Rotax 377, Rotax 447, Rotax 503 proper cylinder alignment required during reassembly.

Aircraft:
All ultralight aircraft using the 377, 447, or 503 Rotax engines.

Incident report:
Several ultralight pilots have reported rough running engine, and or engine failure. Upon closer examination it was found that the aluminum intake manifold, had cracked at the bolts used to fasten it to the cylinders.

Suggestions:
The cause of the problem, is that when the Rotax engine (377,447,503) is disassembled and then reassembled a special alignment tool is needed to properly align the cylinders before torquing the heads. If the cylinders are not properly aligned then the holes used to mount the intake and exhaust manifold are off ever so slightly.

The proper procedure is to first align the cylinders using the alignment bar, torque the head bolts, replace the fan shroud cowlings, and then replace the exhaust and intake manifold.

Without first aligning the cylinders before torquing the heads, the cylinders do not line up with the holes for the aluminum manifold, then when the manifold is tightened, because the holes are off ever so slightly the manifold cracks.

This allows air to enter the engine causing a lean mixture and overheating. It is strongly recommended that if you are removing the manifold, cylinders, or heads that you properly align them before retightening to proper specifications.