An inside look

For this review, I disassembled the BT-64A and evaluated it from the inside out. I found it easy to work on; the only unusual tool required was a gear puller to remove the flywheel.

For a CDI engine, the flywheel, ignition coil and associated bracket incur an obvious weight penalty, and there really isn't much that can be done about it. The flywheel positively engages the crankshaft with a tapered fit and a woodruff key. A hex nut and washers lock the unit into place. The engine has the innovative ATM module, whose size and weight are basically the same as the standard coils' on other CDI engines. The aluminum mounting bracket suits the crankcase design, and mounting bosses are cast in the front half of the case-again, a little extra weight.

The crankcase amounts only to the parts required to support the crankshaft and the one-piece cylinder jug. The ball-bearing races are press-fit into the case halves, the edges of which have been machined for a really clean, tight fit. One half has pins to align it with the other half. I think this is pretty cool. The parts fit together tightly and need only thin gaskets-no gasket goop. Each case half is finished with a grease seal that protects the bearing races and lubricates the crankshaft ends that protrude from the case.

The crankshaft is a very stout assembly; needle bearings support the conrod at both ends. Whereas other gas engines allow quite a bit of conrod travel at the piston wristpin, the BT-64A has thrust collars that limit lateral play-more evidence of the engineering expertise behind this design. The cast-aluminum piston has two piston rings that ensure a positive compression seal. The hardened wristpin is press-fit into the piston and held with two spring clips.

The BT-64A's cylinder bore is chrome-plated for durability, and the machine work and casting are clean and precise. The cylinder is painted with a heat-resistant paint that looks super. An angled exhaust port on the cylinder looks reminiscent of the 3W50i. This curious departure from the norm isn't mentioned in Fuji literature, but Cactus Aviation claims that this angled port enhances power. I'm not making any claims here, just noting the difference.

Induction parts include the Walbro HDA-211 carburetor and a cast phenolic adapter/heat-isolator block that uses threaded inserts for the capscrews that hold the assembly together. The Walbro uses a diaphragm to create positive fuel-pump pressure without having to lose exhaust pressure; the main benefit of this is that the fuel tank can be farther from the firewall-preferably near or on the model's center of gravity. The angle of the carburetor is very well suited to typical aircraft throttle-linkage installations. While the carburetor angle is great, in my opinion, the carb's linkage arm is still too short. A longer arm would offer a more gradual throttle change with the servo's linear movement. For now, I'm happy that the throttle arm resembles an aircraft control arm and not the industrial configuration Walbro usually uses.

WEIGHT DETAILS

IGNITION PARTS & BRACKET: 8.3 oz.

FLYWHEEL, KEY, NUT & WASHERS: 10.7 oz.

CRANKCASE & CAPSCREW (8): 9.8 oz.

CRANKSHAFT ASSEMBLEY, BEARINGS & SEALS: 19.4 oz.

PISTON, WRISTPIN & RETAINERS: 3.2 oz.

CYLINDER JUG: 16.3 oz.

WALDRO ADAPTER BLOCK & CAPSCREW: 6.2 oz.

TOTAL: 73. 9 oz. (4.61 lb.)

Copyright Air Age Publishing Jun 2004
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