Lotus Elan

Effort required to rotate crank without head

PostPost by: "John Jacobs" » Wed Oct 19, 2005 5:13 pm

I am rebuilding my engine for the first time, and am concerned about how
much effort is required to turn the crank when all pistons are fitted (but
not the head).... It seems particularly stiff.

Can anybody offer a rule-of-thumb or torque indication of how freely (or
otherwise) the crank should turn?

I gapped my rings to 0.012" (on a 83.3mm bore) according to the manual
specs. Could this be too tight?

Any advice much appreciated,

John Jacobs
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PostPost by: lotuselan2 » Wed Oct 19, 2005 5:17 pm

New rings will feel quite stiff on the bores. Did you use any lube on the
bores before installing the pistons? If not try a few drops of 2-stroke oil
in each bore. Give it some time to work around the ring gaps. I think it
will loosen up.



Ken

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PostPost by: "Sean Murray" » Wed Oct 19, 2005 5:49 pm

John,

I trust you checked that the crankshaft rotated freely before you fitted the rods and pistons ?

Did you need to grind off the rings to get the 0.012" gap, modern pistons are usually supplied with the gaps optimally set.

You should be able to rotate the assembled crank and pistons reasonably freely with a socket on the crank pulley and standard 12" bar, I don't have atorque figure.

Sean Murray
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PostPost by: gwnorth68 » Wed Oct 19, 2005 6:55 pm

Did the crank turn freely and uniformly before the connecting rods were
installed? What are the bearing clearances? (mains and rods). If these were
all satsfactory the pistons will probably loosen up with a little oil. You
should be able to turn the crankshaft assembly without serious strain using
a box end wrench on the crankshaft pulley nut (3/4").
Tom Moore
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PostPost by: M100 » Wed Oct 19, 2005 7:15 pm

Without the pistons you should be able to turn it over with just a
light finger touch on the crank webs. With the pistons fitted, using a
torque wrench on the crank pulley bolt you should be able to turn the
engine over with about 10 lb.ft max (5 or 6 lb.ft is more typical on a
competition engine build) In a pinch you could get away with twice that
but its all wasted power.


Martin
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PostPost by: dlbarnes1 » Wed Oct 19, 2005 10:00 pm

In David Vizard's book "Tuning Twin Cam Fords", in regard to checking crankshaft alignment and bearings (before the rods are connected) he advises that the crank should rotate with a torque of about 1/4 lb ft. I have found this to be a viable number with a polished crank, all dimensions within tolerance, and appropriate lube on the bearing surfaces.
Dave - 72 Sprint DHC
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PostPost by: Arno Church » Thu Oct 20, 2005 6:19 am

Little oil is the word, otherwise you will glaze the bores and the rings will take ages to bed in. A light wipe of oil is what is needed.
Serious race engine builders ( not all of them !!!) wipe the bores with paraffin , start the engine and give it two revs up blasts
Rings are bedded ! OK they rebuild their engines a little more frequently than us
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PostPost by: twincamracing » Thu Oct 20, 2005 3:32 pm

.... It seems particularly stiff.
Can anybody offer a rule-of-thumb or torque indication of how freely
(or otherwise) the crank should turn?

I gapped my rings to 0.012" (on a 83.3mm bore) according to the
manual specs. Could this be too tight?

John,
A bare short block should rotate with less than 18 ft lbs torque, less
is better. You should be able to slip the crank pulley on the snout and
turn it with your hands.

Lay crank in well oiled main bearings and check for true with a dial
indicator. When torqued to spec you should be able to spin the crank
with your fingers.

Installing rods, again, liberally oil bearings snug up rod side to side
to pull the rod cap on evenly before torquing, torque cap in steps.
turn crank with each install and check for binding or too much
resistance.

Different ring sets have differing amounts of drag on the bore, .012"
gap is good, don't be complacent thinking modern ring sets are
correctly gapped for the application (performance ring sets tend to
need filing) someone in shipping may have slipped an oversize ring in a
std package.

I normally dip the head of the piston in a tub of oil to lube the rings
and lands along with a couple squirts in the bore, smeared around with
my hand. By the time you get the rest of the engine together most of
the excess has run down the bore. You will also crank the engine for
oil pressure before installing plugs, poor ring sealing will not be an
issue from excess assembly oil (but it could be from incorrect bore
finish for ring application).

A wealth of pertinent engine assembly/blueprint information is
available in text and is applicable to all four stroke engines. Try
Smokey Yunick's Power Secrets or Waddell Wilson's Racing Engine
Preparation for basics.

Cheers,
Scott




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