I found that using a 5.7" rod on a 3.48" crank doesn't leave ANY dead spots.

The dial sweeps every twitch of the crank rotation.

Maybe it's a little more obvious with a 6" rod for instance?

 

Isn't 'top of piston travel' and 'crank TDC', the same thing? Anyway, I find the use of an indicator to find TDC is more accurate. The idea is to take a degree wheel reading at a given (.010" for example) before piston TDC, as piston is coming up, and take another degree wheel reading at at the same indicator figure, as the piston is going down. I consider it a rule to rotate the crank in only one direction. Spliting the diff is still what you are accomplishing, without reversing crank rotation. To each their own, or whatever turns your crank.

 

Powerhouse makes a tool to do it the easy way.
POW 351088
$229.00+shipping
It will give ya the correct measurement from the main bore for each or both sides without all the math.

 

I agree with taking two dial readings off TDC and back calculating TDC is a fine way to do it also. Honestly I've never compared accuracy of all said methods. Do whatever will give you repeatable results.

 

Actually the stop method is the most accurate way to get TDC... you can do it with a dial indicator but if you stop the piston 1/2" down from the top on both sides and get TDC you will be 100% dead on then.

In reality the only time the aveage guy needs to check for TDC is when he is degreeing in a cam, other than that you only need to do it to zero deck the block.

Bret

 

I thought he wanted to check deck height,as was said in the original post.

 

Yeah but the only reason to check it is so you know how much to mill of the block to make it right..... other than that WTF do you need to know it for? Right?

 

Checking for the sake of checking, help determine CR upon assembly, or to check the end result, AFTER machining, to determine what you ended up with. Thus it is also a post machining 'inspection' procedure. I prefer to take nothing for granted, thus I inspect any machining procedure.

Referring to the bold print Bret, you are contradicting yourself. Same paragraph, it need not HAVE TO be zero deck. For the top paragraph, reversing crank rotation to determine TDC, AFAIC, is encouraging 'slack' induced errors, making it a technically unsound method, even though 'slack' should theoretically be nil. In the real world, slack will be minimal, making that method acceptable.

 

Not really Arnie..... I'm saying that you check for TDC when the person who is zero decking the block is measuring the block dimensions. You can double check him which is not a bad idea, but two things to think about then.... if you find it off how much is acceptable? I would say a .001 is a pretty good tolerance on decking a block so if you measure it within that you can't "fix" that. The other thing is if you are having the block machined by someone you don't trust, WHY ARE YOU PAYING THEM MONEY in the first place?

If a shop doesn't check the 4 corners of the block to find out how much has to be taken off and they just skim a side based on the measurement of one hole, they didn't do their job correctly and if that's "zero decked" to them you are basically SOL on that block unless you want the piston out of the hole. If you are interviewing them prior to doing work, I'm sure if they are confident in their work they will show you a block they have just done and let you measure it. In all reality you can easily check for a zero deck if you bring the piston to TDC and feel the top of the block and piston with your finger.... you can tell if it's acceptable that way. A depth mic is the way to go in reality.

BTW if you have "slack" in your rotating assembly then you have more problems than what we are talking about on here.

There are lots of things you can theorize about on how to make perfect, the problem is in the real world you can't do those things, and if you can you don't have the money for it. I know how to deck a block and my results get that puppy within +.001-.0000 deck height everytime I have checked so why should we argue about it?

P.S. in the thread the other day where I said you were getting "all wiggy" I think you've hit a apex on your posts today of being wiggy, if you need a definition of that. Just chill out a little bit and not jump down everyones back.

Bret

 

Tolerances are set on anything that interacts with something else, whatever that tolerance may be. You decide when to deviate, and when not to.

Not aware of anyone that pays a machinist to make mistakes. To be redundant, anything machined for me, (even BY me) is checked and/or inspected in some way. When I state something is correct, I know it is, as I have verified it. I've been around machinists for almost 40 years Bret, and I have been around some of the best anywhere. I have NEVER met a PERFECT machinist. As long as the POTENTIAL of a mistake is there, the work is inspected. That is just the way I am. So you are stating... because you trust him, (and pay him) it is GUARANTEED, he will machine it correctly? More power to ya. If you have found a 'perfect' machinist, hang onto him. Not many of those around. Humans/perfection is an oxymoron in my book.

The place I use, needs the dimension from crank to block deck, and that is it. As the block is mounted off the crank bore, and the equipment is set to cut parallel to crank, the lone possible error is in the initial setting for the cut. If one corner is on (or off) all four are.If you reread that post, I attempted to stress the preference not to use that method, as I consider it technically unsound, not for actual slack. May want to reread that part of my post.May be a 'regional' term. Not in the dictionary, so I'll take your word for it, though I'm still a bit unclear on that.

 

My this has got more interesting than I thought. FWIW I was checking deck height to how much the block needed decking. I will also check it again after the block returns from the machine shop, not necessarily because I don't trust the machinist, I do or I wouldn't be giving him the work. I'm assembling the short block myself and I want to learn and have a good build sheet in the process. At any rate thanks for all your help.

I do have another question however. I have a cam degree kit that includes TDC piston stops, but I guess I'm not totally sure I see how the piston stop is used to determine TDC???

 

Read up a few posts to where Bret (SStrokerAce) is describing the piston stop
vs. dial method.

Essentially, setup up the piston stop so that the piston rests fairly deep in the
bore.

Connect the degree wheel to the crank and fix your pointer to the wheel.

Rotate CW until the piston touches the "piston stop".

Write down the number on the degree wheel.

Reverse the crank rotation until the piston touches the "piston-stop' once again.

Write down this number.

I believe at this time, you add the numbers and divide by 2.

The resulting value is exactly TDC.

Remove the piston stop and rotate the crank until the pointer reads the
calculated value.

Someone needs to confirm this process. I've used a dial for the longest time to set TDC,
so I'm not 100% sure.

 

Close....

with that method you want to get both numbers to be equal degrees from TDC.

For example 35 degs BTDC and 35 degs ATDC, will put TDC at 0, which is what you want.

Bret

 

Now I understand. Basically you use the piston stop to create a known reference point that you hit from both "sides", (BTDC and ATDC) . Doesn't matter where the stop is exactly, just that it is the same number of degrees from both. Half of that will be TDC.

Thx.

Mark.

 

One more note when using a dial indicator to check the deck height.
Try to get as centered over the piston pin as possible or you will get
a deviant measurement due to the piston rock in the cyl.