Cylinder head

[7122alanParticipant @7122alan]

Hi

Could any one tell me if it is possible to have a new one of these made { does not have to be red} ,it is a cylinder head of a .75 cc model plane engine .

classed as a diesel engine the hole in centre should only be 5 BA little hole 10 BA ,or would it be possible to fill with alloy solder drilled and re tapped .

It is off an engine from when I was 12 (66) now so it’s old

cheers in advance

 

[Andrew TinsleyParticipant @andrewtinsley63637]

Not quite sure what the 5BA and 10 BA refer to. It sounds as though the threaded hole in the (red?) cooling jacket has worn badly. I assume it is the threaded hole for the tommy bar / compression screw.

Soldering would need special solder for aluminium. I would simply drill it out and make up a compression screw with a larger diameter thread.

It would suggest that the contra piston is too tight in the bore. This seems to be the most common reason for the compression screw, threaded hole, to wear. It is a simple job to ease the contra piston to give a better fit. If you have a lathe then this is a straightforward, but messy, job.

Andrew.

[noel shelleyParticipant @noelshelley55608]

It should be possible to repair ! one way would be an oversize threaded bush and tap to size needed. Another would be to make an oversize contra piston thread and same in head. Good luck.  Noel.

[7122alanParticipant @7122alan]

Hi

The 5 BA is for the contra piston 10 BA for limited stop pin on head 5 BA is stripped ,tried to re thread failed miserably now it has a 1 BA hole ,( yes tried a few times I’m no engineer 😀

don’t have a lathe wondered if anyone on here could make a replacement .

[Clive FarrarParticipant @clivefarrar90441]

In my oppinion remaking the whole head is overkill and way more difficult than putting a new oversize thread in the head for a new compression screw.

If the contra piston is too tight now is the time to get it out give the rim a LIGHT buff up with Brasso to ensure it moves smoothly.

fitting a bush or a helicoil are also possible alternatives.

 

Good Luck.

[JasonBModerator @jasonb]

There does not look to be much metal left between the now 1BA hole and that for the pin. At a minimum you are likely to have to go upto 0BA or M6 maybe a bit more to 1/4 x 32 to get a full depth thread and they could all break into the smaller hole.

[Andrew TinsleyParticipant @andrewtinsley63637]

The head looks as though it is from a DC Sabre. The later models had a pin stop to limit the amount of compression that beginners could use. If you needed a smaller higher revving prop, then often the stop pin would prevent you from getting the optimum compression ratio. IMHO the pin is best left off the cylinder head.

Andrew.

P.S. Looks as though the head could do with re anodising.

[JasonBModerator @jasonb]

At 0.75cc is it not a Merlin?

The flats have been a bit chewed up in the past too.

[7122alanParticipant @7122alan]

No it’s not the Merlin it is the super Merlin , red head and red prop spinner both were .76 cc the sabre was 1.5 cc the spitfire blue head 1cc and the rear induction 2,5cc Rapier .
That engine is 54 years old with bits missing , but I got it running till compression lever stripped’
I have no means of repairing it

[7122alan]

On 7122alan Said:

[…]

I have no means of repairing it

I don’t know how hot these things run, but as a conservation project I suggest you try fitting an insert, with a plain outside diameter and a 5BA tapped hole : Make it a snug fit and use one of the  Loctite retaining compounds.

[Careful use of an electric drill would serve as a makeshift lathe.]

It may, of course, blow out when you start her up ! … or you might get lucky.

A little bit of fettling with needle files would tidy those bruises.

MichaelG.

.

 

[Grindstone CowboyParticipant @grindstonecowboy]

Expanding on Michael’s idea, is there space inside for a small lip on the insert? That should stop it blowing out if the Loctite isn’t up to the job.

Rob

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>Probably too hot for loctite unless just a cosmetic non runner, but a headed bush pressed in from underneath might work. Without a lathe the OP is going to struggle.</p>

[Michael GilliganParticipant @michaelgilligan61133]

Struggle is good for the Soul … especially if one succeeds !

[Careful use of an electric drill would serve as a makeshift lathe.]

MichaelG.

[JasonBModerator @jasonb]

But it would be difficult to put the head into the lathe to clean up the hole sufficient to get a half decent surface, might manage with a drill to make the bush. I also feel temps will be too high even for something like 648 so to get a suitable press fit the OP would need to be handy with the drill and graver.

[JasonB]

On JasonB Said:

But it would be difficult to put the head into the lathe to clean up the hole sufficient to get a half decent surface, might manage with a drill to make the bush. I also feel temps will be too high even for something like 648 so to get a suitable press fit the OP would need to be handy with the drill and graver.

I was only suggesting he use the drill as a lathe to make the insert !

MichaelG.

[7122alanParticipant @7122alan]

Well thanks for the interest , seems a bit scary to me, nearest thing I have done is sharpen my chainsaw blade.

Not sure what I’ll do could go on eBay buy another for spares etc .

Till then I’ll keep looking

cheers

[7122alan]

On 7122alan Said:

Well thanks for the interest , seems a bit scary […]

Then please don’t do it … Such non-standard methods are clearly unsafe if you are not confident/comfortable.

MichaelG.

 

[sprocket 3Participant @sprocket-3]

Yes, a threaded repair bushing with a ‘top hat’ lip shouldered up against the underside of the aluminium head/cooling jacket would work ok. The only problem for the OP is he doesn’t have a lathe or a milling machine to accurately ream the stripped thread hole square with the top of the head and to a good tolerance fit for a bushing….I’m sure that could be salvaged but it isn’t something I would tackle without a lathe or a mill.

I think the high temperature Loctite retaining compounds would cope with the heat of on of these little diesels aero engines….I know I have to get components that have been bonded with high temp retaining compound a lot hotter than one of these motors run at to disassemble the parts.

[sprocket 3Participant @sprocket-3]

These Davies Charlton aero engines were manufactured in a factory in Hills Meadow industrial estate(Isle of Man) which is just about 5 minutes walk from where i live.

In the 1960s my Father ran a non ferrous scrap metals business and he used to buy all the aluminium alloy swarf and scrap from the D.C. factory.

edit to add: The old D.C factory building is an engineering training facility these days.

[7122alanParticipant @7122alan]

Methinks a lathe is in order , only 55 yrs ago at school I last used one .

machine mart here I come 😁

[7122alanParticipant @7122alan]

Can anyone recommend a decent lathe ,had a look but overwhelmed by the choice ,300 mm between posts would size I want with thread cutting

pref pre owned as I am to old to learn and would not get use to new one to make it worth buying

[Author]

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