Leakdown Test - How to determine TDC on 2-5

[Bmartin]

I'm attempting to perform a leakdown check on each cylinder.  Since the flatheads only have access to the #6 piston top, is there way to determine TDC of cylinders 2-5?  I am not sure if you could use the status of 6 to determine the other 4 or not.  Engine is in the car and running, access to the valve covers is extremely difficult without pulling the fender.  Thanks.  

[keithb7]

Get a longer wood dowel. Small diameter that fits down in your pipe plug hole iin the head over #6. Stoke can't be any longer than 4 3/4 so get a 8" or so long dowel. Loosen all spark plugs to relive compression. Turn engine fan by hand. Look at rotor under distributor cap.  Note where wire to #1 spark mounts in the cap. Usually around 7pm on a clock. Scribe top round edge of distributor housing if you like. Then move the distributor cap out of the way. Turn engine over by hand to just before the rotor points to plug wire position to #1 cylinder. Slow down, watch your dowel in #6. When it reaches the very top and starts to come down again, back the engine up slightly. Stop where the dowel is at its highest point. That's TDC #1.  Mark your fan blade if you want. Reference it with a mark on the rad maybe. Look down at the timing marks on the crank shaft. 0 should be right under the pointy arrowhead that is attached to the front timing cover. A flash light is a big help here!

 

We know the firing order is 1,5,3,6,2,4. Every 120 deg of fan blade rotation (1/3 of a circle) the next cylinder is at TDC. Watch your marked fan blade. Turn it 120 degrees. Then you are on TDC #5, again 120 deg to TDC #3,  Then turn it another 120 degrees then you are on # 6. By this time you have covered a full 360 degrees. The mark on your fan blade should line up exactly where your mark is on the radiator again. You've come full circle. Your dowel in the hole #6 is at its very top again. The pointy thingy on the timing chain cover is at 0 on the crank pulley again. Keep going, 120 then you are on# 2. Then finally 120 again to #4. 120 one more time and you are back at #1 TDC. Fan blade and rad marks line up once again and wood dowel is back at the very top where it started.

 

To be "fer sure, fer sure" you cold pull side valve covers off and check the pair of tappets for cylinder you are aiming for. The tappets will both be loose, the amount of valve tappet clearance spec. You can pull and wiggle the tappet upwards 8 or 10 thou. Then you know both valves are perfectly closed at your cylinder of choice.

 

Might as well set those tappets when you are in that far! Nothin' to it by now.  You've already cussed up a storm likely by now. Hurt your back. Dropped two screw drivers and a wrench 3 times. Congratulations you're 90% there to completing a full tappet set yourself!

Edited March 5, 2022 by keithb7

[Bmartin]

Thanks for the process.  I'll give that a try.  Hopefully saves me from pulling the fender for now.  I need it mobile for a while yet.

 

[Bmartin]

I was messing with this today and it was not making sense.  Then I realized the fan pulley is not the same diameter as the crank pulley.  So marking the fan pulley wont work.   It needs to be the crank.  Guess I will give that a shot next.  

[belvedere]

Right, 120 on the balancer, not the fan.

[Bmartin]

Got this done as best I can right now.  

1 - 10%, through crankcase

2 - 15%, through crankcase

3 - 7%, through crankcase

4 - 10% through crankcase

5 - 20-25%, some through crankcase, maybe some through exhaust, hard to tell

6 - I bought the engine from the builder, and when it arrived the #6 sparkplug was threaded in at an angle.  Threads are hosed up.  I threw a new plug in it when it arrived and have not touched it since.  Plug looked good when I pulled it for this.  I can't get a good seal around the tube for the check.  It was around 25% with the leaking threads.  So I'm just going to assume it would be good if I got a seal.  

 

#5 - seems to have an issue, although I'll need to think on how I can further diagnose.  Interesting that it is one of the further pistons from the fuel pump.

[keithb7]

My understanding is the valves furthest from the coolest water, right out of the water pump, tend to run the hottest. Cylinders 5&6 are siamese'd and no coolant flows between the 2 adjacent cylinder walls. Exhaust valves run much hotter than intake valves. As valve train parts wear over time, tappet clearances become tighter. Tighter tappet clearances means the valves spend less time in their seats. This is when the valves are able to cool down, when sitting in their seats. This is why regular maintenance of valve tappet clearance is vital. A valve that runs hot, the sealing surface will pit. Left alone a valve will eventually burn up. This pitting causes valve seal issues. Naturally as exhaust valves pit, this leaking robs horsepower. When the engine is not running and #5 is at TDC, both valves are closed.  When pressurized with compressed air, if you hear air escaping at the exhaust pipe you have a leak at the exhaust valve seat. You can further diagnose by: Checking valve clearances as they sit now.You can remove cylinder head and test seats for leaks using kerosene or diesel fuel. You can remove the valve to inspect.  You can use machining dye to check mating surfaces.

Edited March 26, 2022 by keithb7