I probably have the old radiator as supplied by Zenith in a FWF, vertical orientation, kit ca 2007 - 8.

I understand that there is a "new improved  and heavy duty"  radiator that would fit the engine / cowl of my CH 701:  kit 7-6638, Rotax ULS (sn 5648830) I currently have, possibly,   pn 955-697. I will check the parts list when I get to the hangar.  In the mean time I need ideas.  Have seem radiators for Rotax on the Ebay and other sites.

I am desperate to get a little more cooling for the hotter sumer days in Idaho - 85 degrees plus.

My head and oil temps are right at the limits and sometimes into the yellow - I have done the following:

  • put a "scoop" on the NACA duct opening to increase air flow to the oil cooler
  • Faired a 1 inch lip and opened up the bottom of the cowl exit
  • made fins for the oil filter
  • Made a scoop and vanes for the air intake of the radiator to straighten flow
  • Sent a query to Zenith about their recommendations

Still the termps seem to hang on the higher end of the green limits or into the yellow.

Any ideas????

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I would consider testing differential pressure across the oil cooler as a first step if you have not done so already.  This can guide further action.  The same goes for the pressure across the high / low pressure sections of your cowling as well.  A basic instrument will do such as:


If the answer is not immediately obvious after that, I would give a call to Bill Genaro at airflow-systems.  That guy is a wizard.  Amazing.  Really.  And was extremely helpful and generous with his time and expertise in my case.  He is the go to guy for these sort of issues.

Good luck and let us know how it works out.


Not that it helps any but I deal with the same setup and issues. I have also tried most of the same things with only small gains on the cooling issue. 


I attached several photos of the external work I've done on the cowl...under the cowl I have wrapped the exhaust pipes and added reflective insulation to the oil cooler air intake box...on each stepl have had some modicum of success but have not gotten the temps down substantially. I plan to phone Zenith to answer my original question about the radiator I was supplied and the "heavy duty radiator".

I also am lost when I research the pelethora of oil coolers.

Note to Bob, which one did you use


Also this one that was left out...I plan to straighten out the fins...not that oblious when looking at the radiator but it shows in the pics


I had a long hard learning lesson getting my Rotax 912 CH601HD to cool properly. Then I threw out the racing car automotive oil cooler and installed a true aircraft grade engine oil cooler. Aero-classics Aviation Remote Mount Oil Coolers. A year of grief and multiple experiments / adjustment wasted. Bolt on a real aircraft engine oil cooler and instant relief.

My 2 cents & experience. 

Bob, any idea which oil cooler you purchased? Seems to be an endless selection to choose from.

Update to above...talked with Steve and Joyce at Zenith Fri 8JUL21;

  • My kit (7-6638) ca 2007 supplied with rotax rad  995 - 697
  • New kits  for 912ULS CH 701 are supplied with 995 - 698
  • Updated radiator      per Joyce                            997 - 084 $371.00 us 
  • Rotax article  Updated: New Coolant Radiator for Rotax 912i, 912, and 914 series said 997 - 083

Not all of this has been verified but it gives you a starting place as of 10JUL21

I've stumbled upon a company who makes all Aluminum radiators - looks to be Chinese,  but what isnt any more....Alloy-Works.  Guess you just google it and take a look...seems their  536-51304D-BN would be a pretty much direct fit, and a much more effecient radiator at a lower price...don't know. 

Your valued thoughts and experiences please                                       

Update again:

Betting on a hunch, I ordered an "all Aluminum"  Alloy Works radiator to replace the "original Rotax / Zenith pn 995 - 697" as supplied with my kit in 2007.

The radiator pt number 536-51304D-BN from Alloy Works is advertised as; 2 Row Aluminum Radiator for Ultralight Rotax 912i/912/914 UL 4-stroke Engine...hey that's me !!

Got it yesterday and am impressed with what I see so far... going to hold it up to the installed radiator...if it's not a direct drop in I will get out the TIG and make it a drop in replacement.  This is experimental right ??? Can't remember the exact price but it was right around $150 with discounts delivered to my door...pictures to follow when I get time.


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