Aaaack! An odd prime number! How about never! Seriously, on the 308-2V (101 tooth belt), you'd have to spin the crankshaft 202 times (to spin the cam pulley 101 times) to have the marks line-up again. I'd give a human a .000001% chance of doing that without losing count even if they wasted the huge amount of time to do that. On the 308QV (96 tooth belt), you'd have to spin the crankshaft 32 times (to spin the cam pulley 16 times) to have the marks line up again (and even that wouldn't be easy to do).
This older thread is relevant: https://www.ferrarichat.com/forum/threads/why-arent-the-belts-marked.335919/#post-140806417 It’s designed so that the same teeth on the belt don’t come around to the same teeth on the sprocket every time.
Yeah I just ran the calcs back of envelope then threw it at grok.. for my gt pulley design. It's 484 rotations when everything synced up again... Hahaha... That's nuts!
Marking cam belts is not necessary and just adds confusion...as it did in this case. Once you have placed the crank/cams at TDC/marked the cam pulleys in relation to each other, remove the old belt/install the new belt. Worrying about cam belt tooth count and putting marks on cam belts/wondering when the belt marks will align again has absolutely nothing to do with cam timing. Though I guess it might be construed as an interesting mathematical exercise.
This reassuring discussion for the OP reminds me of work I did on an old bevel drive Ducati. You had to rotate the crankshaft 10 times for the indexing marks on its lower bevel gears to line up again. Ducati uses a “hunting tooth” principle to mitigate wear by preventing the same bevel teeth from engaging each time the camshaft is loaded by the valve springs. Specifically, Ducati use 21 and 30 tooth bevel gears on the lower pair, and 20 and 28 tooth bevel gears on the upper pair. The crank:cam ratio remains 2:1, in that (28x30)/(21x20) = 2. I imagine this is the same philosophy so that cam belt tension variations are spread evenly over each belt.
Done it many times on 83 QV...........16 rotations of the crank to get marks on the belt lined up. Yes its tiring.........especially at 70. I sleep better knowing everything is back in place.
Steve, yeah....I was just taking an educated wild ass guess on the rotations (never did it or felt the need before). Then when I counted the 101 teeth I was like...OMG!
From an engineering "design goal" or optimization standpoint... The belt tooth count is likely an objective wear parameter, but kind of "doesn't matter" from a belt change standpoint. I could be wrong, but: The first (primary) design goal, or optimization: For every 360 deg of crank rotations, the cams rotate exactly 180 degrees. In the 308 engine, the 50% ratio is achieved by a combination of the transfer gears behind front (main seal) cover, and the ratio of the size of the lower small cam "drive" pulley to the main upper cam "driven" pulleys. Another objective is longevity... via even/distributed belt wear characteristics. As an example, picture the same engine with a longer stroke via connecting rod length, with a higher block deck height: In a minor (minimal height change) case, the available "dynamic range" the idler pulley range could easily compensate (it would present as major belt stretch, but acceptable to accommodate) But, if you were to go higher on deck, (or raise the cams a bit more) we would exceed the "dynamic range" of the tensioner assemblies... which would necessitate a longer cam belt with one or two (or more) teeth. Other parameters come into play: longer belts stretch more, belts "flap" on the longer/non-bearing stretch, resonant frequency noise, etc. But in the end, regarding this discussion, I think "number of teeth on the belt" and trying to rotate back to the original BELT marks... doesn't really matter. From a numerology standpoint, a design parameter... but likely irrelevant to a mechanic replacing belts. I'm not sure, but perhaps designers likely plan for a "sweeping effect" across the belts (each belt tooth getting to "kiss" each cog tooth in some repetition) to improve belt longevity and minimize wear to avoid consistent/repeated wear on one area. For those interested, here's a technical paper on "Automotive timing belt life laws"
Yes, I can independently confirm. OEM 2v belts have 101 teeth. I know this because I am now running the Forza Round Tooth conversion kit, and I carefully documented the comparison between the OEM belt system and the Round Tooth belt. By the way, the Round Tooth belt has 121 teeth (20% more than stock) and is roughly 15% wider than the stock belt.
Ok, guys, it’s Paul the originator of this post with a new twist. Plugs out, borescope on the viewing the pulleys and me below turn, turn, turn. Don’t know how many turns, but it was a lot and marks on belts and cam pulleys line up. But here’s the thing, marks on the crank pulleys don’t line up with original marks? What do you think? And for those that think this whole discussion is crazy and before you say it, yes, the crank is at TDC and the one cam I can see through oil fill hole is at TDC Image Unavailable, Please Login Image Unavailable, Please Login
Probably going to suck at explaining this, but here goes. The cam pulley and the drive pulley are obviously not the same dia. So the number of rotations to sync the cam pulley back to the belt is a different number from the lower drive. The math to crunch is finding the least common multiple between all three, the belt tooth count and the two pulleys tooth count. So basically after a given set of rotations the belt will line up with either the lower or upper, but to sync all three... That's the bigger rotation number. There's a bit more with the crank drive ratio to the lower drive.. But that's essentially it.
Concur SMG, I don't understand the logic of being concerned or depending on marks or numbers on the BELT itself (besides engine designers for longevity), but rabbit holes sometimes bury me too...
I have possibly timed or over seen timing of thousands of engines. I have taught it to many Ferrari mechanics. There is one rule whose importance is far above all others "KISS". Keep It Simple Stupid". Marking or caring about belt position is the antithesis of that. Pulley position matters, belt position does not. Quit distracting yourself. Keep your eye on the ball.
We Well, I have to admit. I've been following this thread and ever time I open it to read the new posts I just smile. It's been entertaining.
Hello all. The reason for all my consternation about the belt and pulley marks is this. I followed Ferrari’s procedure for tensioning the belts, but what they don’t tell you to do is take the spark plugs out. So my problem was when I turned during tension procedure and hit compression, the jumped back a bit putting slack on the longest stretch of the belts. So my concern was that the drive pulley could have skip a tooth(s). I don’t think there was enough slack for this to happen, but obviously the consequences could be bad. So by turning until the marks on the belts lined up again, I thought I could verify nothing moved/got out of sink. I’m well aware marking the belts is not a method. I should have marked the camshafts. Anyway, after many turns, all of the marks on all 4 cam pulleys line up again. The crankshaft is at TDC and the cam I can see through oil filler hole is at TDC. So with all the pulley belt marks lining up and knowing the crank a camshaft I can see at TDC, I think I’m ok. Again, total rookie move and I won’t do this again. My only remaining question is now that the cam pulleys and belt line up at original marks, why are the crankshaft drive pulleys not lined up at original marks? After many more turns would all six pulley and belt marks line up?
Because the cam drive pulley has 20 teeth (and rotates 3/4 rev for each crankshaft revolution) and cam pulley has 30 teeth (it rotates 1/2 rev for each crankshaft revolution) -- therefore: The cam pulley lines up with the 101 tooth belt every 202 crankshaft revolutions. The cam drive pulley lines up with the 101 tooth belt every 134.666... crankshaft revolutions (and would be off by 180 degrees when the cam pulley lines up at just 202 crank revs). Both pulleys line up with the belt every 404 crankshaft revolutions (so just do another 202 crankshaft revolutions ) Do us all a favor, next time set the crankshaft to PM1-4 and put a mark on the cam pulleys and corresponding marks on the cylinder heads -- those lines up every 2 crankshaft revolutions (just like the marks on the cams and the marks on the cam caps), and don't worry about the belts nor the cam drive pulleys.
Yes, If you had questions before, this thread certainly didn’t make the process clearer. Best if the mods deleted this thread so it isn’t archived. Lol
One last word(s). To all of those that met me where I was at the beginning of my post/thread, I appreciate all of you very much, and you helped me a great deal. Thank you. And to all of the know-it-alls on the forums that just can’t help to make comments to show their superior knowledge I hope that scratches that itch for you. I think this was a valid topic, that actually had been discussed years ago when someone else had the same question. Obviously, I wish I would have seen that post earlier in my process.
It's seen as a "valuable" thread only because years ago the process to lock/mark cam belts was posted here and it has now become "the thing to do" even though it is unnecessary, wastes time and adds complication and confusion.
I think you have totally misunderstood my comment with regard to your thread. Never meant to say it was an invalid topic. My comment was meant in that while performing a belt service you want straightforward direct, easy to understand methodology , so no mistakes are made. This thread, although very entertaining has evolved into an engineering/mathematical discussion. Not something generally desired or helpful for your first time belt service. I know firsthand because my first belt service on my 85. Once I pulled the cam covers,I ran into an issue where the timing marks were not lining up on the cams and I did not feel comfortable, pulling the cams. Rather than assume I may get it right, I opted for loading up the car, transporting it 300 miles away and spending $5000 with a mechanic who was qualified to use a degree wheel and reset my timing to what it was supposed to be. I feel it was money well spent instead of lining up marks, the former belt service screwed up. if I would’ve stumbled on this thread, it would’ve done little but add to the confusion. Was never meant as a dig towards you.
For a DIYer, marking the pulleys and old belts, then transferring the marks to the new belts assures that when the new belts are installed a tooth won't be accidentally skipped. Yes, not necessary, but it doesn't add complication or confusion assuming you know what you are doing in the first place. When you get right down to it, if you lock the cams and crank, there really isn't any reason to bring the engine to DTC either. That's more of an assembly issue so you know how to position the cams (using the assembly marks).
