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Old 02-25-2020, 11:52 AM   #1
M2M
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Arrow When going for SCAT crank should you lighten flywheel? If so, how much?

When going for SCAT crank should you lighten the flywheel? If so, how much?
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Old 02-25-2020, 12:10 PM   #2
Jim Brierley
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

Yes, most of us lighten them by machining them for a V8 clutch and removing some metal from the back side of the wheel, resulting in a wheel that weighs 35-37 lbs. The last couple that I had done I had machined for a diaphragm clutch because they are much smoother. A lot of guys think the flywheel must mate-up with the crank weight. The two are very stupid and one does not know what the other weighs or is doing. Lighter weight will give you faster acceleration, and deceleration but the horsepower does not change.
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Old 02-25-2020, 01:48 PM   #3
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

The torque factor changes with less kinetic energy. Too little kinetic energy can lead to rod stretch if rpm gets too high too quick. Scat crank sometimes like to crack. An aluminum flywheel would likely be overkill but a trimmed stocker as was mentioned is about as good as it gets.
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Old 02-25-2020, 06:21 PM   #4
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

So Jim recommends 35 to 37 lb finished weight for an A flywheel from an original 65Lb.
The B flywheel is original 57lb , what weight should we shoot for lightened?
Also a “B” question regards fly wheel, what are the 3 large holes for in the clutch plate face?
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Old 02-26-2020, 01:39 AM   #5
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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Originally Posted by Jim Brierley View Post
Yes, most of us lighten them by machining them for a V8 clutch and removing some metal from the back side of the wheel, resulting in a wheel that weighs 35-37 lbs. The last couple that I had done I had machined for a diaphragm clutch because they are much smoother. A lot of guys think the flywheel must mate-up with the crank weight. The two are very stupid and one does not know what the other weighs or is doing. Lighter weight will give you faster acceleration, and deceleration but the horsepower does not change.

Jim, how does gearing affect the equation? For example, a guy comes to you with a car that has 3.27 hi-speed gears, which obviously weakens it's hill climbing ability. In this situation isn't a heavier stock flywheel better than a lightened? Does a heavier flywheel help in pulling hills? If no, tell us why.


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The torque factor changes with less kinetic energy. Too little kinetic energy can lead to rod stretch if rpm gets too high too quick. Scat crank sometimes like to crack. An aluminum flywheel would likely be overkill but a trimmed stocker as was mentioned is about as good as it gets.

You've seen or heard of SCAT Model A cranks cracking? I've heard of problems with SCAT T cracks, a Swiss guy broke two of them but that was probably because he installed them himself without knowing what he was doing.
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Old 02-26-2020, 02:18 AM   #6
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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I believe Henry's objective with the heavy flywheel as with about any other car or tractor of the time was to keep some inertia working when the car (etc.) slogged through a big mud hole or up a small hill. A lightweight flywheel would not be able to keep the car moving through a short period of heavy loading on the low powered engine.

Today's roads don't offer the same challenges as the roads of the time these cars were built. Lighten that sucker as much as you can without cracking. Your rods will be fine if you don't drive it like a teenager. There is a thread on here that deals with that and gives dimensions for cutting.
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Old 02-26-2020, 07:52 AM   #7
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

Getting back to the OP, its my understanding, as limited as it is, that a counter weighted crank requires at least 12# being taken off the flywheel.
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Old 02-26-2020, 08:47 AM   #8
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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When going for SCAT crank should you lighten the flywheel? If so, how much?
This makes no sense to us.

We have plenty of engines using the Scat crank or the Counterweighted Ford crank and never have we had to lighten a flywheel for what people are trying to explain. Since I have always balanced engines we've never had an engine run wrong because a flywheel was too heavy with a Scat crank.

Sounds like someone is creating extra work for the machine shop .
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Old 02-26-2020, 10:24 AM   #9
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

just my $0.02

The question should be what is the intended purpose/use of this vehicle?

Some of the opinions here are to change the characteristics to make it easier to drive, higher performance and even just to keep it stock.

Having the flywheel too light can make driving a heavy car a little more difficult requiring slipping the clutch more and feathering the throttle. Some may or may not want that.

Depending on the modifications to the engine, the best you will get is a compromise between simple to drive and more temperamental.

If you are involved with a club, take a few test rides in cars with some of the modifications to get a feel. J

FWIW, yes, I have had a couple broken SCAT cranks come to my shop. Obviously abused and well, not the correct application.
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Last edited by johnneilson; 02-26-2020 at 10:28 AM. Reason: additions
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Old 02-26-2020, 05:17 PM   #10
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

In building a fast road engine for my speedster I had the flywheel reduced to 35lbs (fitting a diaphragm clutch) and the crankshaft had counterweights welded to it and all fastidiously balanced
Within 5000 miles it was discovered that the crankshaft was cracked (it had been crack tested before fitting)
I then fitted a Burlington crank, but continued to be plagued by loud timing gear chatter and vibration periods (possibly due to the modifications) despite fitting a matched set of bronze and steel timing gears.
Fitting a harmonic balancer pulley removed all those issues like magic and I can only hope that also it will contribute to the reliability and long-term life of the engine?
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Old 02-26-2020, 08:37 PM   #11
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

Harmonic balancers are a good thing, period.
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Old 02-26-2020, 09:00 PM   #12
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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Fitting a harmonic balancer pulley removed all those issues like magic and I can only hope that also it will contribute to the reliability and long-term life of the engine?
Wensum, is that a Murray Horn damper you have? Nice piece if it is. I have one sitting here. One of these days I might put it on...
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Old 02-26-2020, 10:22 PM   #13
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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Harmonic balancers are a good thing, period.

Who sells these? Are they easy to install? What do they cost?
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Old 02-26-2020, 11:05 PM   #14
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

While this subject comes up all the time, I do not subscribe to it, to each his own.
I would make a suggestion, while the motor is apart, replace the key in the crank with an alloy key. The stock key is soft and have sheared off a couple.

It is not the end of the world, but can be a bugger to replace.

John
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Old 02-27-2020, 02:53 PM   #15
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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Originally Posted by Y-Blockhead View Post
Wensum, is that a Murray Horn damper you have? Nice piece if it is. I have one sitting here. One of these days I might put it on...


Yes, being a Kiwi it was an obvious route to follow, although like many I was very doubtful that it would make a difference to my problems.
In fact it was a miracle!
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Old 02-27-2020, 02:59 PM   #16
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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Who sells these? Are they easy to install? What do they cost?


These are made in New Zealand "These dampers are made by Murray Horn in Levin N.Z. His Company is Engine Restorations Ltd. His email is horn@xtra.co.nz
Murray says this is the best way to contact him.
The dampers are a straight fit to replace the standard Model A pulley. An added benefit is a shallow reverse scroll in the rope seal area to help contain oil.
"
The cost was around NZ$300 (much cheaper in US dollars.) The pulley probably requires the radiator to be removed and the front of the engine lifted above the front cross member. the pulley is a little deeper than the original pulley and comes very close to the front spring U bolts and so the fan belt needs to be fitted at the same time.
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Old 02-28-2020, 01:20 AM   #17
Dan McEachern
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

Making a statement to remove x number of pounds from a flywheel can have mixed resuts, because what you should be concerned with is changing the moment of inertia of the flywheel. Taking the weight off at different locations (diameters) greatly affects how the moment of inertia is changed. Taking 5 # off near the center has a much smaller effect that taking the same 5 # off near the ring gear. Just something to be aware of.
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Old 02-28-2020, 02:11 AM   #18
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

[QUOTE=M2M;1856010]Jim, how does gearing affect the equation? For example, a guy comes to you with a car that has 3.27 hi-speed gears, which obviously weakens it's hill climbing ability. In this situation isn't a heavier stock flywheel better than a lightened? Does a heavier flywheel help in pulling hills? If no, tell us why.
/QUOTE]

A heavy fly wheel is like taking a run up to get up the hill.
About half way up the hill the energy runs out as the engine slows down.
You plant the foot. And now you have to fill that heavy flywheel with energy, and find enough energy to accelerate the car up the hill.

Without that heavy flywheel, at the half way point, almost all energy when you plant the foot goes into getting the car up the hill, so it takes off like a rocket. However, with the light flywheel you've had less of a run up at the bottom!
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Old 02-28-2020, 11:48 AM   #19
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

[QUOTE=updraught;1856728]
Quote:
Originally Posted by M2M View Post
Jim, how does gearing affect the equation? For example, a guy comes to you with a car that has 3.27 hi-speed gears, which obviously weakens it's hill climbing ability. In this situation isn't a heavier stock flywheel better than a lightened? Does a heavier flywheel help in pulling hills? If no, tell us why.
/QUOTE]

A heavy fly wheel is like taking a run up to get up the hill.
About half way up the hill the energy runs out as the engine slows down.
You plant the foot. And now you have to fill that heavy flywheel with energy, and find enough energy to accelerate the car up the hill.

Without that heavy flywheel, at the half way point, almost all energy when you plant the foot goes into getting the car up the hill, so it takes off like a rocket. However, with the light flywheel you've had less of a run up at the bottom!
That is an excellent explanation!
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Old 02-28-2020, 12:12 PM   #20
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Default Re: When going for SCAT crank should you lighten flywheel? If so, how much?

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Originally Posted by M2M View Post
Jim, how does gearing affect the equation? For example, a guy comes to you with a car that has 3.27 hi-speed gears, which obviously weakens it's hill climbing ability. In this situation isn't a heavier stock flywheel better than a lightened? Does a heavier flywheel help in pulling hills? If no, tell us why.
/QUOTE]

A heavy fly wheel is like taking a run up to get up the hill.
About half way up the hill the energy runs out as the engine slows down.
You plant the foot. And now you have to fill that heavy flywheel with energy, and find enough energy to accelerate the car up the hill.

Without that heavy flywheel, at the half way point, almost all energy when you plant the foot goes into getting the car up the hill, so it takes off like a rocket. However, with the light flywheel you've had less of a run up at the bottom!

Okay, so is it correct to say that if someone is running 3.27:1 hi-speed gears they should maybe stick with a stock flywheel since hi-speed gears reduce hill climbing ability? Correct?
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