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TR2/3/3A Crank balancing

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CJD

CJD

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I've never taken a TR3 crank to be balanced before. I was sitting here looking at the crank, and it occured to me that it is perfectly symetrical. That got me wondering...I know, that's dangerous. But my question is:

Normally, with the V8's I have worked on, the reciprocating weights and the rotating weights are matched for all the pistons and rods. Then weight is added to the crank journals to equal 1/2 the reciprocating weight + all of the rotating weight. But this applies to an asymetrical crank with 90 degree offset journals.

With a symetrical crank like the TR3, it would seem to me that the reciprocating and rotating weight of the rods and pistons is not a factor. So long as these weights are matched, they will balance themselves out of the equation.

So...can the TR3 crank be balanced by spinning without adding weight to the journals?
 
And...I just got my answer...

https://www.dsmtuners.com/threads/correct-my-crankshaft-balancing.425327/

Our cranks are spun absolutely bare, with no bob weights at all. You can install any weight piston or rod, and the crank stays in balance. The rods and pistons just have to match each other. Coming from the V8 world, that seems pretty cool! From a practical side, it means I can have my crank spun now, and not have to worry about buying the piston set till later.

For those with a real interest, the first link I posted explains why these engines will never run as smoothly as other engines. The second order vibrations (which are a result of the upward and downward accelerations of the pistons being different because of geometry), cannot be balanced out.

Of note, the second order vibrations come from the piston accelerations...so the lighter the pistons you use, the less the vibration.
 
John,

While you may be the only one talking you are not alone. :smile:

Some of us are out hear "listening" but unfortunately don't have any knowledge to add. When I rebuilt my engine in 1984, I did take the crank, connecting rods and pistons to the local speed shop to be "balanced". I seem to recall that it was not that expensive but I'll have to dig up my receipt.

Scott
 
Art, unfortunately it often does!?!

Scott, the balancer still needs the pistons and rods to match everything, just like you did. I used to balance my own cranks...which is why I had an interest in this topic. I no longer have access to the machine that spins the parts to balance them, but I still prefer to match the weights of everything else myself. It just takes a decent scale. Shops have to work fast to make money, so they shoot for a +-1gram. When I do it myself I can get them much closer than that. I then give the bob-weight number to them, so they don't have to grind on the pistons/rods and do the calculations. Of course, since we get those second-order vibrations that can't balance out, getting too close in these engines is likely a waste of time.

I just got excited when I figured out that I can wait on purchasing the "big ticket" piston and sleeve set for a while, and still have the machine work finished up. Guess I'm easily amused...you know, with talking to myself and all!!
 
You do need to bring in all the parts that are attached to each end of the crank, for a good balance job. My crank balance included the flywheel, pressure plate (not clutch plate), and front pulley. You would want to include the fan extension nose piece (that is a big heavy part). I don't know about the fan itself. I don't have an end-of-crank fan so didn't face that challenge.

When they balance all parts together, they actually make a mark on your flywheel and pressure plate so that you assemble them all the same way as they did when they balanced them all. They did quite a bit of grinding on my pressure plate to get it balanced.

Very soon, I'll actually know how it feels to rev a "finely balanced" engine.

Pat
 
Once again, a final follow-up to close out this thread.

I had the crank balanced at a local speed shop, and according to Pat's instruction, did it all. I specifically asked that they balance the crank alone, and then add the parts one at a time, rebalancing as they added each new part. If the reasoning for that isn't obvious, it ensures that each individual part... like flywheel, clutch, fan extension...are all individually balanced. That way if any one or a combination of parts has to be replaced, it will not throw the balance off of the entire assembly.

Finally driving the balanced engine regularly. A balanced engine is so much smoother running. I never rebuilt the TR3, merely removing the pan and heads to check the wear and clearances, and putting it back together with new gaskets. It ran great, but the shifter would rattle on the highway, even with the little anti-rattle pin in it, and even the key chain would sit there and rattle.

The TR2 has none of that! This shifter has no anti-rattle pin in it, but it doesn't rattle a bit. Holding your hand on the shifter while cruising barely gives a vibration. I used to keep my hand on the shift lever of the 3, just to quiet all the rattling noise. I know there are second order vibrations in a 4 banger, but they are obviously very minor in the big picture...or our engines have enough iron to keep them under control.

In conclusion, if you ever want to know if it is worth spending the $100-$150 to balance your motor...the answer is "Oh yeah!"
 
If you work with a typical chemistry scale (Ohaus, etc.), you can balance pistons, rods, bearings, etc. to +/- 1/10th gram. It takes patience, but the results are nice.
 
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