The International Cessna^® 170 Association

Just found a bad stud that holds the tachometer housing to the accessory case. The stud pulled out of the case. Looks like the threads are stripped in the case. Shown on pages 16 & 17 in the parts catalog, item # 7, part # 401870. (engine code "G"). What are my options on this. The parts book shows the studs are available in oversize, .003", .007", and .012". Does this mean both ends of the stud are oversize, or just the end that goes into the case? The end that goes into the case is 1/4"-20, while the exposed threads are 1/4"-28. The other 2 are fine. Engine is an 0300D with the old style accessory case.

If the stud actually pulled the threads out, you are past the point of salvaging it with an oversize stud.
A helical insert is probably the answer.

Is it legal to use a helicoil AND an oversized stud if the standard size and helicoil won't hold?
#curious

If you install a fresh helicoil/insert....then install a standard one and you won't need an oversize stud.

Thanks for the replies. Looks like we'll go with the heli coil insert.

Be certain to install a new garlock oil seal inside the housing when you're finished. (Replacing the tach drive housing will not position the old, hardened seal accurately enough in it's former position to prevent oil running up the tach cable and into your cockpit.....and remember to face it the correct direction to keep oil inside the engine.)

gahorn wrote:If you install a fresh helicoil/insert....then install a standard one and you won't need an oversize stud.

My question was more of "may" vs "can" or need to.
Is it legal to use a helicoil with an oversized stud in the event that the hole is really borked?
Or is a helicoil only allowed with the original sized bolt?

Thanks, George. That's what made us remove the tach flange originally, to correct an oil leak there. Hopefully, this will correct the leak.

bagarre wrote:

gahorn wrote:If you install a fresh helicoil/insert....then install a standard one and you won't need an oversize stud.

My question was more of "may" vs "can" or need to.
Is it legal to use a helicoil with an oversized stud in the event that the hole is really borked?
Or is a helicoil only allowed with the original sized bolt?

I'm not familiar with "oversize" helicoil/inserts.... so how would one install an oversized stud and not add stress to the standard helicoil/insert?

As for legality...this will be in the eye of the repairman/inspector to return it to service, IMO.

gahorn wrote:

bagarre wrote:

gahorn wrote:If you install a fresh helicoil/insert....then install a standard one and you won't need an oversize stud.

My question was more of "may" vs "can" or need to.
Is it legal to use a helicoil with an oversized stud in the event that the hole is really borked?
Or is a helicoil only allowed with the original sized bolt?

I'm not familiar with "oversize" helicoil/inserts.... so how would one install an oversized stud and not add stress to the standard helicoil/insert?

As for legality...this will be in the eye of the repairman/inspector to return it to service, IMO.

I think that answered my question. I was thinking oversized meant the next size bolt.
What I am hearing is, an over sized AN3 bolt (for example) is not an AN4.
Thus the helicoil question makes no real sense.

Studs in aircraft engines are generally a slight interference fit, and are intended to stay in place when the nut is tightened or removed.
If the threaded hole pitch diameter is slightly too large (a few thousandths of an inch), a stud with a slightly oversize pitch diameter can restore the proper fit.
The section of the stud which the nut threads onto is still standard size.

If the threads are ripped out of the base material, the helical insert can restore the fit. The proper size drill is used to completely remove the old threads, the special tap is used to thread the base material, and the helical insert installed. The internal threads of the helical insert will be correct for a standard stud, if the pitch diameter of the tapped hole was correct. If the standard stud is loose in the helical insert because your new threads had oversize pitch diameter, an oversize stud may restore the proper fit.

The O300A overhaul manual allows for the use of helical inserts to restore threaded holes.

Actually, special oversize helical inserts with outer pitch diameter .003 oversize can be obtained, but are usually not readily available.

If you have the bad luck of ripping a helical insert out of the base material, then a Twinsert may still save your part. You drill out the threads, thread with the special tap, install a special outer insert, then the special inner insert. Again, not generally readily available, and not inexpensive, but if you have enough edge distance it can save an expensive part. They can also be used to relocate a hole, if the original drilled location did not meet the print requirement.

In a case as above, if the hole has to be enlarged beyond the limits of a "Twin-sert", how about a bushing if the remaining edge distance is great enough.