Author Topic: Dragonfly engine rebuild  (Read 1326 times)

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Offline Sgtbiro

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Dragonfly engine rebuild
« on: 02 Oct 2018 at 21:26 »
Hi can anyone recommend someone that could do a complete engine and gearbox rebuild on a Dragonfly

Cheers kevin..

Offline Eric S

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Re: Dragonfly engine rebuild
« Reply #1 on: 04 Oct 2018 at 09:21 »
Same for my Aero 1937 ...

Offline AndyMorgan

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Re: Dragonfly engine rebuild
« Reply #2 on: 24 Nov 2018 at 18:09 »
I will be rebuilding mine shortly, what do you need?

Offline Eric S

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Re: Dragonfly engine rebuild
« Reply #3 on: 25 Nov 2018 at 16:43 »
As far as I am concerned, now I only need 6 or 12 engine head bolts.

Offline AndyMorgan

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Re: Dragonfly engine rebuild
« Reply #4 on: 27 Nov 2018 at 20:37 »
I should have a set of head bolts somewhere, I'll take a look and let you know. Just moved house so things are chaotic at the moment

Offline Eric S

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Re: Dragonfly engine rebuild
« Reply #5 on: 28 Nov 2018 at 11:53 »
That would be more than welcome !

Offline AndyMorgan

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Re: Dragonfly engine rebuild
« Reply #6 on: 28 Nov 2018 at 22:20 »
Please PM me your address and I will dig them out and I will pop them in the post or drop them off next time I am in NCL

Offline Roys rider

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Re: Dragonfly engine rebuild
« Reply #7 on: 02 Dec 2018 at 08:37 »
Andy: I get the impression that you have built Df engines before, if so you might be able to help with a problem I have. The engine I put together a few years ago turns out to be so tight the kickstart wont turn it. I can turn with a spanner on a clutch nut or the crankshaft nut without much difficulty. When putting it together I didn't measure the end float on the crank and I can't see how one would create any. Could this be the problem? Advise from anyone would be appreciated as I am up against a wall.
Cheers
Peter

Offline eddie

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Re: Dragonfly engine rebuild
« Reply #8 on: 03 Dec 2018 at 16:19 »
Peter,
          We have already had some discussion on this matter in a previous thread. With the engine and gearbox separated, if the engine turns over OK, I doubt there is any problem with the crankshaft end float. If the kickstart operates correctly with no engine to turn but jams up when trying to turn the engine, I would check out the kickstart gearing in the back of the gearbox (it is a weak point on the Dragonfly). Remove the cover for the kickstart auxiliary shaft (the round cast alloy plate at the back of the 'box, held by 3 countersunk 3/16" screws). This cover has a boss on the inside that locates the auxiliary drive shaft. The boss has a slot for a pin that prevents the shaft from turning. Make sure the 2 halves of the boss are in tact, as overloading the kickstart can result in one half of the boss breaking away and letting the auxiliary shaft float and jam - often also doing irreparable damage to the shaft's front location in the main gearbox casing.

   Eddie.

Offline Roys rider

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Re: Dragonfly engine rebuild
« Reply #9 on: 05 Dec 2018 at 11:53 »
Eddie
Thank you for responding. I have checked the gearbox and the pin and boss are fine. I have now removed the timing cover and both cylinders, and the tightness is still there. The conrods move freely on the big ends so we can discount those. I'm thinking that back in the days when these engines were made,  the equipment used to machine the parts were a bit aged, the guys did their best but often selective fitting and fettling were needed, neither of which I applied 30 odd years ago when I put this together from NOS pieces. Therefore it could be that one or more of the bushes are tight. I read in one of your excellent responses that 3 thou is useful to let the oil in. Should I be looking for this sort of figure.
My next job is to take the crankshaft pinion off and see if the camshafts are free, if they are the problem lies with the crankshaft. Your idea for a single front bush seems logical and would be nice to do however I don't have a lathe nor the skills to use one so will stick with the original.
Fingers crossed.
Peter

Offline eddie

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Re: Dragonfly engine rebuild
« Reply #10 on: 05 Dec 2018 at 13:51 »
Peter,
          If you have the timing cover and barrels off, and the tightness is still evident, it could be either the crank or cams that are tight. Try removing the front crankshaft nut and use a puller to lift the crankshaft gear a little - if the engine frees up, a shim is needed between the oil retaining plate and the shoulder on the crankshaft (some oil retaining plates have a step to provide the clearance). If the engine is still stiff, remove the camshaft idler gear, and check again. If the crank is still stiff, operate the clutch to pull the crank backwards, and then check with feeler gauges check for clearance between the front main bearing and the crank web (5 thou is ideal). If the crank is till stiff, the front main bearing is probably on the tight side (2.5 - 3 thou is ideal). Tight front main bearings have been known to turn in the cases - in which case, the locating pin deforms the flange of the bush, and tightens it even more. If the cams are tight, the cases will have to be split to effect a cure.

Regards,
               Eddie.

P.S.   While you have the barrels off, check for end float on the big ends. The front rod needs 10 thou or less end float, otherwise most of the oil will be thrown from that bearing and starve the rear big end. If your engine was built from spares 30 years ago, it probably has a reconditioned crank so it's length may be a little bit suspect, and need re-shimming.

Offline Roys rider

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Re: Dragonfly engine rebuild
« Reply #11 on: 06 Dec 2018 at 20:30 »
Eddie
Whooppee!!
All those reading this will know how it feels to solve a puzzle that has been niggling for a long time. Now thanks to our resident expert, thank you very much Eddie, all is revealed. I even poured Eddie a beer however as he wasn't near enough I had to drink it myself. Cheers Eddie. Yes it was the washer in front of the crankshaft bush nipping up on the crankcase. So a little shim will solve the problem. I could kick my younger self.
Tomorrow I should have time to do the measurement checks you have recommended. The crank spins easily so the bush must be fine. I will try to check the crank float and definitely the front big end clearance, as I have read your piece on oil starvation.
Thank you again Eddie.
Peter

Offline Roys rider

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Re: Dragonfly engine rebuild
« Reply #12 on: 19 Dec 2018 at 21:37 »
and then the euphoria evaporates and reality sets in. Eddie in his PS mentions 10 thou or less on the front conrod. Phew that is OK. HOWEVER he also drops a hint about re-con cranks, which this is, and shimming. 5 thou at the front he says: I have 70 that's some shim. The crank being so far back is of course why the oil ring fouled the front bearing in the first place. Isn't hindsight a wonderful thing.
Any recommendations?

Offline eddie

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Re: Dragonfly engine rebuild
« Reply #13 on: 20 Dec 2018 at 10:36 »
Peter,
          If there is excessive clearance (70 thou) between the front of the crank and the front main bearing, you will have to remove the flywheel and clutch release mechanism. Then gently tap the crank and rear main bearing forward until you have the correct clearance (5 thou). At this point, check the depth from the rear main bearing housing to the actual bearing. Compare this to the depth of the spigot on the clutch release - then make a shim/shims with a thickness equal to that difference, and fit it/them between the outer track of the bearing and the clutch release. Before fully tightening the flywheel nut, make sure you can still get some free play on the clutch operating arm. If all is OK, fully tighten the flywheel nut. If you cant get the split pin hole to line up, leave the split pin out rather than backing the nut off to line up with the split pin hole (as this can result in the flywheel eventually working loose). With the shim inserted behind the rear main bearing, all the clutch thrust will be taken directly by the rear main bearing instead of being transferred through the crank to the front main bearing (causing the tightness you have experienced). Adding this shim will result in a much freer rotating engine, and also prevent engine temperature from affecting the clutch adjustment.

  Regards,
                Eddie.

Offline Roys rider

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Re: Dragonfly engine rebuild
« Reply #14 on: 22 Dec 2018 at 09:24 »
Hi Eddie
Thank you very much for this. I have had a look at the exploded parts diagram and I think I understand what you are saying. I am sure it will become obvious when I have the clutch off. I hope to have a few minutes today or tomorrow.
This knowledge you impart is invaluable and we are all very grateful.
Merry Christmas
Peter

 

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