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Building a FM 118 crankshaft?


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Old 11-01-2013, 01:19:33 PM
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Default Building a FM 118 crankshaft?

I own a pattern and I've even machined a few castings myself but I can't afford to do them in any kind of a production run any more. I was looking at the thread on smoke and flames showing them build a crankshaft for a ship engine in pieces.

http://www.shipsnostalgia.com/guides...s#Introduction

This got me to thinking why not? So, I'm having some journals cut out in a water jet and ordered some cold roll. I'm going to build a few and see how they do.

Has anyone here ever tried this before? Any input would be appreciated.

I will take pictures along the way and will show the final product.
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Old 11-01-2013, 02:27:12 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: Building a FM 118 crankshaft?

A lot of motorcycle cranks are built this way, of course with multi cylinders getting every pin indexed just right is the trick. A single cylinder should be pretty easy. Are new cranks, or reground cranks hard enough to find to justify this?
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Old 11-01-2013, 04:09:04 PM
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Default Re: Building a FM 118 crankshaft?

As I tried to say, I have made them before. 70+ of them. To my knowledge I made the last ones. It's not fisable any more. Welding them is disaster, they will break. I think I would trust this more than a regrind for sure. We just never weld and grind a FM crank. Others might, we don't and am not considering starting.
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Old 11-01-2013, 06:31:21 PM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: Building a FM 118 crankshaft?

If I were going through this much trouble I would order better than cold roll, 4140 or one of the alloys, someone on here will probably have recommendations. If you order ground and polished and the correct reamers it would be an easy to machine job. Tack the throws together, bore and ream, press together. I would leave the rod pin and the main shafts long, machine a few thousandths off the ends for and inch or so so it lines up before the press fit starts, press it through and cut the excess off. Make a snug fit dowel to hold the main holes in line while pressing.
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Old 11-01-2013, 06:38:15 PM
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Default Re: Building a FM 118 crankshaft?

JB, wish I could spend some time with you. Sounds like you have some good ideas. I do plan on machining the journals at the same time but was planning on welding the assembly together. 4140 would be nice, I'll think about that for later if the first one works out. Cold roll would be better than the stock FM crank though. (My cranks were nodular ductal) Wish I knew more about reaming? I don't. I plan on making a light press so they go in straight with big counter boring so the weld penetrates deep and also gets it down out of the way.

It did cross my mind to put the main shaft in in one piece and after welding the outside cut out the middle. Similar to your dowel suggestion.

Do you care to tell me more about what you refer to as reaming? I have various Sunnen hones, would that work?
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Old 11-01-2013, 07:31:38 PM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: Building a FM 118 crankshaft?

You can buy reamers in standard sizes, an undersize like you would need would be expensive. Sometimes a metric size will be close. You drill or bore the hole to within a few thousandths and ream it to size, correct size hole, good finish, no measuring. The Sunnen hones should work, I have an old Sunnen pin honer that I use to get an accurate bore with a nice finish. Welding wouldn't hurt but I would try to get a good press fit. If you press the main shaft through in one piece I would make it oversize at the throw, turn the rest down so you don't have to press all the way. Turn two beveled grooves for the weld area. Cold rolled doesn't machine real well, usually kind of gummy.
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Old 11-01-2013, 08:00:14 PM
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Default Re: Building a FM 118 crankshaft?

Leaded steel does make a nice finish. I do alright with cold roll if I spin it fast.

This will be a learning experience but that's what I like.
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Old 11-01-2013, 08:27:44 PM
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Default Re: Building a FM 118 crankshaft?

I have thought about this for some time but haven't had the time to try it myself. It seems to me that it would work - cold rolled steel should be much stronger than cast iron, shouldn't it? As long as you can weld it up without distorting too much, it should work. My book has the FM 118 crank pin diameter at 1.936 to 1.937; that should make it easy to start with 2" stock and grind it down after you've finished welding. How big are the mains (under the taper roller bearings)?
I had originally thought of turning the ends of the rod down to 1 1/2" and only have to bore a hole that big in the flat pieces, but that would prevent your idea of starting with one piece for the main shaft and cutting it in two after welding. That's a great idea!
If you've got some scrap laying around, I'd make a "learner" crank out of that, weld it up see how it works - i.e. how much runout does it have after your "done"? Unfortunately, I don't have a bunch of 2" round and 1" plate to play with myself.

The other thing to check is how much space you have between the main shaft and the rod bearing journal. If I looked at the right info, a 6" stroke would have a 3" offset (main to rod). With 2" material, that would give you 1" between the two. Some cranks have a lot less stroke and the rod and the main overlap, which makes it hard to build one up by welding pieces together.

Are you planning on welding on the counter weights or cutting the side pieces out of a large piece of stock? Which brings up another question - what about balance? I know a single cylinder engine isn't going to be perfectly balanced (without countershafts), but you need to get the weight close enough that you don't wind up with a terrible jumping engine.
How thick are the side pieces? I assume you have an existing crank to measure.

---------- Post added at 07:27 PM ---------- Previous post was at 07:25 PM ----------

Leaded steel is horrible to weld. I think 4140 is a kind of chrome alloy steel (cromoly).
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Old 11-01-2013, 08:33:29 PM
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Default Re: Building a FM 118 crankshaft?

Not an expert and I haven't made any big crankshafts but I've used two methods to make the ones I've done.

Method 1: Waterjet cut a blank then turn it in a lathe. This is time-consuming but I've had good results. The 30-60 replica engine is an example of a crank like this.

Mdthod 2: Cut out and machine the crank cheeks and pins then press the whole works together. I don't like welding them because of the distortion (and I'm not a very good welder). Examples of this kind of cranks are The Homebrew Engine and The Homebrew Hvid.

Go to my webpage to see details.
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Old 11-01-2013, 08:57:09 PM
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Default Re: Building a FM 118 crankshaft?

Great info. Keep it coming.
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