TDC Sanity Check

Hi Ferrari Folk,
I'm carrying out my first belt change on my 84 308 QV, and just wanted to get a second opinion on the TDC markings. From the images I found here and some Google research, it appears on the flywheel the line at PM 1-4 is before the stamping. But on my car the line seems to be after the PM 1-4 stamping. Can anyone confirm that this is the same and I have this correct as TDC before I pull the old belts? I've attached some images of the flywheel markings and the cam marking at this position. Thanks in advance. BB
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Brock, Unless you are changing the cam timing (not very accurate from the factory) the main thing is to lock the cams so they don't move when you take off the old belts. The exact positions of the crank and cams is not critical as long as you lock them all for the duration of the change. If you are checking valve clearance or adjusting the cam timing then the easy way to lock them is a piece of paper under one cam bearing cap (for each cam.) Otherwise there are good DIY lock designs here on Fchat.

Remember to check for oil leaks and play in the lower drive pulleys while the belts are off. If you want to switch to HTD/GT2 belts I have a set of pulleys in Singapore and often know people going to Oz.

 

Really poor advice not to have engine at TDC during a belt change.

To the OP. The mark on the flywheel reflects engine position only. The mark on the cam represents cam position only.

 

I like to start a belt change at TDC just in case something goes wrong you have a clean starting point. Just saying.....

 

Accidents happen. Cams slip. Sometimes carnks turn. It isn't a perfect world. You need to know where everything is supposed to be so it can be put back there. Thats why there are marks.


USE THEM.

 

Yes, you have the crankshaft at 1-4 TDC. The position of the actual "mark" (the line) has moved around some on different models/versions relative to the PM1-4 lettering because of other markings (for example, the -3 deg ATDC on your flywheel meant that they had to shift the PM1-4 lettering to be in front of the mark so as to not overlap into the -3 deg stuff).

 

"The exact positions of the crank and cams is not critical" is what I said-- you don't have to be exact. Generally around compression TDC is better as if it does move a bit there will be no contact unless left in the incorrect position after the new belts. I can see from the inlet cam through the oil filler and the mark on the flywheel that you are fine in this position-- hope this helps.

 

Your flywheel TDC is on the mark, but that cam looks to be off by a degree or two from the mark. If you clamp the cams in current position for the belt change, you'll still be off by the same degree or two after the belt change, the same as before the belt change. If you wanted to go the step further and degree the cams you could likely (possibly) get better cam timing, but that's a fair bit of extra work.

 

The picture of the flywheel shows some parallactic deviation. It's still not exactly TDC. I generally aim for getting the pointer and mark spot on, then compare cam marks. I always have the help of a second pair of eyes when turning the crank on the opposite side of the engine.
And of course I never remove the belts with the crankshaft NOT on TDC. Especially on the carb engines this is the best position, because the cams don't jump, as opposed to the 4V-engines.

Best from Germany
Martin

 

Cheers all, I think we have consensus that this is TDC position and thanks Steve for confirming the reason for the marking change. I did see the 3 deg ATDC marking but didn't think about the spacing issue when they stamped the flywheel

I also noticed that my cam timing mark is a little out, but I also saw this on other images here of TDC when a cam belt change is being done? Maybe its a little belt stretch on the old belts? I'm not keen to mess with the cam timing, the car runs as it should, I don't want to introduce other complexities at this stage

BB

 

Looks OK as it is but if you don't have them then also make your own marks on the cam pulleys & the crank pulleys as well, they will help you a lot to see what's going on if something moves. The front bank is the most likely to do so just because of the position of the cams at TDC.

I can't imagine why you would not put the thing at TDC before starting a belt change.

 

Brock, with each cam tooth being 12 cam degrees (or 24 crank degrees) you have to get a fair bit of movement to be one tooth off. One helpful tip I saw on here somewhere is to mark the cam pulleys so that you can see if they move. I put white dots adjacent to each other (on a straight line between cam centres.) This helps as you can’t see all cams’ timing marks with the covers on.
You can also mark adjacent teeth on the cams and old belts before removal and then count teeth and transfer marks to the new belts before fitting. Not sure if this is the case with the 4V but for the 2V I had to both compress and lock the tensioner and loosen the 3 tensioner mount nuts most of the way in order to get the new belt on.

 

There are also marks on the cams as well as pointers on the backing plates outside of the valve covers. On the 1-4 bank there are two peep slots between the belt cover and backing plate and on the 5-8 bank you can see the pointers by looking down on the back side of the plate. These are not extremely accurate but good enough for a quick and dirty verification that you are at TDC for #1 rather than #4 (if you consider TDC to mean top of the firing stroke) Normally if you see the 1-4 intake cam mark through the oil filler cap you can assume all the others are right as long as nothing weird has happened but those marks are there if you want to use them.

As noted earlier there's no point in concerning yourself with minor cam and cap mark alignment because if you are off even just one tooth it will be totally out of whack and obvious.