Join Date: Jul 2008
Location: Huntington, New York
Cam belt Tensioners
Warning Long Post.
This year I have had a lot of problems with my Milano in lemons racing. The
first race of the season the head gasket gave up (original). No damage but that
entailed some work. I took the opportunity to cut the head(.024") and lapped the
valves and uprated the valve springs (30% stiffer). It all went back together
well and based on my seat of the pants dyno I was up some noticeable power. The
cam in this car is a used one I found on eBay an RJR 570/744 it works very well in my
motor. Leak down on the motor has always been really good (2% to 6%)
As some of you may know I have the rev limit set at 7200 and a set of used eBay
IAP headers that we welded a balance pipe to. The pipes go into a gutted center
section muffler and then two pipes out the side. I replaced the plenum AFM hose
with a big smooth hose going to the AFM and a cold air feed to a KN style
filter. The motor is tuned pretty well, cam timing optimized. It's was on a
Chassis dyno 3 years ago and it had plenty of fuel for what it could breath. For what its worth the dyno said 145HP and I think about 140 ft/lbs of peak torque before I worked on the heads. But most
importantly we drive like we stole it. The harder you drive it the better it
runs and believe you me, I hammer this motor hard.
So back to the problem: It's all about the mechanical tensioner's. Six hours
into the race at NHMS the outer spring in the tensioner fatigue cracks and we
lose some tension it skips and its game over. OK, Now I do a proper valve job
with guides and new valves and I am getting good at this. I bought a NewWay kit
and port match and some other work as well. Remove some pistons and inspect the
bottom end everything looks great. So I put it all back together and while I am
talking about getting a hydraulic detensioner I cannot locate one in time for
the race at Summit point. So 5 hours into that race one of my drivers hears a
pop and bang and the power goes away and the car is smoking a bit. We bring him
in and after a bit of jerking around I see the 1,2,3 bank cam is out of time by
one tooth. Sure enough the new mechanical tensioner outer spring is broken. So
we pull it apart screw the tensioner together and reset the cams and send the
car out. The powers OK but the car is smoking blue smoke pretty heavy and I
decide rather than grenade the motor better to retire.
BTW I am tensioning with a fixed tensioner so that the belt along the long length on the tension side
can not quite be turned 90 degrees, you agree?
So it turned out, I had a few bent valves on the 1,2,3 bank and the bent intakes
valves had pulled my guides out on 2 cylinders so oil was leaking down the bore
the valve guides press into. Fast forward and I have repaired the damage.
It seems as though the little bit more of spring rate on my valve springs has
changed things radically for the mechanical tensioner. It is being rocked around
with some sort of resonant excitation which fatigues and cracks the outer spring
that goes to the bimetal element. So I need to tension the belt differently. I
see a few choices.
1. Wind the mechanical tensioner up to it installed position and use 5mm bolts
through the blind holes in it to turn it into a fixed tensioner
2. Use a rebuilt hydraulic detensioner which I have.
3. Use the hydraulic detensioner with the oil feed port block off so there is no
4. Design and build my own fixed adjustable mechanical tensioner using the
plastic wheel and bearing from the stack of used tensioners everyone has.
5. Take the hydraulic tensioner apart and tap a thread where the piston assembly
is now located for a pusher screw.
6. Attempt to make a ZAT style tensioner, probably the most work to do it
Perhaps I missed something? Anyway I am spamming a few people with this message
to see if the collective wisdom can help me and also cross posting this. I have a few photos I cut & pasted of some examples of other peoples tensioners.
Links to pictures or instructions on other sites are also welcome.
We are racing at NHMS in Loudon NH on October 26th through 28th if any Alfisti in New England
want to stop by.
Thanks in advance.
1972 Berlina Vintage Racer, 1987 Milano Platinum (LeMons Car), 1982 GTV6 EP Race Car, 1974 GTV Vintage Race Car, 2012 VW GTI stage II, 1966 Corvette Coupe 327-350, 2007 Triumph T100 Bonneville. Dearly departed: 1971 Fiat 124 Spider, 1983 Saab 900 Turbo, 2004 Saab 9-3 Aero, 1995 BMW 540i Sport, 2014 Audi A4 (swamped)
Last edited by GregSef; 10-18-2012 at 09:07 AM.