How to find TDC?

[18Fiatsandcounting]

Okay, so lacking a dial gauge, here's what I did this afternoon:

Removed spark plug #1. found a piece of metal rod in my garage that fit down into the spark plug hole and stuck out about 4 inches when the piston was near TDC (according to the cam marks). Found another bracket made of plastic that I could secure to one of the extra bolts on the cam cover. Tapped a hole into this plastic bracket and put a screw in it so that the screw stuck out of the bottom and almost contacted that metal rod sticking out of the #1 plug hole.

OK, here's where it gets ingenious. I hooked a "buzzer type" electrical continuity tester between the body of the car (ground) and that tapped screw in my plastic bracket. So, when the #1 piston goes up, the rod touches this screw, electrical continuity is established, and my continuity tester buzzes. So, by adjusting this screw in and out, and rotating the engine ever so slightly around TDC, I can hear (by the buzzer) when the piston is at its maximum upward motion (TDC).

Eh? Eh? Whadya think?

-Bryan

[RRoller123]

Friggin brilliant.

[dinghyguy]

i have used a similar technique to set hydroelectric gate guide steel prior to concreteing. We used a plumb bob in a bucket of oil to damp the movement (40ft high wire). Then using an internal mike, a headset , a battery and two alligator clips we clipped the alligator clips such that adjusting the Micrometre we could hear the buzz when the mike touched the wire and then we could read the values, record them and confirm the guides were vertical (or not).

So an internal mike might also be used and you could measure the piston travel per tooth rotation of the system

But yes a good approach. Clearly you have too much time!

Dinghyguy

[18Fiatsandcounting]

Thanks, fellas.

I do think a dial indicator is probably easier and better, but I continue to think about my electrical contact idea. Like what if you took an old spark plug, removed the porcelain innards, cut off the tang at the tip, and epoxied a 1" or so plastic dowel down the center of the plug. Tap that dowel for the bolt of your choice, about 3" or so long. Turn the crank so that piston #1 is near TDC, screw in this spark plug thingie, connect a continuity buzzer between that center bolt and ground, and by turning that bolt you can hear when it just touches the top of the piston. Play around with the crank and how far this bolt is inserted until you find the point where it buzzes but turning the crank ever so slightly in either direction stops the buzzer.

Of course, be very careful not to ram the bolt through the top of your piston....

Whether a dial indicator, the above idea, or the tried and true visual check of a pencil in the #1 hole, it all depends on how much piston movement there is around TDC as to how accurate it is. If the connecting rod bearings on either end have a fair amount of clearance, these TDC measurements won't be that accurate. But perhaps just as accurate as you'd ever get with that lame mark on the timing belt cover that has so much slop in how you position the cover as to only get you within plus/minus 5 degrees anyway.

-Bryan

[SteinOnkel]

If the connecting rod bearings are so shot that you can see it on a Dial gauge, I would argue the engine is due for a complete rebuild.

[18Fiatsandcounting]

Agreed! With the timing belt off, if you can turn the crank by hand tens of degrees from TDC before you feel the piston ring friction = Not Good. But hey, isn't that what oil additives are for....?

-Bryan

[MadBeez]

So, lots of good info in this thread. I'm in the home stretch with my 1608 rebuild and now I need to get the right sized shims in the buckets. I have "the tool" and now I need to calculate the sizes for the replacement shims. But, after measuring, they seem to be pretty close to what they should be. What I need to know is how close is close enough.

Here's what I found (all in .mm):

Exhaust: should be .48 mm
#1= .35 - .37
#2= .40 - .45
#3= .35 - .40
#4= .45

Intake: should be .43 mm
#1= .30 - .35
#2= .35 - .40
#3= .40 - .45
#4= .45 - .50

https://www.mm-to-inch.com/inches-to-mm.php

What happens if the clearance is too small like exhaust #1? Would you bring them all to .48 and .43?

Thanks, Rich.

[geospider]

Well,, you have come this far. now get this right. too much clearance, you get the valve tappet clatter,, too tight, valve hangs open (to some degree).
not hard at all to get these right. I adjusted them to right in the middle of the range; although did not change if it was within the range ( i'm not that crazy,, although Bryan may beg to differ)..

Geo

[18Fiatsandcounting]

...I'm not that crazy,, although Bryan may beg to differ)..

No crazier than any other Fiat owner!

The original specs for the valves were in Metric, so 0.45 mm for the intakes and 0.50 mm for the exhaust. Sometimes the clearances get converted from Metric to English and then back again, and so they might differ from the 0.45 and 0.50 mm.

Since tappet shims usually come in increments of 0.05 mm, that's about as close as you can expect to get. My feeling has always been that 0.40 to 0.50 for the intake and 0.45 to 0.55 for the exhaust is fine. Sometimes you get lucky and hit it right on the mark.

So, on the exhaust side, #1 and #3 look a little low, and on the intake side, #1 and possibly #2 look low. None of these look dangerously low to me, but if you can get them one or two increments closer without too much trouble, I'd do that.

By the way, all this assumes the valves have fully seated if you've had a valve job done (a few hundred miles perhaps), and the engine is completely cold when measuring.

-Bryan

[Nut124]

Regarding valve clearance. Recommended clearance varies based on who made the cam. For factory cams, if the clearances are over 0.010" and not more than 0.020" and there is no mech clatter, I would leave it alone. If any clearance is less than 0.010" or is over 0.020" and if there is clatter I would adjust.

As to how to determine TDC - does anyone else not do it like this, with a TDC finder plug?

You buy or make your own TDC finder spark plug. This is a plug with a rod that extends down to act as a piston stop.

You turn the engine to where both valves are closed or close to closed as seen thru the spark plug hole, then insert the plug. If valves are wide open, they may interfere with the plug. Then turn the engine back towards TDC until it stops at the plug. Record deg reading at deg wheel. Turn the other direction until it stops at the plug at the other side of TDC. Record the wheel reading. Adjust wheel position until CCW stop deg equals CW stop reading. With my home made plug, the stop is at about 38deg before, after TDC.

Never turn the engine CCW without the belt tensioner properly set.

With this plug, the crank comes to an absolute and hard stop as long as the plug is long enough to stop the piston at least 20deg from TDC. This method is very accurate, allowing precision way better than 0.5 deg if the wheel is properly centered.

Below are a few pics of my TDC plug. This is an old spark plug with the center punched out and then a bolt epoxied in at a slight angle. I like the angle so the rod points almost straight down when in the head and stays away from the oversize valves. Using the black mark to orient the plug so.




Once you have the TDC established, I would make an attempt to install a reliable TDC marker, or verify the factory. I added a home made timing pointer on front oil seal cover.

Another way for TDC, if you remove the timing belt, is to check the crank free turn at TDC. In my engine, this was about +/-5 deg or so. I think this reflects piston ring slack in the groove. This method is not as precise but will tell if you are way off.

[MadBeez]

Thanks guys, I'll get out "the tool" and do it right.

~Rich

[Nut124]

With the valve adjustment tool, if I recall correctly, when doing the exhaust side, after inserting the tool at full lift, one needs to turn the crank CCW rather than CW or the intake valve will run into the open EX valve.

Just make sure that the belt tension is OK or cams will jump out of time when turning CCW.

[Nut124]

...for finding TDC.

If you do not want to mess with a degree wheel, mark the crank pulley for the CCW and the CW hard stops. You can then measure the TDC spot on the pulley as half way between the two stop marks. This is assuming you have some fixed pointer you can use to mark the pulley.