Hai gebissen

Ok. I used a 10mm Snap On allen. It fit perfectly across the top of the T/D piston with no room to spare. I realize this isn't as precise as using a caliper but it is close enough to tell me it is extended considerably beyond the recommended limits.

Hai gebissen

Looking back at my posts from three years ago when I first installed the racing belt and Porkensioner, it measured 9mm of extension before and after rotating the engine. Cam gears, oil pump gear, water pump pulley and obviously tensioner roller were brand new, crank gear was not. Others have commented that the crank gear looks fine, so hard to imagine it is considerably smaller than new. Really suggests the cam gears are closer to the crank than stock, meaning heads are cut, except my less than ideal head measurement vs. provided specs suggests otherwise. I am tempted to replace this entire engine with a known good unit, but first I think it would be easy to string a new non-racing belt and observe tensioner extension (and travel pattern) again.

SeanR

I have done dozens of them and not once, ever, was there a loose belt up on dis-assembly. I've checked dozens part way through the belt cycle, and not once, ever, was there a loose belt up on checking.

One note, I do not think that belt looks in great condition, the tops of it are all scuffed up. I'd like to see the printed portion, if there is any left.

Hai gebissen

For the record, my belt took considerable effort to remove.

EDIT: Of course I removed the covers and turned the engine to 45 before removal.

jcorenman

You are correct, that new belts are a very tight install with a PKT. After running they stretch some. And as you rotate the engine, the PKT takes up slack and keeps the belt tight.

But that that is not the looseness that Imre is talking about, and which I have experienced also.

See my posts #32 and #96. As the engine heats up the block expands, which makes the belt path longer. (The racing belt also shrinks a little as it heats up). This tightens the belt, which pushes on the tensioner and slowly compresses the tensioner piston, which gives up some of its belt to compensate. All well and good.

Now park the car and let it cool overnight. The block shrinks, the belt expands a little, and slack develops all around the engine. You would expect the tensioner to take up all of this slack, but it does not. After the engine cools the belt on the driver's side in particular is left loose enough to push off the gear with a finger-tip.

I've seen this myself, as has Imre, and I suspect anyone else can also: All you need to do is warm up the engine, park the car overnight, and then -- without rotating the engine -- remove the driver-side belt cover and have a look.

If you rotate the engine to 45-deg (or TDC) before removing the covers, as most of us do, you will never see it-- rotating the engine shovels all of the slack to the tensioner.

What happens is that, without rotation, the spring in the tensioner is not strong enough move the cams or crank in order take up slack on the far side of the engine as the block cools. The tensioner takes up the slack in the belt segment from crank to 1/4 cam, and that's it. The remaining 3/4 of the belt goes slack as the engine cools.

The expansion over the belt-path is around 7 mm if my math is correct. CTE for Aluminum is around +23 (10^-6 per deg F), Kevlar is -3, path length is around 2 meters, I took temperature as 40F to 180F.

I can imagine a Conti belt stretching that much, so in fact may not go literally slack-- just like a rubber band wouldn't go slack. The stock Gates belt is stiffer and stretches less, and the racing belt is promoted has having virtually no stretch. But less stretch means more slack as the engine cools.

Does this happen, at least sometimes? Absolutely.
Is it a problem? I don't know, I originally thought it was unnerving-but-harmless, now I am not sure. It certainly seems like a few mm of literal belt slack would be enough for the teeth to climb out of the gear.

(The resistance of the oil pump would help, but at startup with no oil pressure there is virtually no resistance).

jcorenman

Have you ever parked a warmed-up car, let it cool overnight, and then removed the belt-covers without rotating the cold engine? Once you rotate it to 45-deg (or whatever), you've moved the slack to the tensioner side.

SeanR

That's pretty much the way I take them all apart. Wait until they are stone cold, take everything apart so I can see how it all is and then rotate to TDC.

Imo000

Next time you work on one, try what I did and report back. The belt will slide off if you do it the way I did it. This is not a matter of looseness but low tension.

Mrmerlin

As strange as it sounds I think Imoos info should not be discounted.
These observations as first might seem unimportant but actually could provide clues as to whats happening with the belt run as temperatures change from a running condition to a cold condition.


I Also check belt tension on cold engines and notice some parts of the belt run with more slack than others this will change when the crank is turned.

Anyway, some more thoughts about what has been posted

After looking at the crank drive gear sitting on the table there are witness marks of corrosion on the gear teeth .

Now if this gear was put back onto the belt run with rusty faces this could account for the powdered rubber thats adjacent to the crank drive gear.

The rust would wear off the rubber as the belt was turned on it,
the rust would also polish down some as well.

With the belt teeth now with less tooth rubber this would make the belt fit onto the gear teeth even more loose than a new belt has.

NOTE look carefully at the belt as its strung on the crank drive gear note the small areas on afte flanks of the belt it appears there is clearance.

NOTE the previous remark about the racing belt having more play when meshed with the crank drive than a regular belt has.

Also my previous suggestion that with a 5 speed car putting the trans into gear,
then parking the car like this,
if the car was to roll slightly to the rear this could turn the crank backwards,
thus possibly collapsing the piston in the tensioner slightly ,
As the engine cooled this play could increase on both sides of the crank with the tensioner side having a bit more of the tensioner being compressed.

Then when the engine was started with a brittle belt and a weak battery the slow rotation may have been able to make the crank jump time just enough to bend the valves thus the hard to start just after the initial starting attempt.

If any of what I am postulating is true,
I think it makes sense to install the roller guides under the crank gear for the chance time when the belt might be more than loose enough to jump time till the slack was taken up.

the flyin' scotsman

Jim..........your observations are at minimum worth the time and effort to investigate.

My GTS had a complete belt overhaul with all new gears, new Gates belt, new water pump and new PK system.

Shall report back asap.

ptuomov

Interesting, and very logical thinking.

Some questions. [EDIT: silly me, believing any of the fairy tales about the racing belt having been made of Kevlar. It's a fiberglass belt per the manufacturers specs.] First, the racing belt is not entirely Kevlar. Furthermore, it may be plied or something like that. So I don't think one can just assume that it has the same coefficient of thermal expansion as Kevlar. Or is that -4 alpha value a measured value? Still, probably less than fiberglass reinforced belt, so your point stands on the direction of the difference between belt versions. [EDIT: Everything in this paragraph is a misinformation that I naively believed after reading it.]

The second question I have is about cams. Depending on which position the engine stops, the valve springs are going to exert some force in one direction or another on the cam gears. I would think that the force from valve springs will either pull the belt tight between the cam gears or add to that slack, depending on the crank angle that the engine stopped. This independent of the thermal expansion. Am I thinking about this correctly?

If I am thinking about this correctly, the issue really isn't the cooling of the block. It's the amount of overall slack in the belt at different temperatures (regardless of at which temperature the engine was turned off) and whether the camshafts are in a position at which the passenger side bank valve springs push the camshaft to rotate clockwise and the driver side bank valve springs push the camshaft to rotate counterclockwise. Right?

Hai gebissen

The spot where the car was parked slopes very slightly towards the nose of the car, so it is impossible that the car rolled to the rear.

Are new crank drive gears available? Strange I didn't get one back in 2013, unless they were not available or insanely expensive (though I spent nearly $600 on cam gears alone, so I doubt cost was the reason I didn't replace it).

Hai gebissen

When I initially installed the new Gates racing belt, Porkensioner, cam sprockets, water pump, and oil pump gear, the engine had not been run in at least a year. I strung the belt, released the tensioner, and measured 9mm of extension. I turned the engine over and found the same extension. I eventually replaced a faulty starter relay and turned the engine over with the starter, and still observed 9mm of extension at the tensioner. The engine still had not run under my ownership.

I expressed concern about this extension exceeding the 7mm max, and aware of the heads being replaced (and possibly milled) Ken did suggest slotting the holes accordingly. I decided to leave it alone, and of course also deleted the seemingly superfluous idlers at the crank gear. In retrospect I should have made the modifications and left the idlers in place (and probably just used the standard belt).

ROG100

I am pretty sure there is no Kevlar in the RTB.

marine928

Great mystery, really cool observing car sleuths at work. Can't wait for requisite " moral of the story is" conclusion. Einstein would be proud.