The DeHavilland Cirrus 1/6th Scale - Rebuild

5 Bears Home

The teardown, cleanup, and ultimately the reassembly of the Cirrus engine continues...

With the intake manifold removed, more castor gum is evident. Each of these intake ports splits into a "Y" and feeds two cylinders apiece. Note two head hold-down bolts run right through the intake ports at the mouth. Below each port, the hold-down bolts carry a pair of viton rings cut from tubing, creating a seal, and preventing the induction system from drawing ambient air from the area below the head.

The cylinders were turned from mild steel. The roots of the fins had a bit of gum but nothing excessive. The tappets, visible at the base of this picture, were in bad shape, with half of them frozen and non-functional.

Removal of the timing gear cover was next. The model engine carb is held with a "pull bolt", a section of rod which was bored in place with the timing cover. The boring produced a hemispherical section of the rod. When the carb is inserted, and the pull bolt tightened, it effectively locks the carb in place.


With the cover removed, the drive pinion and the camshaft gearing is visible. Being 4-stroke, the gearing is 2:1, with the camshaft rotating at 1/2 the speed of the crankshaft.

These gears are small. I cannot remember if they are available commercially, but I do remember cutting these gears with a dividing head and a homemade cutter, using Pat Loops's gear hobbing method as described in Strictly IC magazine. The lightening holes were drilled... I thought that they looked cool.

The tiny studs are 4-40, produced from scale hex bolts available from Cole's Power Models.

Both the front and rear crankcase ends are bolted to both the upper and lower crankcase pieces. With those tiny bolts removed, the bottom of the crankcase can be pulled off of the engine. I was pretty scared at this point, concerned that it would reveal severe corrosion on the crankshaft and piston rods.

Thankfully, that first glimpse revealed just a tiny bit of corrosion on some non-critical areas.

The crankshaft is revealed. Supported front and rear with roller bearings, the center is supported with a plain bearing made of formed bronze shim stock sandwiched between the upper crankcase and an aluminum bracket. Every plain bearing and rod has a hole for lubrication.

The hole at 12:00 leads to the intake manifold.

Pistons two and three are set here at BDC. You can barely see the bronze shim bearing surfaces which are the "meat" of the sandwich formed by the slit connecting rods.

The connecting rods were an interesting exercise. Both the big and little ends were turned spherical... in other words, the big ends you see here started as fairly big square stock, split, reconnected, then turned into a 3D sphere. The bores were cut, then some final exterior profiling finished the connecting rod. The result is one of the prettiest shapes that can be generated short of CNC.