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Mail From: "Owain Lloyd" <(email redacted)>

i thought i'd throw this one at the list as i'm alway scarred of
damaged threads

as i may have mentioned before, one of the threads in my left side
engine mount bracket is damaged. it turns out the thread is ok in the
rear half of the casting but damaged in the first half of the hole.
the stock 3" bolt does not get enough threads into the hole to torque
up.

my choice is:

a) use a 3-1/4" bolt which will fill all the threads and does (i've
tested) torque up perfectly well
b) drill and tap the bracket and fit a recoil insert (i can't find a
supplier of keyserts in the uk)

i'm leaning to toward a) unless, in their wisdom, any listers think
damaged threads can somehow spread or half the engine mount thickness
is insufficient for this purpose.

many thanks.
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Mail From: (email redacted)

I would think that if there is room and you can get at it that you could use
a longer bolt and where it pokes through put on a lockwasher and nut. It
that will work it should be at least as strong as it was originally.
M



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Mail From: "Owain Lloyd" <(email redacted)>

the 1/4" longer bolt will not poke through by more than a fraction.
the existing threads appear sufficient to hold it with this extra 1/4"
inch.

thanks.

On Sun, Apr 6, 2008 at 8:43 PM, <(email redacted)> wrote:
>
>
> I would think that if there is room and you can get at it that you could use
> a longer bolt and where it pokes through put on a lockwasher and nut. It
> that will work it should be at least as strong as it was originally.
> M
>
>
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Mail From: drmayf <(email redacted)>

Seemingly not addressed here is just how much load might be placed on
these motor mounts and the three bolts holding it to the frame. It has
been forever since I removed mine (pre 1974) so I don't remember if
these three bolts are 5/16 or 3/8. If they are 5/16 the cross sectional
area is 0.0767 sq inches. Published yeild on Grade 5 bolts is 120,000
psi and shear is taken to be 60% of that number. So shear for one bolt
would be int he neighborhood of 72000 psi. That works out to be about
5500 pounds per bolt to shear it off. The notor weighs about 450 pounds
per so and that is divided by two. So even with a 10 g load the max
loads would be around 2250 pounds per side. And there are three bolts if
I remember correctly. Plus this is a clamped joint with friction so that
makes it even more conservative.

So yeah, Owain, run a tap through the threads on the motor mount plate
to make sure you don't bung up any more threads or a bolt and put in a
bolt that will go at least flush with the through side surface.
Naturally if the bolts are 3/8 inch it is even better. So put a bolt in
it and don't worry about it.

mayf
Owain Lloyd wrote:

>the 1/4" longer bolt will not poke through by more than a fraction.
>the existing threads appear sufficient to hold it with this extra 1/4"
>inch.
>
>thanks.
>
>On Sun, Apr 6, 2008 at 8:43 PM, <(email redacted)> wrote:
>
>
>>I would think that if there is room and you can get at it that you could use
>>a longer bolt and where it pokes through put on a lockwasher and nut. It
>>that will work it should be at least as strong as it was originally.
>>M
>>
>>
>>________________________________
>>Planning your summer road trip? Check out AOL Travel Guides.
>>
>>
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Mail From: "Thomas Witt" <(email redacted)>

Like Mayf, I too can't remember the mount - exactly. Is there enough room to
simply drill out the threads altogether? Then, with a longer bolt just use
a nut external on the mount to tighten the bolt with?

Tom
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Mail From: Theo Smit <(email redacted)>

The threads in question are 3/8-24, so having 1/4" of good thread would
give you six full threads. I forget what the torque spec on that mount
is, but you won't be able to get anywhere near the maximum torque that
the fastener would normally be rated at. The rule of thumb (from Carroll
Smith, IIRC) for full strength fastener engagement is that the thread
should be fully engaged over one and a half diameters, which would be
9/16".

The thing you're concerned with here is vibration related fretting and
having the threads eventually wear due to localized high stress. If you
use the longer bolt, then loc-tite (blue) it in place so you can at
least be reasonably sure it will stay there even if you can't tighten it
all the way to 50 foot pounds or whatever the proper torque is.

Theo

Thomas Witt wrote:
> Like Mayf, I too can't remember the mount - exactly. Is there enough room to
> simply drill out the threads altogether? Then, with a longer bolt just use
> a nut external on the mount to tighten the bolt with?
>
> Tom
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Mail From: "Owain Lloyd" <(email redacted)>

many thanks for all the ideas.

- owain.

On Mon, Apr 7, 2008 at 5:15 AM, Theo Smit <(email redacted)> wrote:
> The threads in question are 3/8-24, so having 1/4" of good thread would
> give you six full threads. I forget what the torque spec on that mount
> is, but you won't be able to get anywhere near the maximum torque that
> the fastener would normally be rated at. The rule of thumb (from Carroll
> Smith, IIRC) for full strength fastener engagement is that the thread
> should be fully engaged over one and a half diameters, which would be
> 9/16".
>
> The thing you're concerned with here is vibration related fretting and
> having the threads eventually wear due to localized high stress. If you
> use the longer bolt, then loc-tite (blue) it in place so you can at
> least be reasonably sure it will stay there even if you can't tighten it
> all the way to 50 foot pounds or whatever the proper torque is.
>
> Theo
>
>
>
> Thomas Witt wrote:
> > Like Mayf, I too can't remember the mount - exactly. Is there enough room to
> > simply drill out the threads altogether? Then, with a longer bolt just use
> > a nut external on the mount to tighten the bolt with?
> >
> > Tom
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