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Author Topic: Os120FT Gemini ~ petrol conversion. Any ideas  (Read 1201 times)

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Offline Area 51

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Os120FT Gemini ~ petrol conversion. Any ideas
« on: February 06, 2017, 21:35:10 PM »
I have the Os120FT gemini Mk1.
It is new to me, the guy I got it off suggested it may have been modified for petrol.
The chap he got it off stated he had performed some changes, but was unhappy with the "heat" generated; the motor could be run on glow again if wanted. The guy I got it off did nothing with it and its sat in a box for a few years..

The motor does not show much sign of wear at all, there is a casing attached to the rear of the crankcase, this is not present on the Os images of the rear of this motor.
The stock crankcase appears to be flat and of "all in one" casting.
 
This casing is a good fit and appears to be a manufacturer/cast piece to suit this motor/crankcase, not after market. It has four screws through that hold it rather poorly to the crankcase.

Any advice greatly appreciated, I have searched the web for 120FT conversions and had no success.

Thanks a lot in advance,




Offline alangorham

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #1 on: February 07, 2017, 09:50:11 AM »
FWIW I've got a Gemini MK1 and it definitely doesn't have that housing.

The pic with the housing removed shows that a drive shaft has been added on the back of the cam drive gear. My immediate guess would be that the housing is a fuel pump. Could this be right?

In any case it would seem that if you made sure the housing was sealed up you could still run on glow. Does it still have the original carb fitted?

Offline Area 51

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #2 on: February 07, 2017, 11:12:21 AM »
Hi Alan, thanks for taking the time to reply.

It does have the carb and all the bits as far as I can see.
I had not thought about a fuel pump Alan, it has two outlets at 3 and 9 o'clock, a nut type attachement, sheered off wire in the middle, I was told by the guy I got it off he thought these powered the glow plugs or sparks in the case of a conversion?
There is one other outlet from the casing and this is at the 6 o'clock bottom with a screw in it..

I will look and see if there is any tell tale signs of an addition to the rear of the cam gear! I did think this stuck out too far to fit the original casing you refer to on yours and that can be seen on various pictures.

I would think this the cam gear is supported by a bearing or something if the correct end plate/back plate was fitted? Would this be right?

Aprat from this palarva, it does seem very low time..  $%&


Offline alangorham

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #3 on: February 07, 2017, 12:18:41 PM »
The cam gear does have a modification. The alloy boss with the square shaft coming out is the added part.

You are correct that the standard engine would normally have a bearing to support the end of the shaft.

The white plastic (aceeal/delrin?) lining to the extra housing has chambers and outlets that made me think of fuel related things - although I'm not sure a pump would be needed...

You've then made me think that this added housing could be a contact breaker to fire the spark plugs, but I can't see any contacts or cams that would do it.

I'd be tempted to run it as a glow engine first for simplicity and prove it all works before any conversion work.

Offline Area 51

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #4 on: February 07, 2017, 12:53:52 PM »
Hi Alan, thanks!

I am wanting to run it as a glow, perhaps my message title is confusing. No interest in the petrol set up.
The two outlets at 3 & 9 o'clock appear to have some sort of wire sheared off, the guy I got it off suggested these went to the glow plugs.

I will look at it again this afternoon and see if that output shaft that has been attached with a screw or anything that can be removed, perhaps it is a sleeve over the original camshaft end. With your comments in mind Alan, if this extension was removed I would not have anywhere for the original end to be mounted as the crankcase covering is drilled out to fit this casing...!

Perhaps a solution is going to be secure the caasing that is attached and run it as glow, as you have said.
In cleaing this rear end up and opening it, it has come to light the mounting screws used into the engine case appear to be short.. they grip nicely and find the bottom of the holes when screwed in without the casing, however once passed through the casing they struggle to find a thread and tighten!


Thanks again for the additional comments and looking in here,  $%& :af :af



Offline alangorham

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #5 on: February 07, 2017, 13:12:01 PM »
Yes, I would do exactly as you say.

If you wanted to remove the extra bits permanently then bear in mind it seems almost impossible to find spares now, so that a rear cam cover will be hen's teeth material.

Good luck!

Offline Area 51

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #6 on: February 08, 2017, 21:07:04 PM »
Hi again, I have been in the rear of it now; see the pictures.

That extension to the camshaft appears to be very firmly fit; a tiny pin hole at the side, possibly heated and shrunk on??

If I were to remove this, the original camshaft would have no where to sit in that extension casing and the bearing contain there in; it needs that extension shaft now to make up the gap to the bearing!

When assembled, I suspect something then fits on the square end of this extension to the camshaft, beyond the bearing in the casing, and spins in the end of the casing.

There appears to be some sort of metal strip around the inside of the rear of the casing, something to spin against? pick up from? at the very back of the casing in the centre, there is another support/bearing location.

I have taken a picture of the outlets from the casing, they have sheared off wire coming from them, the guy I got it off thought/was told these may connect to the plugs?

I cannot rear see much oil being lost beyond that bearing in the casing, it has a seal before the extension shaft goes into the bearing.

Iím thinking of stripping the engine completely now I am this far, chiefly to ensure the is no gunk or blockage in the conrod lube holes etc. (Iíve read this can be a weak spot).

Is there anything to observe or be aware of when stripping and rebuilding a twin 4 stroke?

The casing at the rear of the crankcase has four 8ba x 1Ē screws to secure it; the casing was also sealed with a blue sealant when it came to me.

Cleaning fluid soften this sealant, this has revealed the 8ba screws do not tighten.

Checking this out, the screws go to the bottom of the four holes and tighten, however when passed through the casing they are 4-5 threads short!, one failing to find a thread at all..

I have ordered some 8ba threaded rod, new stainless bolts and gasket material. I will make up some of thread rod to the correct length with a gasket installed and then get it to tighten properly.

Any other observations or ideas appreciated.

Thanks in advance again.

Offline Engine Doctor

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Re: Os120FT Gemini ~ petrol conversion. Any ideas
« Reply #7 on: April 17, 2017, 10:18:02 AM »
Hi If you haven't run it yet then please read on . I bought one of these some years ago . It was fitted in a model and the person I bought it from said it had not been used for a few months  ? I wetted it thoroughly with fresh fuel to loosen it up and then started it . Within a couple of minutes it started to knock . On stripping it down I found that both big ends had a lot of play. so much that the bottom of the pistons were hitting crank !.further stripping showed the the oil holes in the big ends were all blocked with hardened dry castor oil. Luckily the crank shaft journals were not damaged and cleaned up ok .New  con rods are now unobtainable for this engine unless you can find a breaker I managed to find some rods from John Haytree who has now sadly passed away. The Mk11 Gemini rods are completely different and will not fit this engine . So BEFORE you run it , it might be a good idea to strip it down and check that the con rods oil holes are clear then use synthetic based fuel to prevent further clogging. Its not a very powerful engine in glo form so would be even less powerful run  on petrol and as said will get very hot . I replaced mine  with a Sc 160 twin that ran rings around the 120 . 
PS. i now keep one of the damaged rods as a Key ring to remind me about good old castor oil  ;)
PPS I forgot to mention that the large bearing fitted to the center of the crank is a OS only part and couldn't  be matched up by bearing specialist and back then OS had discontinued it !
« Last Edit: April 17, 2017, 10:23:09 AM by Engine Doctor »
Take off's are optional ,landings are not !


 

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