question on locating TDC

[cantfindausername]

First off, HI!

Wish I'd found this forum sooner! Looks great and seems very informative!

I have seen the thread in the other section about how to find TDC, however, I'm a little confused on how the protractor comes into play.

Could someone explain the process for me?

I thought I had located tdc but was unaware of the dwell period.

Thanks in advance,
Ant

[Rich Ellingham]

I think you may have nearly answered your own question. The linear movement of the piston against crank degrees is that of a sine wave. Thus at the peak or trough of the wave (tdc and bdc respectively) a change in grank angulation makes a very small change in motion. Thus to estimate the TDC at this point is not adequate because say 5 degrees either side of true TDC may show no noticable movement in the piston.

There can be no guess work in engine assembly, so this is where the protractor comes in. If you use a protractor and a DTI (or a positive stop) say to put the piston at a height a few more dgrees away from TDC then read the gauge and note the travel. Say 7mm, put the protractor on the crank nose (you need a sturdy pointer that can point to the outer edge of the protractor) and zero it in line with the pointer. Then turn the crank past TDC (or wind it the other way for a positive stop) until you gain the same DTI reading the other side of TDC. Note what the protractor says e.g. 20degrees. The true TDC will be half of 20, thus wind the crank to 10degrees (protractor reading) and mark the pulley - this will be true TDC. I would say its best to measure in the center of the piston to avoid any piston rock distorting your DTI measure.

If this is incorrect I'm sure Guy will correct this, but I'm pretty sure this is correct.

Rich

[cantfindausername]

ahh ha!

I see now. The protractor I was getting confused about is the degree wheel, right?

This is my set up at the moment...


Its suddenly clicked into place about what I need to do to find TDC. I'd read a few write ups on how to find it, but its just never sat right with me until now.

Thanks for your help!
Ant

Feel free to check out my site to see my engine build project amongst other things car ralated...

[cantfindausername]

This is my plan... and a query.

1- Fit rigit pointer, plunger DTI (in center of piston) and degree wheel.
2- Rotate crankshaft backwards till the DTI reads X mm ADTC.
3- Note the reading on the degree wheel.
4- Rotate the crankshaft in the running direction till the DTI reads the same X mm reading BTDC
5- Note the reading on the degree wheel.
6- TRUE TDC is the mid point between the 2 readings.
7- Rotate crankshaft in running direction to the mid moint
8- Reset degree wheel to 0 degrees without turning the crankshaft.

If this is correct, then the only part I have a query on is No 2. Would this be right? Turning the crank backwards, rather than keeping it in the running direction? Would there be a difference?

[Guy Croft]

Direction doesn't matter getting the crank at tdc. But it does to an extent when you are setting or checking cam timing with belt/chain/gears on etc.

GC

[cantfindausername]

Thanks for clearing that up Guy.

I'm so chuffed I found this site. I've learnt loads more than I already knew!

[Guy Croft]

Hi

it is very nice of you to say that - and already - we have attracted many, many expert members here.

And that of course is precisely what GCRE.com is about - a respected knowledge base. Free. Getting it published before the expertise is lost.

I have deferred writing another book for now, only got one pair of hands, maybe the site will become sufficiently good to obviate the need.

GC

[cantfindausername]

I know the sort of thing you're after. On my site I am documenting the stripdown, and rebuild of my engine so it can be used as a reference library for others.

This is my first engine build, and this is just the sort of thing that is going to help me. I hope to stick around and learn alot more and hopefully one day be able to put something back into the community.

Ant