Seems as though most everyone is putting aluminum pistons in their engines except a few hold outs. I have read all the reasons why this is done but I have a question. Given two cars (or more) one with aluminum and the other cast iron pistons, more or less stock engine, driven in the 0 to 35 mph range, both hills and flat, which would be more likely to break a crank? Or what has your experience been?

Very interesting question Mark. I too will be interested in reading the responses.

I don't know of anyone making new oversized cast iron pistons, so short of lucking into some good used ones in the size you need, going to aluminum is about the only choice if your block has to be bored. I have to wonder how many broken cranks today are due more to dogging the crap out of an engine never intended to take it, than cast iron pistons.

I'd suggest the key items to think about is:

1. the condition and type of the crank being used (is it a SCAT, Dubats or EE?)
2. Quality of engine rebuild
3. Quality of the driver (do they lug the engine?)

Aluminum or cast iron piston is way down the list but being lighter I guess aluminum puts less stress on the crank.

I would think broken cranks are mostly caused by driving a 100 year old car to hard.There are probably some because of some defects but the majority is to much on a 20 hp engine.
To many guys want a hot rod T instead of just building one from a later era car when you could have a real hot rod.

Considering we are talking about a stock crank which has no counterweights, you do not interfere with the balancing of the crank itself, so an aluminium piston would put less strain on the crankshaft so should in theory result in less broken crankshafts..

In theory reality and theory should be the same, in reality however....

I would think that broken cranks are caused by driving 90 year old engines. I run the heck out of mine, lug them and put more miles on engines than most here, haven't broke a crank yet. The cranks are early and late type, no EE's.

What I think makes a crank last is balance and alignment. We drive at speeds that would have been hard to do back in the day due to road conditions. Changing oil often and keeping the engine is good working order.

I would agree with Leo. Lighter pistons = lower rotating mass. Less mass would equal less stress. If I needed pistons and the cast iron pistons were free, and the Aluminum pistons (especially if they were top-quality forged Aluminum) were $100 each, I'd use Aluminum. I'm not rich; doing the job once correctly is far cheaper then having to do it over.

Not new to T's. My grandfather and my uncles told me about broken crankshafts back in the day when they were still making Model T's. In those days you could take it into a Ford dealer (have it towed in) and pick it up the next day ready to go. So I'm sure they cracked with cast iron pistons.
Norm

John
I'm curious, why worry about forged pistons in a nominally stock T?? If you are making a HP per cu in, sure

Heavy rods, light rods, heavy pistons, light pistons. These things have to make a difference to the crank shaft stresses, which in turn must make a difference in the likelihood of the crankshaft breaking. The question becomes, How much of a difference do these things make? The heavy weight rods probably make a considerable difference, raising the likelihood of a broken crankshaft quite a bit. The rod runs freely, held between the crank and the piston, throwing its weight around to both. Three of the cycle strokes push that throw one way, while one stroke (the power stroke) throws the weight in a different direction). One stroke throwing the weight of the rod one way while its neighboring stroke throws its weight the other way tries to twist the crankshaft. The heavier rods will try to twist the crankshaft with more force that the lighter rods will. Heavier rods, pushed hard, probably will break crankshafts much more often than the lighter rods.

Why all the talk of rods? When the question is pistons?
Simple.
On the surface, pistons have a similar effect. More weight thrown, more pressure dumped onto the crankshaft. However, there is what turns out to be a considerable difference. Unlike the rod, the piston is being forced through the cylinder with tight rings and little oil. The drag of the rings in the cylinder acts very much like a shock absorber. It does not eliminate the forces of the thrown weight of the piston. But it does greatly reduce the shock and stresses reaching the crankshaft. Since it is the piston that is held back. it is the piston who's weight is most mitigated to the crankshaft. Therefore, I suspect the likelihood of breaking a crankshaft by using cast iron pistons instead of aluminum pistons (all other things being equal) probably increases only a very small amount.

Proper pan and block alignment is probably the most important thing to most reduce your chances of breaking a crankshaft. The floating transmission shaft is probably one of the best things you can change to further reduce crankshaft breakage.
The next most important thing, would be to simply drive the car correctly. Lug the engine as little as possible, don't over-wind the engine by driving too fast (says the guy that liked driving his center-door sedan at 55 mph). And remember, any increase to power and performance adds to the likelihood of a broken crankshaft. That includes carburetor (simple upgrades all the way to the extreme), compression increases, improved lift camshaft, bigger valves, better manifolding. Anything. They all put more stress on that little crankshaft.

But, I like model Ts. Right down to the crankshaft. I have never had a model T with anything other than a model T crankshaft.

I'm thinking Wayne nailed it.

Clearly, problems happen. But if you drive your model T like they are supposed to be driven..(about 25mph) the stock crank will keep on whirling around and provide excellent service. I've been on tours where the tour leader likes to run up against the 40mph mark. I refuse to run those speeds. I won't even run 35. The cars were meant to run in the 20 to 25mph zone.

The power stroke is the one stroke which puts the most strain on the crankshaft. That should be the same regardless of what type pistons you have, however, domed pistons or high compression head can increase the power stroke. I agree that the bent crankcase or extremely worn 4th main would be the hardest on the crankshaft. Also, a mistake to tighten the center main when things get out of alignment. If the back of the transmission drops, the crankshaft will move upward in the center putting wear on the center main. If you tighten the cap, it keeps that center up against the block which causes the crankshaft to flex with every turn of the crankshaft resulting in metal fatigue at the weakest point in the shaft.

Alignment, alignment, alignment.
Norm

Wayne
Thank you for the "endorsement " of the floating transmission shaft. I truly appreciate your understanding
I have expanded Dan Hatch's license to be world wide other than Canada

Les, You are welcome. I like what I have seen so far of it. To me, it just makes sense. I worked most of my working career on the edges of engineering. I have been in places where I have seen how much "inflexible" structures really do flex. Structural steel and concrete bridge girders for one. Being up about a hundred to a hundred fifty feet on a communications tower, feeling it move, then looking down to see the ground moving around---. I just can't believe that the best straightened model T pan in the world does not flex enough under all the stresses it gets hit with, to adversely affect the crankshaft. (Of course, the bolted together '26/'27 block and hogshead helps a lot also.) Besides, I have straightened a few pans over the years. I know about how much one must bend a pan BEYOND straight, to have it relax back to straight.
Your floating transmission shaft has to help a lot.
Good luck to Dan Hatch!

John K. and Mike B. both said that one reason Model T crankshafts break is that they are 90-100 years old. I believe that's true. If there is any misalignment anywhere in the engine/transmission assembly (and most of them have considerable misalignment), that will impose some flexing movement upon the crankshaft.

Tom Lieb (the owner of the Scat crankshaft company) gave us a very informative presentation about crankshafts at the Sunflower Crankers Winter Clinic a few years ago. He said that, as with any piece of metal, if you bend a crankshaft back and forth enough times, it will break. MOST Model T crankshafts are bending back and forth some amount every time the crankshaft turns each revolution. Some last 100 years and some don't.

When asked whether a Scat crank would break, Tom's reply was, "Yes, eventually."

I'm no engineer, but did have a bit of engineering education during my two years at Chicago Technical College, and gained a bit of "practical" as a detail draftsman during my four years of working as such in the engine/transmission group at the International Harvester Advanced Engineering and Research Center in Hinsdale, Illinois way back in the '60's. Accordingly, I do know just barely enough about engines to be dangerous!

I would add just a bit to what Norman Kling said about the power stroke being "the one stroke which puts the most strain on the crankshaft". What I would add is the fact that "lugging" any internal combustion engine, especially the Model T Ford with that little "hairpin" crankshaft, is just asking for a broken crankshaft. That is because with the piston moving downward on the power stroke so slowly when lugging is when the cylinder pressure is absolutely the highest, and that's one thing that can and will break a crankshaft. So, all that to say,....by all means, avoid "lugging" no matter what! FWIW,.....harold

..... I think maybe that's what Hal Davis meant by,...."dogging the crap out of an engine never intended to take it"! (:^)

To add to this, the power stroke is the only stroke that pushes the crankshaft forward. All other strokes apply pressure in the reverse direction, just not as much pressure.
So, every time a power stroke fires, it is trying to twist the crankshaft in the forward direction, while all three other cylinders are trying to twist the crankshaft the other direction. That poor crankshaft is getting tweaked forward and backward constantly when being run, even at a sitting idle. Add the road jerks and bumps, and you can begin to understand how much stress it really gets.
I still like to run original crankshafts. And although I do like to drive my model Ts faster than most T owners do? I don't really push my cars very hard. It is one of the reasons I like the various auxiliary transmissions I have run over the years. I like the gear choices to go fast on the flats, or climb hills at an optimal rpm. Then I get a brass car and don't want the extra tranny because it is not brass era correct.
Hmmpf. Some people.
Drive carefully, and enjoy, W2

I don't understand how the power stroke pushes the crankshaft forward.

Mike,

I'm sure Wayne means forward as in the normal direction of rotation, the other 3 strokes are resisting normal crank rotation.

I wonder what percentage of broken crankshafts happen in stock engines versus those with modifications that increase compression.

Gary -- If that's what he meant, I'm good with that.

Steve
The majority of the broken cranks in this area were on totally stock engines. None of the cranks in significantly modified engines have failed here

Crankshaft stress can be reduced considerably by having enough space to put a bearing between every cylinder. This is only seen in some very expensive engines. For reasons of weight and space, they choose to beef up the crankshaft and put one bearing for every two cylinders.