I have a broken alternator bracket, and it isn’t the first time. What’s going on?
Search strings: alternator mount bracket, mounting bracket, aft brace, alternator mounting bracket, alternator mounting brace, link.
Technical Editor:
When the original bracket was installed, it had a tendency to overload the aft case bolt. The case bolts cannot be full-value torqued, as they are threaded into the magnesium case. That’s part of the reason why they require the locking tab to keep them from backing out. The least bit of looseness there causes the aft area of the case pad to chafe; then the bracket cracks. Since it isn’t usually discovered right away, the chafing worsens before the bracket completely breaks, is discovered, and gets replaced. Since the pad is now chafed, the new bracket no longer gets adequate base support, and prematurely breaks. And so it goes, through a succession of new-break-new (or break-weld-break). I have never seen a welded alternator bracket that lasted more than a matter of mere hours.
The prevention and/or cure is to ensure that both the forward alternator mounting bracket AND the aft bracket brace are installed together. Lycoming developed the aft brace to support the rear side of the mounting bracket. It makes use of one of the aft boss holes that was formerly used by a (much longer) generator mounting bracket. This new brace was never added to the Beech parts book; but it is called out in Beech Service Instruction SI-0698-359. Many (if not most) of the plane owners never had this SI complied with. It is also possible that the bracket was left off during past maintenance. If that is the case, the existing alternator mount bolt will look like it is about 3/16” too long. If the bolt looks like the proper length, with no extra spacer washers anywhere to help line up the cotter pin holes for the castle nut, then you will need a longer AN7 bolt when you add the brace. It is usually wise to purchase a new bolt, nut, and washers anyway, when this work is being done. The original bolt is nearly always both chafed and corroded, due to the environment. A worn bolt makes it difficult to maintain proper belt tension, as there is too much clearance between the bolt and the parts. Bolt and nut corrosion brings a different set of problem down the road.
I cannot guarantee the exact length bolt you need, as they can vary depending on the alternator, the front brace link, and the new bracket. I have encountered different thicknesses of metal. The original bolts were ‘STD’ series Lycoming bolts; i.e. STD2069; but the AN7 standard hardware bolts are fine, and they have a better length range availability. The STD bolts are necessary when they screw directly into the engine case, as they have the needed coarse threads; but the alternator mount through-bolt doesn’t enter the case. You can get the bolts, nuts, washers, and cotter pins from Aircraft Spruce; they are not terribly expensive. You can measure your existing bolt, and order one the same length, plus the nest two longer sizes. The bolt that SHOULD work is an AN7-43 (see below), but I cannot guarantee it. The -43 is currently under $5 from Spruce; you might want this one plus the two next-shorter bolts. The washers, nuts, and cotter pins are dirt cheap from Spruce. There is also a Beech laminated shim washer, P/N 169-910017-43 that can be used to shim the alternator. This shim is called out in Beech Service Instruction SI-0487-359. I have not found it necessary; which is a good thing, since Beech-RAPID doesn’t have any (and they were priced very high).
The bracket and lock-plate parts are listed below. You can call Rick Scarborough at Graham Engines to purchase them; 770-252-4995. Rick can get you the main mounting bracket, the aft brace (link), and the locking tab for the two main bracket bolts. Just tell him that you are a BAC member, and that I sent you to him for a direct purchase.
76906 – Alternator main mount U-bracket (Aviall P/N 30-07A21443)
LW13365Y – Alternator aft mounting bracket brace (‘Link’)
MS20074-05-05 – Link drilled-head mounting bolt
LW13365 – Superseding alternator aft mounting bracket brace (‘Link’)
LW133654 – Alternative alternator aft mounting bracket brace (‘Link’)
73383 – Main mount (U-bracket) twin-bolt lock-plate
AN7-43/M – Alternator mounting through-bolt, drilled shank
AN960-716 – Standard washer
AN960-716L – Thin washer, for shimming
AN310-7 – Castle nut
MS24665-302 – Stainless-steel cotter pin, 3/32” by 1” (AN391-3-16)
Make sure that the alternator belt lines up straight across the flywheel pulley and the alternator pulley. You can use the shimming washers, or the laminated shim (if you can find and afford one), to space the alternator fore and aft as needed, to obtain proper belt alignment.