MP Jet 040 - Gudgeon Pin removal

[Paul Jefferies]

Does anyone have any experience of dismantling an MP Jet .040? I am trying to replace the crankshaft but I can't get the old one out because I can't get the con-rod off the crank pin. I have removed the liner but, unlike a Mills, there is not enough play to "jiggle" the whole piston/conrod assembly out in one. It looks as though it was assembled by inserting the gudgeon pin through the hole in the crankcase where the carburettor assembly screws in but there seems to be no way to remove it by the same route....... I am sure there must be someone out there who has had this problem before so any advice would be much appreciated.

Paul

[Denis Watkins]

Is it this one Paul

LETMO MD-0,6 has some unusual construction nodes - this is given by the then knowledge and production capabilities of the post-war years. In particular, it is a crank mechanism - LETMO has a "hook" handle without the counterweight - the rod is very short in order to achieve the smallest dimensions, the piston in the lower dead center fully enters the space where the crankshaft is in the normal engine. Although it is very convenient in size, it has huge vibration problems - the crank mechanism can not be balanced at all. Another unusual design feature is the front right-side lid - the lid at Letma 0.6 tends to be still loosened, and a special pin wrench is required for tightening through the holes in the head. 

 

I read the following as the head screwing off?

 Long-stroke engine - Drilling / stroke 8/12 mm 
- Concept "T"Piston-driven rinse-out wash 
- Rich ribbed head of threaded cylinder turned from rod 
- Crankshaft mounted in sliding bronze case 
- Split nozzle with fuel needle of 2 mm diameter, M2 thread 
- Needle lock with clamping nut 
- Coated insert and piston made of special gray Cast iron 
- a slim turned steel connecting rod

Edited By Denis Watkins on 05/08/2017 20:16:17

[Paul Jefferies]

Yes, that's the engine. I can't take the piston and con-rod out together so I need to separate them by removing the gudgeon pin.......... but I can't remove the gudgeon pin with it all in situ. There has to be a way to do it but I just can't see it!

Paul

[Denis Watkins]

It says in the list above Paul

The crankshaft is mounted in a sliding bronze case?

Could moving the crankshaft fore or aft not give you more room?

Also the pictures show the head fins removed?

[Paul Jefferies]

I have removed both the head fins and the cylinder liner, leaving the piston loose in the crankcase but still attached to the crank via the con-rod.

I think the "sliding bronze case" is a less than perfect translation from the Czech and it refers to what we would call a bronze bearing. It does have a bronze bearing which was pressed or sweated in but it certainly doesn't slide........ There is a small amount of fore and aft movement of the crankshaft in it's bearing but not enough to allow removal of the con-rod. It is very nicely made to quite close tolerances!

Paul

[onetenor]

You need to make a press tool from thin steel .A ring that will slip over the piston with the two ends joined with a nut and bolt through the overlap .A hole to clear the gudgeon pin diametrically opposed.Slip the ring over the piston with screw end against the pin than tighten the srew to push out the pin. You may have to juggle the whole thing so that you can line things up through the ports It may help to solder the nut and joint together with the nut on the inside this will enable the screw to be removed and inserted through a port .I'm sorry I can't send a pic but a bit of thought will show you what I mean John O/T

[Paul Jefferies]

John, thanks for your suggestion, I think I understand what you are getting at but having made up the ring there would be no way to get it over the piston down inside the crankcase and then, because at tdc the gudgeon pin is still below the top of the crankcase, there would be no way to access the screw in order to turn it ......... or have I not understood you correctly?

[Engine Doctor]

Hi are you sure the conrod won't come off the crankpin with the crankpin at 10 or 2 o'clock position or even bdc ?Fiddly yes but it must be doable as there is no way that gudgeon pin can be removed with the piston /conrod  assembly in position. Think of it as one of those Chinese puzzles you get in a cracker . The gudgeon pin should be tight fit. Pins designed to be removed through the inlet passage or through a hole in the crankcase are usually floating pins an hollow to allow a tool to be inserted and withdraw the pin but this is classic old design and a tight fit. Good luck and let us know how you get on.

As for a tool to press it out , that's a no goer as there is nowhere for the pin to go at the front of the engine.

Edited By Engine Doctor on 06/08/2017 09:57:48

[Denis Watkins]

Quick one Paul, your photo shows a hole at Gideon pin height on one side of the crankcase

Their advertised photo shows a hole at the other side?

Is their a blind hole/ filled in opposite the hole you show.

Alternately, if the motor is not precious, could you drill a drift pin sized hole opposite, then drift the Gideon pin out through the existing side hole

Tap a thread and seal your access?

Only if the motor is not precious

Edited By Denis Watkins on 06/08/2017 11:01:16

[Paul Jefferies]

'Done it !!!

Thanks everyone for your suggestions..... Denis, the engine isn't "precious" as such and I think your idea would have worked but I would be reluctant to drill holes in it unless it was the very last resort. Engine Doc you were right, it was a bit like a Chinese puzzle....... I put it at bdc and jiggled the big end, the piston and the crankshaft plus a little more force than I felt comfortable with and it went. Having fitted the new crankshaft, the next challenge was putting it all back together again. The tolerances on these engines are incredibly tight and it really didn't want to go. The problem was that the end of the crank pin has very sharp edges and, because it wasn't perfectly aligned, it tended to dig into the much softer aluminium big end. In the end I had to use very fine wet and dry to take the sharp edge off the crank pin and with some oil and a little persuasion, it went back in. I have now run the engine and it is "as new"!

Many thanks to all for your input.

Paul

Edited By Paul Jefferies on 06/08/2017 16:49:26

[Richard Jones 14]

Hi there

Even though I have yet to reach this stage I have read this thread with more than a little interest. They are lovely little engines easier to handle than a Mills and more power availability, they just seem lacking that bouncing under compressed charm of a burping of a Mills.

My question is, where do you get a new crankshaft from? When the first one broke John Hook replaced it FOC, now that I need another one he doesn't know where I will find it.

In 50 years I have never had one crankshaft go until now, is it a common problem, or have I just been unlucky?

I only used it lightly in a Tomboy and never crashed, honest.

thanks

Richard