paul lacroix wrote:
>
> Hello list,
>
> I've been following with much concern, the discussion about which
> REPLACEMENT rod bolts to use during a TR6 engine rebuild. I say concern,
> because I've just completed buttoning up the bottom end of my first ever
> engine rebuild, and I simply re-used the original rod bolts after cleaning
> them and applying some locktite thread sealer. I torqued them down to spec
> without incident and never considered for a moment using NEW fasteners.
> Have I missed something important here??? Could someone please enlighten
> me.
Okay, Paul, there is one issue (which may be of concern after buttoning
up the bottom end <g>). To explain takes some basic materials science.
Bear with me. All materials have two important features: the point at
which they yield, and the point at which they fail. Each of these are
measured in terms of tensile strength. Now, rod bolts. The purpose of
rod bolts is simply to keep the rod cap and the rod together, to keep
the bearings in position around the rod eye. Picture this: high rpm,
inertial forces want to make the rod and piston keep going up as they
accelerate toward top dead center. The rod cap prevents this, and so do
the rod bolts. The higher the rpms, the higher those inertial forces
(these forces grow by the square of the rpm). Shut down the throttle
suddenly at high rpm, and you add the forces of vacuum to that existing
inertial load. If you're past the rpm where those forces exceed the
ability of those bolts to resist the forces, the bolts give. That's the
yield point. The bolts have stretched. In a generic sense, each time
you exceed that rpm in the future, the bolt stretches more. Finally, it
stretches to the point where it fails by breakage, because it's been
stretched to its limit. Rod separates from cap, rod and piston go where
they want to. <g>
The rod bolts are sized and graded so that they don't do the above under
normal usage. But, let's say, the DPO was racing the blonde in the
Camaro to impress her and exceeded the recommended rpm. The bolts
weren't up to the imposed load and stretched. You've never abused the
car, and have babied it. You never reached a point where, after the rod
bolts weakened, you exceeded their existing strength. They don't yield
further, and they don't break.
And, you put the engine together, rebuild it and think it brand new, and
break it in and then rev to the redline. The already stretched bolts
aren't as strong as they were when new, and stretch some more under the
loads you've placed on them. Do that a few times, and the engine comes
apart, quite suddenly, when you least expect it (while stoplight racing
that same blonde in the Camaro that suckered the DPO <g>).
The easiest way to check rod bolts without going through the trauma of
disassembling the engine is to pull the pan and check the length of the
bolts. If they've yielded, they've stretched. That means they're
longer, period. Find a couple of new rod bolts, and measure them with a
pair of dial calipers, then calculate the average length of the two.
Then measure the installed bolts for length (you should be able to
measure each because a little of the bolt should be proud of the
threaded bore in the rod). If it's appreciably longer than your new
bolt, it's stretched, and ought to be replaced. But, the engine probably
doesn't have to be completely disassembled--just remove the rod bolts
(plural) from that rod and replace with new bolts, then torque, settle
the caps against the rods as recommended, then re-torque. If you've
done this enough, you can also feel as your torquing a bolt if it's
yielding as it's being torqued. A bolt which requires a number of
re-torquings to meet specified value is probably yielding each time it's
torqued and ought to be trashed.
> stand in my workshop...if I need to change these bolts I can do it with
> relative ease. Are new rod bolts really necessary, and why?
Only if the above conditions apply....
Cheers, Paul.
--
My other Triumph doesn't run, either....
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