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One tooth shorter timing belt, how tight is too tight?


Feb 22, 2010
Portland, OR
I've got a shaved head and block, as well as a thin HG on my B21F, and my OEM timing belt was so long I was right at the end of travel on my tensioner. The belt was pretty loose, but I never jumped teeth and ran it for 7000 miles.

Someone here advised ordering a belt 1 tooth shorter, which I did and installed. Now I'm at the other end of adjustment, nearly completely compressed on the tensioner. The belt was damn near impossible to install and is pretty tight, although it does seem to obey the normal 45 degree twist rule. On the test drive, I noticed a supercharger whine above 2600 RPM, but just barely audible in the cabin. At idle I can hear it if I'm right at the timing belt cover. I first thought it was the tensioner roller bearing, so I installed a fresh one but the sound remains.

Some web searching indicates about a 50/50 split between people who claim the supercharger whine indicates the belt is too tight, and people who claim it is normal break in and goes away after ~500 miles.

Looking for some opinions here. Obviously don't want to risk a break, but the OEM is so loose I never wanted to rev above 4500 RPM for fear of skipping a tooth.
I don't know the answer, but I moved it to Performance as it may get read by someone who has experience with this.

Lawrence Knox (aka fidel) may have an educated opinion on the subject. He worked with Gates to get the stronger timing belts produced and he's been around redblocks a long time.

How much have you taken off the block and head? How thick is the HG? I remember Rob took .100" off a 530 and ran it on a B23 (that may have been decked, don't know) with a stock HG and a stock belt that was borderline too long I think, but it never skipped a tooth and it got revved often.
0.026" off the head, 0.016" off the block, 0.027" HG. I know it doesn't sound like much, but the tensioner was indeed at the end of its travel pretty much from the beginning. The OEM timing belt was definitely looser than it would have been with a tensioner in mid-range, but obviously not loose enough to jump yet. I don't know what the stretch cycle is for these timing belts, but I kept thinking that as the OEM one stretched, it would only make the issue worse and worse.
My B21ft was running 0.050 off the head, zero off the block, with a 0.027 cometic and didn't have any t-belt tension issues. Weird. Sorry, I know that didn't help you. Good luck!
Same issue but could not get the ford belt (one thooth shorter) on. To dam tight
Running floppy t belt also
That's probably the best idea. Make it just thick enough to put the tensioner back in the center of travel where it goes. Make a couple of them if you plan to keep this car/engine through the next belt change.
Have you considered a smaller idler on the tensioner, when using with the smaller belt? or a larger idler for the OE belt?
Have you considered a smaller idler on the tensioner, when using with the smaller belt? or a larger idler for the OE belt?

Maybe it's because I'm more comfortable designing and having a machine shop make me a sleeve, but I'm leaning toward a bigger pulley with the OEM belt. If I do that, I only need to source one part for future maintenance; the sleeve. I can have a machine shop make me 10 and be set for the rest of the car's life. If I find a smaller pulley, I'll need to remember where I got that, plus where I got the 1 tooth shorter belt.

Has anyone sourced a smaller idler? That doesn't seem trivial, because you'd need a smaller OD while maintaining the same ID for movement around the center stud. There are limits to how thin you can make roller bearings.
Right but isn't the center stud like m10 or something? I'm sure a bunch of them are right around that size.

^ This is what I was getting at.

There are many automakers that use tensioners with slotted holes, you just need to find one that fits and is the right size. Go down to the Vatozone and see if they will take you in their stock room and go through some tensioners with a pair of calipers. Then just jot down the OEM part number for future reference.
Hmm i never really though about doing something with the tensioner
I'd love to to hear what comes out of this..
I think I am misunderstanding what you're getting at. The M10 center stud is inside the roller bearing, so the bearing can only change position within the limits of its ID. How does the slot overcome this?