Constructing the Nemett Bobcat

[Adam Gregory 1Participant @adamgregory1]

Hi

I want to have a go at making the Bobcat engine which has started in ME. Part two opens with "mill a suitable aluminium block", my question is what would be a good grade of aluminium to use. I have the book by Malcolm Stride on miniature IC engines which came as a gift for my subscription to ME. In that Malcolm suggests to use LM4 for the crankcase, but this is a casting grade of aluminium. I looked all over the drawings but can't find any reference to what type of material to use.

Also is there going to be published soon a cutting list (not sure if that's the right term) for all parts required to make the engine, so all material can be ordered together to save on carriage costs?

[Adam Gregory 1Participant @adamgregory1]

[JasonBModerator @jasonb]

HE30 or 6082 as it is now known will be fine for the majority of the ali parts, thats what I used on my Firefly.

I may make the single myself

J

[Mike WainwrightParticipant @mikewainwright87512]

A cutting list would be useful as the drawings do not specify the material for some of the parts.

[KWILParticipant @kwil]

Has anyone else started out on this Bobcat Project?

[JasonBModerator @jasonb]

I'm Still thinking of doing the single but having recently started a double siz eEaston & Anderson it won't be this side of xmas. But then again I did see a rather nice looking Kiwi on the cover of teh opposition recently.

Are you thinking of doing one Ken? Make a change from all that old tooling

I'm still wondering where all the 50 sets pf materials for teh Firefly have gone, only seen one other made from the mag.

J

Edited By JasonB on 15/10/2012 16:58:30

[KWILParticipant @kwil]

Jason, already on the way

[Martin-tygParticipant @martin-tyg]

I started the Bobcat, made the crankcase from HE30, seems to have worked out OK.

Maybe a dimensional error on the crankshaft nose piece, shows bores of 19 and 22mm for the two bearings, both bearings are 19mm. Used some T6 Aluminium for this piece, turned and bored just lovely.

Next excitement is the crank, cannot find the EN8DM in flat, lots of milling to get it from round. Any ideas on alternative bar? Agree with material list at start of day, saves cost and delays.

Martin_tyg

[KWILParticipant @kwil]

Martin,

Inner bearing is a 61801 which is 21mm dia OD and 12mm ID. Nose and crankshaft drgs OK it is the bearing list that is wrong! Nose drawing clearly shows 21mm counterbore on the inside.

Yes I machined a crankshaft blank from 30mm dia EN8 as I could not find any flats. Sory no suggestions on alternative. Have almost completed the journals but it is hold until after Sandown MEX because of prpep for that.

K

[JasonBModerator @jasonb]

M-Machine do 1" x 1/2" EN8 bright flats.

J

[KWILParticipant @kwil]

Drawing errors,

Gudgeon pin is from 5mm silver steel and not 6mm as shown on drawing.

E clips of 2.3mm size are 6mm thick and so the locations are too narrow.

[JasonBModerator @jasonb]

Should that be 0.6mm rather than 6mm thick?

Actually I think the drawing shows 24mm inside the grooves and 0.5mm outside with no width specified for the actual groove.

Edited By JasonB on 03/12/2012 16:34:32

Edited By JasonB on 03/12/2012 16:35:05

[KWILParticipant @kwil]

Jason,

At least someone is awake, however, yes it should read 0.6mm thick for the e clip. Drawing definately shows (in all cases) 0.5mm wide with 0.5mm from end dimensions. Drawing do not just show the end lengths, just a confusing drawing..

[JasonBModerator @jasonb]

Just looked at the printed version rather than the digital and yes the groove is shown as 0.5mm, you can't tilt the screen and squint along the dimension lines like you can with a bit of paper.

J

[Martin-tygParticipant @martin-tyg]

Re. Making the main bearing diameters, the magazine recommends tight spacers to maintain the crank web spacing. A better method is to make the 3 M3 holes in the webs and make a steel plate that screws onto these holes. Cannot move and should provide good compressional force resistance.

Now making the second crankshaft , started the lathe when I should not have and bent the first attempt! Not best pleased, but **** happens!

Martin

[KWILParticipant @kwil]

I used two brass screws with double nuts, set the screws in place with the nuts locked, however the whole was araldited in, when set, you will not move anything (well reasonable) by compressive forces. You can free the araldite when finished.

[Martin-tygParticipant @martin-tyg]

Hi Kwil

that should do the trick. Hoping o finish the crankshaft soonish and see just how well things turn round! Always an intresting moment.

[Martin-tygParticipant @martin-tyg]

OK, finished the crankshaft, mounted the bearings and assembled. Imagine the joy when it turned with no hint of tight spot! With unbalanced crankshaft rotating the crankcase leaves the shaft heavy side down. Such joy.

[David Clark 13Participant @davidclark13]

Hi There

Just finished editing the Bobcat.

It will be in 4447, 4448 and finish in 4449 as far as I can tell.

regards David

P.S. How many of you are building this engine or the single?

[KWILParticipant @kwil]

Bobcat twin David

[Martin-tygParticipant @martin-tyg]

I'm making the twin.

[Traction manParticipant @tractionman]

Hi

I am collecting the materials etc for the twin. I was going to start construction when all the parts in ME had come out, to see if there were any comments from other constructors.

[KWILParticipant @kwil]

Hi all,

Another drawing error I am afraid. Cylinder shows 5 off M2.5 threads for cylinder head bolts, cylinder head shows 5 off 3.0mm clearance, all on 31mm pcd, ie M3 screw clearance….. You takes your choice I guess!

[David Clark 13Participant @davidclark13]

Hi KWIL

I am confused.

What is wrong?

Cylinder shows 5 off 2.5 mm diameter holes.

The text says these are tapped M3.

If anything, the cylinder head/ liner drawing should say tap M3 but it won't scrap

the component if you drill them 2.5 as this is correct.

regards David

[KWILParticipant @kwil]

David,

Some of use work direct from the drawings and ignore most of the text. If you are going to stick with the text, then that has to be correct as well. The point I am making is that both should correlate, you should not need to keep cross checking. I agree it will not scrap the component, it is just irritating that these differences are there on what has already been made by the Designer, that should have proved the drawings.

[Author]

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