Do you have to have engine at TDC to change timing belts?

[McLovin]

Just changing the timing belts and tensioners.

When you read most DIY accounts, they put the engine at TDC #1.

I understand why, in case a cam slips, you can reindex it since you know the crank position is at TDC.

But is there any other technical reason for doing that?

Or, if you assume that the cams are firmly mechanically locked with cam locks (and assume the crank doesn't move), is there no reason for it?

I ask because on this particular car, getting the flywheel inspection plate off is a pain (several other parts need to be removed to get to it).

It seems like you can just lock the cams up and do the swap, and the position of the crank is irrelevant, but I'm not sure if I'm missing something.

[kaisen]

If it was correct before, and nothing moved, it will be correct after

[Hugh R]

With belts and tensioners, I would think its OK. On my old DB4, I had to use three dial gauges to measure piston TDC, and drop of each cam to within a few thous. It had elongated holes in the cam gears to allow you to rotate and adjust for drop.

[krystar]

timing belt by itself? nope. unless you move the cam sprocket or the crank sprocket manually after removing the old timing belt, it'll stay as is.


however, once you take off the balance shaft belt, chances are, the offweight of the shaft will mean it'll turn itself.

and once that's detimed, the only way to retime the BS belt is to get timing back to TDC.

[speeder]

The only reason is because that is where the marks are. The better question is why you would want to do it differently?

[fastfredracing]

Originally Posted by speeder

The only reason is because that is where the marks are. The better question is why you would want to do it differently?

I can think of a bunch of reasons to set the motor to TDC first, and can come up with no good reasons to do it any other way.
If you think you are saving yourself a few minutes, just wait until you bump , or move something acciedentally, and you now have no idea where you are at.
Guess what you have to do now? That is right, find TDC, but now you have to do it with the cam belt off, and risk the chance of touching valves to pistons while you are turning the crank/cams individually.
Even on the simplest of simple timing belt jobs, that I have done hundreds of, I always set the motor to TDC, and verify the timing marks before I remove the belt.
Best of luck !

[speeder]

Fred brings up another point: if the belt was somehow off by a tooth somewhere, how would you ever know it if you don't use the marks? Painting dots on things would just put you back where you were, IOW, wrong.

[Gogar]

My Toyota 22RE chain stretched and slipped a tooth before I changed it; so I agree while you're in there you need to check your marks.

[stomachmonkey]

Setting TDC is the prep work.

We've all had our share of experiences where we've tried to save time on the prep side only to end up expending 10x's more time and energy than we saved trying to fix our "shortcut".

[stomachmonkey]

Setting TDC is the prep work.

We've all had our share of experiences where we've tried to save time on the prep side only to end up expending 10x's more time and energy than we saved trying to fix our "shortcut".

[red-beard]

Yeah, you wouldn't want to do something twice!

[stomachmonkey]

Originally Posted by red-beard

Yeah, you wouldn't want to do something twice!

One of my favorite proverbs.

"There's never enough time to do it right, but there's always enough time to do it over."

[stomachmonkey]

Originally Posted by red-beard

Yeah, you wouldn't want to do something twice!

LOL, just noticed that, got it.

[Baz]

If Fred says you should do it.....then do it.

[rick-l]

I don't think that on my WRX the engine was at #1 TDC. I think they picked a point for the markings to only have one cam lobe being pressed on to move. 4 cams

[aigel]

The crank can be at TDC or 180 degrees out and any marks you put on it will nicely align in both scenarios. If you lose track of TDC, marks on the crank will be useless.

I don't mess around with cam timing - it is a great way to ruin your engine if you don't get it right. I'd go by the book.

G

[rick-l]

Originally Posted by aigel

I'd go by the book.

Always

Why would you want to take the belt off if the marks to put it back together were not in position.

You are not supposed to rotate the crankshaft or camshafts without the belt on.

On two of my cars they are worried about the intake and exhaust valves hitting each other and bending if you rotate the two camshafts independently (intake and exhaust).

[McLovin]

I took the flywheel cover plate off (it actually came off easy without having to take off any other parts) and set the engine to #1 TDC.

I still can see how for a "lock and swap" it shouldn't really matter, at least in theory. But as we all know, sometimes reality doesn't quite match theory!

[sc_rufctr]

How much effort does it take to turn he motor to TDC?

[GH85Carrera]

When the trusted professional says do it, take the advice.