Finding compression stroke (TDC) w/ head off

[t-west]

Hi all,

So, I can find TDC easily, but hos do I know if it's the compression stroke of 1 or of 6? I'm trying to stab the dizzy before replacing the head. Should I just wait? Can I tell by inserting and watching the pushrods?

thanks,
--tom

 

[addo]

That is basically it. TDC compression the both pushrods for #1 will be at their lowest.

Take the opportunity to confirm that your balancer mark is true and correct. You aren't playing with the cam at present, are you?

Cheers, Adam.

 

[t-west]

Balancer mark looks right on. I pulled my old dizzy and am installing a DSII, but want to get it lined up right now, since I can see TDC.

Ill drop the pushrods in and take a look, thanks.

No, no cam right now. Next summer I plan to rebuild the bottom end, install a cam and some headers. For now, the '78 head, ex. man., the DSII, a 5200 carb, suspension and brakes. It'll be an exciting yet miserable August (it was 108* today).

Thanks addo,

--tom

 

[61Futura]

I would think that both lifters would be at their lowest at both TDC's. The difference being that on the compression stroke TDC, both lifters have been at the bottom for the previous & following 180 degrees of crank rotation. The wrong TDC had the exhaust just close and the intake is about to open.

Steve

 

[StrangeRanger]

At TDC on exhaust/intake, both valves are open.

 

[61Futura]

I would think that that would cause a backfire. I'll have to check that out when I put on my chrome valve cover.

 

[StrangeRanger]

It's called "overlap." It assists in purging the burned gasses from the CC and replacing them with incoming mixture. Look at the specs for ANY camshaft. They will list the exhaust valve closing ATDC and the intake valve as opening BTDC.

 

[Lazy JW]

Almost all engines have some overlap. The only exceptions that I am aware of are industrial engines that have extremely slow top speeds. My 1941 John Deere Model "A" tractor tops out at about 1000 rpm and has zero overlap. Anything that runs much faster than that needs some overlap for best power.
Joe

 

[61Futura]

I've seen alot of posts of people getting the distributor 180 degrees off so it must be a suttle amount of valve opening at the wrong TDC. I also have heard it is a bad idea to turn the engine over backwards when the head & rockers are installed. It probably isn't a no-no with the head off since the cam will turn easily.

Steve

 

[t-west]

The pushrods move so little . . . I think I've got it, but before I fire it up I'll pull the first spark plug and verify the compression stroke is right.

thanks for all your help,
--tom

 

[addo]

#6 is on the opposite phase of the firing cycle. If you lay a square on the deck and measure up to the height of your two pushrods for #1, it should be somewhat less than the same dimension for #6. Say ¼" or so...

That ought to be concrete proof. Otherwise, wind it back ¾ of a turn, and press down on the inlet pushrod as you rotate forward again to the TDC of before. You should feel the pushrod sink.

 

[Vaughano]

I've seen alot of posts of people getting the distributor 180 degrees

Yep, did this myself after I rebuilt my Xflow, but only because I was too lazy to take the rocker cover off (I sealed her up before installing dizzy). It's pretty difficult to confuse the power and exhaust strokes when you have a view of the rockers. I just set mine up by the timing marks knowing it could be 180 degs out.

Mind you I first started the engine some time after I had installed the dizzy, and was surprised to hear the thing back-firing! I'd forgotten that I'd guessed the stroke! No harm done (didn't need that lube on the cam anyway...)

 

[t-west]

Thanks Addo. Got it, though the difference was only about 1/8". I had expected the difference to be more noticable . . .