont need a cooler, but if your like us and you run it very hard and have it heavily modified then its a must.

We have a PRC built 110cc xr100 with basicly every mod I can do to it at thise point. We use the motor in a road race (RS125) chassis for roadracing so the load the high RPM usage is very high. The motor was damaged last race due to overheating and so this install was much needed.

So to start remove the clutch side cover and clean it well.

Put the motor in a vise or something strong to hold it as you will need to drill into the engine cases.

First this is to find the location of the oil passages in the engine cases.

the white Arrow shows the oil passage area. You will see this on both the engine and the cover.


and on the cover


This passage is the output of the oil pump. It flows oil upwards in this channel to the small pinhole at the top (engine side) where it flows to the cylinder head. This is where you need to pickup the oil flow and send it to the cooler.

First this is to drill out the plug in the block used to seal up the hole from the pump passage. Also you can see the location from the pics above.


You will want to use a 1/8"NPT fitting. Use a 1/8"-27 NPT tap and "Q" drill bit. Make sure to start with a very small drill and work your way up. Also put grease on the end of each bit to capture metal shavings so that they dont fall into the oil passage, and blow it out after each drill from the inside of the motor to the outside. This is very important as shavings can damage the motor.

Dont drill all the way into the internal passage, only about 1/3" from the outside edge of the cases is fine.

Later you will install an 1/8" to -4 AN fitting with thread sealer here but do not do this yet.


The next step is to drill the Clutch cover..

The location of where you drill is sort of up to you and the room you have. For our particular installation I choose to drill into the top most area of the oil passage in the cover..


Again using the same 1/8" NPT to -4 adapter and a 90deg -4 AN fitting.

As you see in this photo the oil passage is not only on the cases but on the cover as well. Though this is a narrow passage and to tap this correctly I had to use 2 1/8"NPT taps. One was a standard tap right out of the package the second I cut off the lower 1/3 of it to allow myself to reach the thicker upper part of the tap. As you can only tap down about 1/4" due to the other side of the passage wall.



After this is done again install the NPT fitting and thread sealer.

The next step is to block off the oil passage to force the oil to head out of the first fitting we installed.

As you notice in this photo the arrow points to a machined hole that leads from the pump passage to the oil passage way up to the head. We need to fill this hole so that the oil travels out the fitting.


Here is where my best friend JB weld came in..

Take a small metal rod approx the same diameter as the hole at the base of the shaft where you drilled and tapped for the fitting. This is about 1/16" or so.. Put grease on the rod and slide it down in the hole(this keeps the JB weld from sticking to it), this will keep the JB weld from filling the oil passage up and thus blocking oil flow completely.

Next mix and apply JB weld into the bottom of the passage completely covering the hole, make sure its fairly thick as we dont want it to fail and let oil through. You can see in this pic where I applied the JB weld


Every 10 minutes turn the rod a few turns to make sure the JB weld does not stick to it and seal it all up..

Lastly install the fitting as described above and you are now good to go.



Put on your clutch cover install your engine and start it up.

Make sure to test oil flow by removing the inspection plug next to the spark plug and also remove each of the hoses to your cooler to test flow and pressure just to be safe..




This is one of the best "How to" I've ever seen. Great photos and excellent desciptions. What oil cooler did you buy and how did you mount it on the bike. Any photos of the end product? I would recommend a Kitaco Hi-Flow oil pump to go with this since you'll be pushing the oil through an oil cooler and extra plumbing. An oil temperature dip stick from XR's only website would tell you how effective it is.

P.S. I bumped your gas gauge for this post. Awesome job!

Thanks Hornet... much appreciated for the advice and kind words

I will post a pic of the bike later today..

As for the Oil Cooler. We used a custom EARLS bike cooler and custom braided plumbing from EARLS as well.

As for the Oil Pump, our Oil Pump was heavily customized by PRC (www.xtrememinis.com) when the motor work was done. So its up to the task.

You can read about us and our bikes on that site as well, Austin my son is listed as his featured rider..

That, indeed, is an excellent project, excellent photo-documentation and excellent write-up!

Good work!

I'll also expand it a bit in that this applies to the Honda 100s and 125s going all the way back to the first of them of the early 70s. I'm overhauling my own '72 SL125, and it's exactly the same, as is a '75 CL100 motor I have. They all have the same oil pump/oil passage up to the head.

Again, good work.


Here are some pics of the install. This is a 1994 Honda RS125 GP chassis with the XR100 motor retrofitted. It took a major amount of work to get this to fit, but it works very well. This bike races against most 50cc 2 strokes and has to date never been beat.

this is the bike itself..






It works ok..


Wow :crazy: That bike is ridiculous!

were there any clearance issues with the intake/carb hitting the shock mount of the pre'95 frame?

Rob the sparky

I think the pre 95 frames are fine, but the 95 and newer RS125 frames, will run into problems with this motor install. We had a guy in our club do this swap and it worked out great for him.

Actually yes we did have major clearance issues.

All RS125 frames will require modification of some kind.

The pre95 frames like ours required a significant amount of work.

We removed the shock mount on both the frame and swing arm, then fabricated it back 7" from its original postion, this required both work to the frame and swingarm and renforcing many areas. We also had to clearance the right motor mount spar to allow for spark plug removal.

All told it was many days of work.

I would be happy to speak to you offline if you are looking into doing a conversion yourself you can email me at ericdehaven@hotmail.com

Also if interested this bike is for sale currently, also email for more info.


Okay, I might have it backwards... the newer RS125 frames don't need cut or modified, and the older ones do. I'll have to get a few pic's of my friends RS-100 that he converted, said it was easy and worked out great!

Very Nice


ok, stupid question, but if you block off that hole that feeds up to the head, how does the oil get to the top end?

In stock form the oil flows from the case to the side cover and then up to the cylinder and head. When he added the oil cooler, he blocked off the hole from the case to the cover and redirected it throught the cooler and then back to the side cover which then sends it to the head. This sends the freshly cooled oil to the hottest part of the engine, the cylinder head. The reason for blocking the passage is oil would not flow through the cooler if it was left open.


haha man i want one of those now

Well the bike is still for sale :banana:

if that hole is blocked, isnt that the only passage up to the head? it looks as if the oil is cooled and dumped back into the clutch cover, i still dont understand, where does it go up to the head? sorry for looking like a nimrod, i have built many engines, but dont quite get this one?:banana:

Hi! Am i lucky and anybody have some pics from this topic!!!!???   

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