I finished my Legacy last year near the end of the season and put a couple of flights
on it before letting it sit around the cellar most of the winter. When I fired it up
for the second contest this spring, I knew there was ball bearing someplace that
was in serious trouble.
My first thought was that it must be the front bearing; but when sending for parts
I purchased both of them since I would have it apart anyways. Good call - it was the
rear bearing that actually failed ! I'm pretty sure it failed due to rust inherited from
the crankshaft. After-run oil never seems to get there (it's behind the valve gear).
The replacement went much easier than I anticipated.
o Remove cylinder head ssembly without even removing vale covers
(don't drop the pushrods on the floor)
o Remove backplate.
o Remove piston and rod
(watch out for wrist pin pads falling out)
o Remove cam housing assembly.
o Attach a pair of prop nuts to the end of the shaft.
o Heat crankcase and the prop driver with heat gun.
o With the back surface of the crankcase housing resting on a steel plate, tap on the end of the crankshaft.
o Remove cam pinion and spacer.
o Drive out bearings with a steel punch.
(it helps to heat the housing once again).
o Locate new bearings in counterbores as far as you can by finger pressure.
o Use dirveshaft (without cam gear and spacer) to drive in the rear bearing first.
o Re-insert the crank with pinion gear and spacer.
o Place front bearing over crankshaft and into housing.
o Heat housing one more time.
o Use prop, washer and propnut to pull the front bearing into housing.
o Replace cam housing assembly.
(make sure timing mark lines up with slot in cam gear)
o Replace piston and rod.
o Start piston ring inot cylinder bore.
(be sure to get wrist pin pads in place)
o Push the cylinder assembly down while working pushrod tube into boots on cam housing.
(don't get the pushrods in backwards - tapered end toward rocker arm)
o Make sure all the screws are tight and prepare for bench test.
o Take valve covers off to verify pushrods are seated.
(might as well check rocker arm gap with feeler gage)
Attach backplate, inlet system, and muffler in preparation of test run.
I worried a lot about getting the cam gear in the "right" tooth. The valve overlap
position is near top dead center.
Total time was about 1.5 hr, including photos.
Runs great ! No more bearing noise. Why did I wait 6 months to do this ?
Does anyone have a good method of verifying the valve timing after the engine is
fully assembled ?
PS : Bob Zambelli has the secret for cam timing:
http://stunthanger.com/smf/index.php?topic=20227.0