TDC question

Darkmoon · 04-13-2004, 05:38 PM

Is TDC at the top of the piston travel, or is TDC at the top of the curve on the cam? My engine is on the stand with the covers off waiting for the Pelican Parts Express to arrive.

Thx.

Rot 911 · 04-13-2004, 05:39 PM

Top of the piston travel.

Darkmoon · 04-13-2004, 05:40 PM

Does it make sense to measure TDC with a set of calipers?

Darkmoon · 04-13-2004, 05:41 PM

Oh and thx for the quick help.

Rot 911 · 04-13-2004, 05:45 PM

What is it you are trying to do? You said the covers are off so I assumed you are adjusting your valves. What you want is the Z1 timing mark lined up and the leading edge of the rotor lined up with the notch on the distributor. Do you have Wayne's 101 project book or some other technical manual to lead you through the adjustment?

Darkmoon · 04-13-2004, 05:53 PM

I've purchased and read through the 101 book, how to rebuild book, the performance book, the Haynes manual, the Bentley book, have thought greatly about it, dreamt on it, drank many beers talking about it, and finally have decided to do it. Do it is:

1. Tear off old exhaust and install SSI's. (Exhaust is off and no broken studs - yeah!)
2. Fix oil leaks. PO started using Synthetic oil - I think that caused some issues.
3. New clutch plate. - Oil leaks got into clutch disk and ruined it.
4. Update chain tensioners to hydraulic style.
5. Adjust valves.
6. Put back together.
7. Drive drive drive.

So... after all of the reading - I'm taking time with the laying on of the hands to understand the movement of things fore and aft before adjusting anything.

thanks for any and all help and/or suggestions.

Darkmoon · 04-13-2004, 06:00 PM

I was asking about TDC because it seems that you can't be sure of anything when you buy used. I want to make sure that the points in time are where they are supposed to be up front before going in to see if the valves need adjusted in the first place. It's amazing to read the posts about valve adjustments. I'm glad the engine is on a stand.

john walker's workshop · 04-13-2004, 06:03 PM

the piston reaches TDC before the crank does. so TDC is measured at the crank.

Darkmoon · 04-13-2004, 06:07 PM

What's the dwell of the piston at TDC. I'm thinking the best place to set valves is at 1/2 the distance between the dwell time.

Zeke · 04-13-2004, 06:27 PM

You don't have tp be that precise on where the crank is on a valve adjustment. You are adjusting the valve lash while the cam follower is on the heel on the cam. While this surface is sometimes not perfect, it is close and theres a lot of it. If you look at a cam profile, you will see that the heel, or base circle, is over half of the cam shape. I'm not going to tell you that anywhere on the base circle is good, because that's not true.

What you are dealing with is two valves in the closed position. Neither one will be at the excact bottom of the base circle at TDC. The point is that if the piston is at the top of the cylinder on the compression stroke, it's close enough by a wide margin to adjust the valve(s).

There is actually another method for adjusting valves that finds the opposite of the top of the cam lobe for each valve, or at the center of the base circle, and then adjust the intakes and the exhausts separately. You need a degree wheel to do this, but with a lot of time, you can do this by observation. Measuring the valve lash all along the base circle is even another method, adjusting the clearance at the biggest spot.

There is even more than this with concern to lash and individual valve timing, but I think I'll let that sleeping dog lie. Just put the piston at the top according to the marks on the pulley and adjust them. It's hard enough as it is.

Darkmoon · 04-13-2004, 06:33 PM

Thanks a bunch! ... Will do... It's nice to hear a knowledgable laid back tune.