Minimax engine swap 503 for 447


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Sep 7, 2020
As part of a general sort out of my Minimax 91 (UK version of 1600R). I'm fitting a 503DCDI. Here's the story so far...

So, the issues arising from this are as follows:

1. Its bigger! Not a lot but it is. Annoyingly the 4 mounting studs (8mm dia) are 8mm further apart than on the 447. In practice, this means that the rear pair of mounting holes in the engine mount plate are close enough to form a figure 8 hole! So a new plate is the neatest option - see photo 1.

2 The longer motor together with twin carbs requires a bit of surgery on the left side of the fuselage. 40mm to be cut back! Fortunately this is non-structural so it just requires some neat jig saw work and tidying up. See photo 2.

3. I had one of my M5 Starters fitted on the 447 and this did require further surgery in the form of cutting a pocket in the inner skin of the fuselage side. The extra length of the 503, actually eases this situation as it's further back and thus the fuselage is a bit wider, meaning less relief (if anyone was fitting one with a 503 from scratch) - Did I mention that I make the M5 Starters? :cool: - See photo 3

4. The overall size of the 503 means a tighter fit all round including the area of the cooling shrouds. The existing cooling system of the enclosed engine Minimax is (in my opinion) a bit random. Fan cooled Rotax 2 strokes require an adequate supply of cold air directed in through the rear facing fan housing where the fan forces it around the cylinders and over the heads and out over the carburettors. Cold air currently enters the cowling though a hole in the front and a small scoop. I'm going to make a scoop that fits over the fan housing and directs air straight in via an enlarged scoop in the top cowling (yet to be done). See phot 4 - this is just a quick piece of card in the rough position of the new cut out.

More anon!

Only 8mm longer between.jpg

503 twin carb clearance.jpg

503 with M5 Starter.jpg

Paper engineering scoop.jpg


Well-Known Member
Log Member
Feb 19, 2012
Did I mention that I make the M5 Starters?