The Murphy Builders Site

Is it necessary to ream a hole if a drilled hole provides an interference
fit?

I'm starting the wing and was drilling the spar caps to the main wing spar
today. The manual has the builder drill the holes up through 5/32nds, then
states to ream the holes to 3/16ths inch.

I had ordered several reamers, however the tool list for this chapter does
not specify a 3/16ths reamer, and I didn't have one.

I measured my 3/16th drill bit at .187 thousand. I measured the AN3 bolt to
be .187 thou. on the shank. I checked my catalogs and found a 3/16ths inch
reamer is listed at .1875 thou. I also checked AC 43.13-1B on Bolt Fit and
didn't find anything that requires reaming rather than drilling a bolt hole.

I drilled a test hole and checked the fit of the bolt. It required a push
with the thumb to seat and had no slop. I went ahead and drilled all the
holes. All the bolts are a 'thumb press' fit.

So anyway I was just wondering why? (really, I was wondering if I should
never do that again).

Thanks in advance for any advice.

Roger Hoffman #687R
Eugene, OR USA



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A snug fit is a snug fit which is the objective. I installed these spar
cap bolts with pro-seal or chromate and I don't like a hole so tight
that it scrapes the cad plating off the bolt. I suppose it could be
argued that a reamer gives a more round hole... If memory serves, the
old manual said to drill the standard Rebel spar caps with a #11 drill
bit which is even larger than 3/16".

I suspect lots of Rebels have been built without ever seeing a reamer.
The only mention of a reamer, that I recall, in my manual is for the
wing attach bolts. As suggested by someone here, when I use a reamer, I
always use a one thou undersize reamer which seems to give me a better
fit. Bolt diameters do vary somewhat.

Ken

Roger & Alice Hoffman wrote:

Is it necessary to ream a hole if a drilled hole provides an interference
fit?

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Sorry for the delay in answering your message.

Lonnie

----- Original Message -----
From: Roger & Alice Hoffman <rognal@continet.com>
To: Murphy Rebel Builders List <murphy-rebel@dcsol.com>
Sent: Monday, August 21, 2000 10:50 PM
Subject: Drill vs. Reamer

Is it necessary to ream a hole if a drilled hole provides an interference
fit?

Yes! If the designer/engineer says to ream the hole, then ream the hole.
Drills bits do not make perfectly round holes. Most of our drill bits have
two cutting surfaces and usually leave a scalloped hole. A reamer usually
will have at least six cutting surfaces and will leave a much smoother
hole.

" All the bolts are a 'thumb press' fit."

All that means is that the bolt was touching the sides of the hole in at
least two places. What you want is to have the bolt touching the sides of
the hole everywhere. I usually have to use light hammer taps to get the
bolts throught the reamed holes. Don't worry about scrapping the plating
off the bolt, just dip the bolt in chromate.

So anyway I was just wondering why? (really, I was wondering if I should
never do that again).

True, you should never do that again.

Roger Hoffman #687R
Eugene, OR USA

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