The piston-stop method is common and accurate for finding TDC. You
just have to do it carefully and do it a couple of times to make sure
of your results. The bolt shouldn't be welded. What you want to do is
beat the guts out of a spark plug and thread the inside to accept a
long bolt. Grind a flat on one side of the bolt to release the air
(you MUST do this on the compression stroke) and a rounded nose on the
side that meets the piston. Get an approximate idea of the length Youl
need by bringing the engine to TDC by turning it slowly and feeling
when it stops pushing air with your thumb over the spark plug hole
and then measuring with a narrow ruler.
Set your bolt to a little longer than that and lock it down with a
lock nut (on the outside of course). Bring the engine up on
compression you start to feel pressure with your thumb, put in the
stop in, continue rotation until you feel a solid stop. Mark that
point on your balancer. Then rotate backwards until you hit the stop
again. Mark the point. TDC is exactly in the middle between those
points.
It's not 17 degrees--the stopping point is arbitrary and varies with
the length of the stop. The key is that BOTH points are the same
distance before TDC. This is much more precise than using a degree
whell, and in fact is the way you set up a degree wheel even if the
head is off. The dead spot when a piston travels over TDC is about
three degrees wide. Even with a good dia; indicator the stop method is
more accurate.
On Jun 7, 2008, at 10:16 PM, Mark York wrote:
> Amici,
>
> I'm finally getting some time to finish the final assembly of my
> motor for
> the TR3 racer, the machine shop did most of the work (pistons/sleeves,
> crank, cam, and head), and I'm doing the rest. I called the machine
> shop to
> ask how to find TDC since the flywheel has been replaced with a
> lightened
> one and none of the indicator arrows are there, I was given the
> following:
>
> 1) Rotate the engine until the #1 intake has just closed and the
> exhaust valve is just opening (mark this position)
>
> 2) Rotate the engine 360 degrees, this is approximately TDC.
>
> Now here is where it gets a little weird:
>
> 3) Weld a bolt to the bottom of a spark plug.
>
> 4) Rotate the engine a quarter turn counter clockwise
>
> 5) Install the spark plug / bolt
>
> 6) Rotate the engine clockwise until the piston makes contact
> with the
> bolt on the end of the spark plug, mark this point
>
> 7) Rotate the engine counter clockwise until the piston again
> contacts
> the bolt on the spark plug, mark this point.
>
> The 2 points marked are approx 17 deg before TDC and after TDC.
>
> My questions are: how long should the bolt be on the end of the
> spark plug
> (won't the length make a difference where the piston meets it)? Does
> somebody have an easier way?
>
> The head is mounted and torqued, I really don't want to take it off.
> Doesn't
> this process seem a little Rube Goldberg-ish?
>
> Confused in Seattle
>
> Mark
>
>
>
> PS: Thanks in advance
> _______________________________________________
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Bill Babcock
Babcock & Jenkins
Billb@bnj.com
503.936.7660
www.bnj.com
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