> Thanks Mark,
>
> I really appreciate the time you took to thoroughly explain the leat
down test.
> Am I to assume that with the GT6 the # 1, 3, and 5 pistons are at
TDC
> at the same time when the TDC mark on the crank pulley lines up with
> the pointer, and the #2, 4, and 6 pistons at TDC when the mark is
180
> degrees from the pointer? Also is it the two valves on the number
> three cylinder that are to be 'on the rock' when the # 1 and # 5
> pistons are at TDC? Couldn't I also just use the closed intake and
> exhaust valves of the cylinder I'm testing as a guide to when that
> piston is at TDC?
>
> I saw this done on the TV show "Crank and Chrome" but they didn't go
> into enough detail for novices like me. They implied that with the
> piston at TDC the pressure wouldn't move the piston down. I'm glad
> you cautioned me on this.
>
> thanks again,
>
> Mike http://pages.prodigy.net/mikeross
> '63 Spitfire
> '66 Spitfire
> '68 Spitfire (donor body tub for GT6 convertible)
> '69 GT6+ (donor chassis for GT6 convertible)
> '70 GT6+
>
> ----- Original Message -----
> From: "Mark J Bradakis" <mjb@autox.team.net>
> To: <spitfires@autox.team.net>
> Sent: Saturday, June 09, 2001 2:56 AM
> Subject: Re: How to find "top dead center"?
>
>
> > Oh oh, this could take a while. I'll try to be brief. In truth,
> the answer
> > to your question is quite simple. The number one piston is at TDC
> when the
> > TDC mark on the crank pulley lines up with the pointer. So is the
> number
> > four piston. And 180 degrees of crankshaft revolution will put
> number two
> > and three at TDC.
> >
> > That is not the whole story, however, with an Otto cycle motor.
The
> short
> > synopsis of the Otto cycle is suck, squeeze, bang and blow. And
no,
> I'm
> > not talking about what my wife and I ... uh, nevermind.
> >
> > Suck: intake stroke of the piston - draws in the fuel and air
> mixture
> > Squeeze: compression stroke - compress the mix before setting it
> on fire
> > Bang: power stroke - spark plug ignites the mixture, kaboom!
> > Blow: exhaust stroke - push out the burned gases
> >
> > So on a motor like the Spitfire, and every other four cycle
internal
> combustion
> > engine on the face of the planet [Wankels and two strokes are
> different] the
> > piston in any given cylinder is at top dead center twice during
this
> process.
> > It hits TDC at the point where the compression stroke [squeeze]
> finishes and
> > the power stroke [bang] begins. And it also hits TDC at the point
> where the
> > exhaust stroke [blow] finishes pushing out the burned gases from
the
> last
> > cycle and the intake stroke [suck] starts drawing in a fresh mix.
> For the
> > purposes of this discussion, assume there is no overlap in cam or
> ignition
> > timing, we'll keep it simple for the moment.
> >
> > To run a leakdown test, I suggest you remove the rocker cover and
> all the
> > spark plugs. An assistant will speed the process, but is not
> required.
> > Doing the test on an engine at operating temperature is a bit more
> accurate,
> > but you can usually get the same picture with slighty different
> numbers from
> > a cold, more finger friendly engine.
> >
> > With the rocker cover off rotate the motor until the two valves on
> the number
> > four cylinder, the rearmost one, are 'on the rock.' That is, the
> exhaust
> > valve is closing, ending the 'blow' part of the cycle, and the
> intake is just
> > beginning to open for the 'suck' cycle. With the valves on #4 at
> this point,
> > the number 1 cylinder is at the point between 'squeeze' and
'bang.'
> Both
> > valves are closed and the piston is at TDC. Screw the leakdown
> fitting into
> > this cylinder.
> >
> > Now, here's whare an assistant could come in handy. With the
tester
> in place,
> > one could easily apply the air pressure and get a reading, except
> that with
> > the 100 psi or so of applied air pressure the piston will be
pushed
> down in the
> > bore, rotating the crank and opening a valve a bit, giving you a
> completely
> > useless reading. If the car is in gear at the time, some festive
> movement
> > will also occur.
> >
> > So the trick is to have someone in the driver's seat standing on
the
> brake
> > pedal with the gearbox in fourth, keeping the crank from moving.
I
> have a
> > long hose on my leakdown tester with a valve in it, so I can be
> sitting in the
> > seat, pushing the brake and then crack open the valve to apply the
> pressure
> > to the tester and get the reading. Relieve the pressure on number
> one, rotate
> > the crank 180 degrees and test the next cylinder in the firing
> order. Repeat
> > process until all four are done.
> >
> > In truth, it can be informative to get leakdown numbers when the
> piston is at
> > TDC, the middle of the bore, and at the bottom. This can give you
> an idea of
> > what condition the cylinder walls are in, but is more work, as you
> need to
> > remove the rocker assembly. Chances are you'll find out what you
> need to
> > know with the basic TDC procedure.
> >
> > Anyone still awake?
> >
> > mjb.
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