Twin aero CI/diesel - some advice needed

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Of course, rod failure makes such a mess, it can be very difficult to ascertain exactly what let go!

Back in the day, when VW beetles started blowing away Chevies, rods were a problem. They were typical "I" beams, of forged steel. For a while, the hot ticket was to fill the "I" with weld (yes really!) but after roller cranks with one piece rods became available, the original cap bolts came under the spotlight.

These days, aftermarket rods are usually the less anaemic "H" section, typified by the Carillo, with ridiculous ARPM bolts; one VWDRC champion did four seasons on the same rods and main bearings; if you wish to find out what's going to break, build a devoted drag car that can do 0-60 in two seconds. When it's working.

But I digress. The little twin's screws backed off. Ol' Malcolm modified the design, and solved the problem. He's an active boat modeller, and his engines don't spend their days gathering dust on his mantle shelf.

Presumably, Russell Watson-Will did not encounter the same problem, or used loctite. Malcolm used four screws per rod; Russell must have used two, and probably staked them too.

One last thing, unrelated to rods, is the funny angle of exhaust apertures in the FTPT case. Russell had Myford dividing head, with a 60:1 worm and wheel.
Last built up crank I made I spot welded the assembly held true, with wire welder tacks, very small, then silver soldered. When cold I locktited pins thru the silver soldered joints. I've made quite a few silver soldered cranks and only had one come apart. It was in a Red Wing and I ran out of solder on the last joint and rationalized enough solder had penetrated. I like fairly tight joints for soldering as it makes holding the unsoldered assembly pretty true. I use a triangular file to file 6 or so notches around the inside of the webs to insure the solder penetrates.

I like the ease of making a built up crank and I can use drill rod for the shafts and pins, yields good steel journals, properly sized. My current project is a built up crank with a needle bearing on the rod journal. To avoid overheating the bearing I put deep chamfers in the webs and tapered the ends of the pin. I then wire welded them together, stopping when they started to get too hot. Going to be a fairly high compression engine so time will tell if it holds together. Pinned also.

Note that any sort of welding onto a piece of drill rod will create a super hard spot that may crack. That may not matter given the way you are using it just as temporary alignment, with the true strength coming from the solder, but wanted to point this out in case someone else wonders why they are having trouble with welded drill rod.
LOVING this thread. Thanks for all the replies. Right now im leaning more towards the pressed pin assembly (possibly with a splash of loctite for good measure). My main reasoning is that it would:

1) allow me to make one-piece conrods and fit before assembly. If I were silver soldering, I'd be really worried about melting the rods if i heated above 660deg or so. Also the cleanup would be v difficult as cr*p would be likely be stuck between the crankpin and con-rod which I'd be unlikely to get out.

2) disassembly at a later date would (probably) be possible with some sort of puller should I need to replace the rods.

3) Press fitting is something id really like to learn to do.

So I now could do with a bit of advice on a combination dowel pin or needle roller to reamer size that might work. The pin will be 5mm and the crank web will be made in some sort of high tensile steel. Normally I'd just buy a selection of undersize reamers and do some experimenting but THESE THINGS ARE EXPENSIVE at 30quid a pop!

Also, any pics of a jig just to get my brain in the right place would also be v useful.

Thanks all for the help.

Thanks Peter, so the sequence would be get an on-size 7/32 (5.56) mm pin such as below, ream an on-size 5.5mm hole and open up a bit with a 5.5mm diamond reamer as you suggest above? Any tips on how to gauge the fit?

Another trick I've used before is to use a parallel hand reamer instead of a machine or chucking reamer- and use the reamer's lead to leave a constriction in the bore. That and a hardened commercial dowel could do the trick nicely.
Hi Pat,
I normally make a couple of test samples to establish the required fit. As much interference as possible without yielding the material, i.e. so the gauge still fits after pressing the pin back out of the test sample.
I will assemble my crank in a fixture which will serve both to maintain alignment and control the gap between the cheeks.
I will still finish the main bearing journals after assembly, with the rods restrained by wrapping the assembly in cling film.
With a pressed assembly, there is scope to correct any misalignment after assembly by force.
Hi there, can I ask why you would prefer to make a complicated multi piece crank instead of just making split big end rods? Just curious.
Hi, after completing 5 single-cylinder compression ignition engines that run nicely. I thought its high time to get a twin under my belt. I had a look around and they seem pretty rare! I was looking at the FigTree Pocket twin (plans by Ron Chernich from the Motor Boys plan book) but haven'y made my mind up yet.

I was wondering if any of you folk could give me some pointers / alternatives to consider. Also, there seem to be very few build logs out there so I'm struggling to get my head around machining the crankshaft and making the 2-piece conrods. I understand eccentric turning and have made a bunch of crankshafts for singles but how to make two polished journals out of a single piece stumps me due the the difficulty of getting a finishing tool deep into a cut/polishing with emery paper etc. Any build logs that you know of would be greatly appreciated.

I can help you with this its way easier than multi piece cranks.
check my build thread for my blown two stroke V12 here
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Dean, in my case, the decision was driven by the need to keep the overall size of the engine down.
Split rods need more clearance, which means a larger and heavier crankcase.
I have made engines with both arrangements and I have not found multi part cranks to be any more complicated than multi part rods.

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