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RVG8tor 07-01-2011 05:49 PM

AFP Bracket torque
The AFP FI comes with an anodized bracket that is held in place with 4 socket head bolts, they go into the same holes as the carburetor studs and are 5/16-18 (coarse thread) The AC43 Torque value for this is 80-90 in lbs but the Lycoming overhaul manual has 204 in lbs for this size bolt. The US torque specification for a grade 8 bolt this size is 216 in lbs. The instructions say to use Loctite so I have some High Temp stuff (Red) but wondered which torque you others would use here. I am leaning toward the Lycoming # but wonder whey there is such a big difference between the two specifications. It also seem the US Torque table is a little higher for socket heads even though the threads are the same size, interesting.


flion 07-01-2011 07:12 PM

Go with the Lycoming numbers. This is not the same kind of application that AC 43 covers, nor are those the same kind of bolts.

terrye 07-01-2011 08:28 PM

Bolt torque
Recommended bolt torques are usually based on the yield strength of the bolt material which is why SAE Grade 8 bolts have a higher recommended torque than SAE Grade 5 bolts. SAE does not have a classification higher than Grade 8, but most socket head cap screws have a higher material yield strength than SAE Grade 8, so they can be installed with a higher torque, and the socket head cap screw manufacturers will list this torque in their literature.

If installed as a cap screw, then the installation torque may depend on the material it is screwed into as the limiting factor may be the shear stress of the female thread rather than the shear stress of the bolt body.

If I understand correctly these cap screws go into a female threaded aluminum housing. Therefore I would go with the Lycoming specified torque value for the fastener.

RVG8tor 07-01-2011 10:26 PM

Great help here at VAF as usual


Don at Airflow 07-02-2011 08:18 AM

5/16-18 SHCS torque

Per my pm to you 95 - 125 in-lbs will work on these cap screws.


RVG8tor 07-02-2011 09:02 AM

Update to thread

Originally Posted by Don at Airflow (Post 555372)

Per my pm to you 95 - 125 in-lbs will work on these cap screws.



Thanks after reading your email this morning I was just logged onto update this thread and you beat me to it. Thanks glad I did not already try to torque them otherwise I would have used 204 in lbs.

This is really why I asked the question in the first place there always seem to be some torque values that don't make sense just following the Overhaul Manual. The bolts holding the alternator bracket will be my next research project.


flion 07-02-2011 09:27 PM

You know, my brain was off when I replied to that. Your AFP system should have come with a beautiful install manual that has everything you need to know about the whole setup, including the adapter plate for the fuel servo and its mountings. If your engine did not come with this manual (a three-ring binder by itself and not part of the engine manual), then you should ask Don for a copy. It also has everything about installation, tuning, troubleshooting, etc., for the while AFP system.

TClinton 01-19-2012 01:22 PM

Lycoming alternator bracket bolt torque
Hi guys, did you ever find out what value to torque the two bolts that hold the bracket to the engine for the alternator? My engine is an IO-540. The bolts that I have are 1" long and 3/8" x 18 thread. I would guess about 15 -17 foot pounds. I'd sure like to know for sure though. Also, I would probably use either blue or red locktight even though these two bolts have a two piece washer (they both go through two seperate holes) and it gets bent over as a locking tab.
Thanks, Tony

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