View Full Version : Kitfox Aluminum Radiator retrofit to early S7

Dave S
09-04-2015, 02:46 PM
The old brass/copper radiator that came with the FFW for our S7, back before John & Debra had the company, started getting some moist spots around the tubes where they entered the end tanks.

Anyone who works with cars knows that it nearly impossible to find a genuine radiator shop that can competently repair or recore a radiator can guess I might be looking at a replacement rather than a repair.

Anyway, the radiators that John Mcbean makes for the current Kitfoxes equipped with the rotax are: 1) All aluminum, 2) have a thicker core, 3) Dimensions similar but not identical to the old tech brass radiators - after communicating with John & Debra I became convinced the new radiator was the way to go.

I wanted to make this report so others are aware of the minor modifications needed to adapt the current aluminum radiator to Kitfoxes that had the old brass radiator and I am happy to report the mofifications are minor and straightforward. No need to fear the change.

1) The new radiator is about 3/8" thicker; and, the center ends up being about the same dimension lower in the cowl due to the fact that the bottom is flat all the way across whereas the old one has the end supply tanks protruding down a bit resulting in the center where the strap goes being a little higher. Due to these minor differences in dimensions, I found the old center strap had to be replaced with a new one to allow for the thicker core and slightly lower bottom - easy enough to measure up the differences and fabricate a new center strap.

2) New longer end straps (about 7/8" longer) which run vertically to each side of the engine mount from the radiator had to be fabricated to account for the lower position of the two holes in the mounting tabs on the ends of the radiator compared to the old radiator.

3) The back edge of the NACA vent which delivers air to the radiator had to be trimmed about 5/8" (on this installation) to account for thickness of the new core and slight dimensional differences regarding how the radiator suspends from the end straps. I do have the air filter in the back of the NACA vent and by some confluence of circumstance - The filter location was not a factor with the trimming of the back edge of the NACA vent.

Referring to the original build manual for the FFW - I followed the same order of business in locating the radiator in the lower cowl, accounted for the slight forward location of the front of the new radiator and maintaining the bottom clearance with the cowl & trimming the NACA vent with a small wheel. Next determined the correct length of the end straps and hung the radiator, then determined the correct length and bends for the center strap. Decided to use 1/8" X 3/4" aluminum strap rather than SS strap. When drilling up through the bottom for the three rivets, I used a drill stop - although there is plenty of room - one "oh tinky poo" due to a slip of the wrist would cost too much in terms of $ and unnecessary work.

All in all I have to say the amount of work was not bad at all, the installation and modifications needed are straightforward and I am happy with the end result.

For extra points, the new radiator weighs a bit over a pound less than the old one. I also went from the Evans waterless coolant to 60% Dex Cool which should give a boiling point of 270 degrees F. with the pressure the system operates; and, under standard conditions.

Did a pressure test of the new radiator prior to installation as well as a flush & reverse flush with detergent/solvent/fresh water flush drained and sucked dry with the shop vac. No leaks and very little crud flushed out - just a few flakes due to the welding. Also worth noting the end tanks and core support are fairly heavy gauge aluminum and the welding is solid.

So how did it perform??? My initial test was 90 degrees OAT , 75 F dewpoint, altimeter 29.90 - Full power climbout and the CHT stayed at 220F where I would normally see 240F. No idea how much of that was due to new radiator and how much was due to the coolant change since both were changed at the same time; but, In my opinion, the combination hits the sweet spot:cool:. ......Plus I don't have those incriminating damp spots on the radiator now :D


Dave S
KF7 TRigear
912ULS Warp Drive
St Paul, MN

09-04-2015, 03:50 PM
Nice work, sounds very similar to my experience doing the same thing, very happy with it.

09-14-2015, 10:32 AM
Dave & Others,

Dave, Your report is appreciated. Mostly because of being conservative, (my old radiator tested okay) I have opted for a new radiator from Kitfox (in my continuing new engine install) and have a couple of questions.

Is it necessary to flush out my new Kitfox radiator? If so, is this something that should be done at a radiator shop or can I do this?

There is a threaded hole in one end of the radiator. What is this for?

Thanks much, Skot

Dave S
09-14-2015, 11:24 AM
Hi Skot,

Glad to hear you are making progress on your new engine installation.

Regarding my choice to flush the new radiator....if a person decides to do it, that is something that can be done in your shop...no need to go to a radiator shop. My reasoning behind flushing.......which applies to anything that has an inside that can be flushed (hoses, radiators, tanks, valves, fuel filters etc) - for me anyway.....is to assure that I know for sure there isn't any foreign materials that I can't see which might come loose later. For me it's a builder/repairman issue and the level of confidence that I want to see. Some items, when they are received, have end caps or plugs which can help assure that a spider hasn't crawled inside between manufacturing and delivery, etc.

The threaded hole is 1/8" NPT for the drain - should be located on the left end near the bottom - if you didn't get a pipe plug with the radiator, probably best to use an AN aluminum plug (like the one for the firewall mounted brake resivoir) or a brass pipe plug - (don't use a steel/iron plug in aluminum). Since this is a tapered thread, probably not necessary to have a safety wire (my original radiator had a straight thread with a gasket do that did need a safety wire - new one does not)

Keep up the good work!


Dave S
Kitfox 7 Trigear
912ULS Warp Drive

05-25-2016, 10:16 AM
Murphy's Law strikes again- My SS7 has 210 hours on the 912ULS and it was 2 years and 7 days ago that it passed the inspection. Today I found a leak in the radiator (behind the oil cooler) so the 2 year/200 hour warranty has expired. I searched "radiator" on this site to learn if it could be repaired but thanks to the great advice above I will get the new aluminum one. When I ordered it I was asked "Do you just need the radiator or any fittings to go with it?" I don't know?? Can all fittings, plugs, attachments etc from the old one be used on the new one or should I have ordered additional parts. I don't know what comes with it. Since I am a magnet for Murphy's Law I figure about the time I remove the old one and attach the new I will need to re-order something (unless those more experienced than I can advise me.) AS always...thanks for the help of the Kitfox community.

Dave S
07-07-2016, 04:44 PM

Sorry; but, I missed your query earlier concerning what comes with the radiator. Try to make amends now:o

When I ordered the radiator, Two things came in the package - the radiator and the AN plug for the radiator drain. I think the threads on the radiator drain are the same for the old and new radiator. Only difference was the plug that came with the old radiator was a hex head/brass while the new one was an internal hex/aluminum.

07-08-2016, 01:09 PM
Thanks for your reply. They didn't send me a new radiator plug and I assumed that it was ok to use the old one but now I am wondering if the old radiator was copper or brass should I not put that old plug in a new aluminum radiator?

Dave S
07-08-2016, 02:02 PM

I don't think the radiator comes with a plug - it's a separate line item to order.

I am certain there are differing opinions on this; however, If a person looks at engineering data and potential for galvanic corrosion, brass and aluminum are of moderate concern so there is some potential for corrosion to develope. My old radiator was brass and had a brass plug, The new radiator was aluminum and and an aluminum plug was used. I am inclined to believe the engineers and use the same material for the plug that the radiator is made of.

I think there are a couple types - The aluminum plug I got came from Kitfox had an internal hex for tightening with an allen wrench. I think aluminum square head plugs are available with drilled holes from places like Aircraft Spruce; and Kitfox may have them too although with the tapered threads of a pipe plug, I don't think a person needs to be concerned about a plug backing out on its own.

At any rate, corrosion does not happen overnight and using a different plug for a short time till a person can get a compatible one shouldn't be an issue.

07-09-2016, 09:11 AM
Dave is right... best to use an aluminum plug. When dissimilar metals and moisture are combined it can be a recipe for galvanic corrosion. Using a good thread sealant can mitigate the potential problem by keeping moisture from entering the threads, where the metals are in intimate contact. Consider this also... I think builders switching from the old plastic fuel header tanks to the aluminum ones often transfer the original brass fittings. It's probably not a huge issue because again, the threads should be well sealed and they're not constantly exposed to water. I used all aluminum fittings on the header tank. You can get an aluminum plug from Kitfox, I'm sure, or from any of the aircraft part suppliers, automotive "speed" shops, etc.

05-30-2017, 08:33 AM
Well better late etc. I also have an early model S7 which was finished in 2007 and have gotten 1500 hours on it. Most fun airplane I have ever had.
I was about to get my third old model brass radiator to replace the leaky one when I read Dave's great article about retrofitting the new Kitfox radiator. The results have been phenomenal. I had already switched to 50/50 because of my water temps were running close to 240 degrees. Even then on a 70 deg OAT day I would be over 220 and rising. The CHT's were also high. My water temp is now running 20 to 25 degrees lower and so are the CHT's for all the combinations of OAT/RPM I have tried.
Thanks again Dave..
Rich Little