U20 tensioner epiphany Someone check my math

Post by **dbrick** » Tue Feb 08, 2005 12:02 am

I was just doing a bit of math and came up with this. Please give me your input.

figuring a 5/8" approx piston diameter on the tensioner, Max of 80 psi oil pressure, I get the folowing:

Force = pressure x area
Area = Pi x (radius squared)
.625 diameter x .5 = .3125 radius
(.3125 squared =.0977) x 3.1416=.307 sq inch area of piston
.307 x 80 (pounds of oil pressure) = 24.55 pounds of force on tensioner.

So if I put a spring behind the tensioner that provides about 25pounds of pressure on the chain, I don't have to "wait" for the oil pressure to build to have full tension on the chain.

Does this sound right? Could it be that easy?

I await a decree from His Lordship or one of his trusted minions
(sorry Sid, couldn't resist)

Post by **spl310** » Tue Feb 08, 2005 3:32 pm

I am not that hip on math, so I trust you. The problem I see is that there is already a spring in there, so the oil pressure would add to that spring tension, wouln't it?

If I am mistaken, off with the head of all that disagree with me!

ambradley · Post by **ambradley** » Tue Feb 08, 2005 3:58 pm

Ok, dumb thought popped into my head. When the car isn't running, there isn't much tension on the chain, right? Is that for a reason? Is it maybe intentional that there isn't pressure 100% of the time? Would putting permanent pressure via a stronger spring be a bad thing?

Post by **dbrick** » Tue Feb 08, 2005 6:52 pm

Not downside that I can see, the valve springs can sit compressed for months with no ill effects. Will has his shimed almost rigid, and has had no chain problems I'm aware of.

glennhuff · Post by **glennhuff** » Tue Feb 08, 2005 11:02 pm

The only difference I can see is that oil pressure would remain constant regardless of chain stretch. Spring pressure would necessarily change as the chain stretched. Don't know how much this would affect the timing chain, but just my .02 worth.

Glenn

Post by **spl310** » Tue Feb 08, 2005 11:06 pm

The valves are not pushing on a chain which is prone to stretching...

Post by **dbrick** » Tue Feb 08, 2005 11:14 pm

I was going to still have the oil pressure, only difference would be stronger spring, so pressure on the chain with oil flowing would be the same or slightly more, only startup pressure will change for the better

I got an extra timing set with tensioner, chains and sprockets for $15.00 on ebay, so if i make a mistake, have spares.

I already have "death rattle" scars on the "L", so I should hear the difference with and without stronger spring. I'll put a glob of grease on it and see if the chain hits. I found there was no tube in the oil filter, big suprise!
Might actually be starting the motor this month!!

garth · Post by **garth** » Thu Feb 10, 2005 2:32 am

I suggest the reason the engine designer chose a hydraulic tensioning system over a spring based tensioner was longevity and reliability. As the engine comes up to temperature the timing chain length increases. Hence the tensioner adjusts to a new position. Your math and calculations are correct; the tensioner provides approximately 25lb of force on the chain. This force is adequate when the engine is new and under moderate load. Assume that the chain performs well for 50k miles. The tensioner must then accommodate the combined displacement due to: the growth in chain length, the wear on the contact face of the tensioner and most importantly, the whip in the chain caused by high RPM and loading. These effects probably require the tensioner to adjust through a displacement of 0.500 -0.750 inch. The hydraulic system can accomplish this at a constant force of 25 lbs. Available spring technology in the mid 60's could not provide reliable predictable constant force. If the designer knew what the 50k mile demands on the tensioning system were going to be, he could have selected a spring to deliver 25 lbs at the anticipated displacement (wear). However the initial force on the chain would be far in excess of the 25 lbs, resulting in the accelerated failure of the chain with its well documented consequences. The hydraulic system, although more complicated and expensive promised longevity and reliability the cheaper spring alternative could not deliver over its operating life.

I apologize if this explanation seems some what tedious.

I wish I understood this concept when I rebuilt my first U20 35 years ago. I am currently rebuilding my second U20 and I anticipate replacing the tensioner face material with a self lubricating low friction composite and upgrading the timing chain. The goal is a system that will perform worry free for 200k miles.

SLOroadster · Post by **SLOroadster** » Thu Feb 10, 2005 3:26 am

I put some shims behind mine so it wouldn't rattle at start up. Both the upper and lower tensoners are shimed, and is far easier than finding a spring. Mike Young made the suggestion.

Willl

n51hj · Post by **n51hj** » Thu Feb 10, 2005 10:38 am

What did you use for shims? How much did you shim it?
Thanks in advance.

nomadtrash · Post by **nomadtrash** » Thu Feb 10, 2005 2:19 pm

I used a zip tie on my race car. Check out the photo.

http://community.webshots.com/scripts/e ... ity=zuqXPV

Worked like a charm and didn't require any major surgery.

Post by **dbrick** » Thu Feb 10, 2005 11:48 pm

The basic goal here for me is to eliminate the slop on a cold startup and prevent the chain from slapping around. Once the oil pressure is up, it's back to Datsun's original engineering. The 25 pound spring should neither increase or decrease the force on the chain once the oil pressure pushes the piston out and the force will be the same no matter how far out the piston is, assuming no leakage.

As far as I know, there is no problem with the chain tension once the engine is running and up to full oil pressure. I would guess the cam timing would retard slightly as engine torque pulls the slack onto the other side of the chain. As far as Will's solution, as long as the shims don't exert more force on the chain than the hydraulic pressure, I can't see any bad points, he's just setting a minimum. If it were shimmed up too tight, I would guess that the wear on all parts would be accelerated.

By the way, except for the obvious things like breakage or excessive sprocket wear, how worn is worn out? Is there a spec or rule of thumb?
My spare set has pretty minimal sprocket wear, judging by the tooth profile. I know from motorcycle chains that a worn sprocket is Very obvious the tooth actually hooks as the load side wears away.

Reason I'm asking is if I'm going to change the timing set, better now than after it's all together.