Holly Buddy Inline Twin

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pat_pending

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Hi,

as I was looking around for a next project, and keen to try a twin aero CI engine, I got chatting to @edholly who has an enviable collection of beautiful engines (and equally enviable petrol-head history that makes me very jealous). A plot was hatched to take the 'mud map' sketches of his successful and running Holly Buddy Inline Twin and make a complete set of sharable working plans and cad models.

After a bit of back n' forth and some long evenings on Autocad Fusion 360, I'm ready to start making some swarf (and hopefully not fill up the scrap bucket too much!). If the HB single is anything to go by this should go like absolute stink. The single turns ~13,500 RPM on an 8x4 prop and thats not fully broken in!

Some features of this design:

  1. Beefed-up front crankshaft, bearing and prop driver over the single to handle more power and look better spinning a larger prop.
  2. Centre split 'bobbin' bearing made from HE15 aluminium sitting on a lip in the crankcase to locate it before securing with a grub screw. The bobbin will be a light press fit so should be airtight and OK for occasional disassembly. I have seen designs of the Sparey and Taplin twins with an 'O'ring on this part but don't think its necessary here as long as the fit is good. Also, going down a deep Youtube rabbit-hole on correct 'O'ring usage, I think the oscillating pressure in the crank case during running not be optimal for one of these in any case.
  3. Dual crankshaft porting to act as a double FRV.... a MRV if you like :)
  4. Venturi + NVA off the Nalon Viper
  5. Hot ends (piston, conrod, liner, contra, muff) from the HB singles.
Here is the artists impression below. I will be making and checking the plans as I go along so by the end of it I should have something accurate/usable.

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It's swarf time! Squaring stock still takes me too long...

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Mark out the crankshaft cavity with the DRO. I take all measurements from the same datum surface to ensure the best accuracy I can.

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Using the trusty 4-jaw again, I was going to use a faceplate to hold the workpiece but didn't have the mounting thingies I needed... Quite a bit of stickout from the chuck but a nice flat surface to hold square. Perhaps ill use that dusty faceplate next time.

This was quit a deep bore so needed to take what seemed like 100s of light cuts to ensure it was parallel.

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treated myself to a fatter 16x150mm boring bar for this task with larger CCMT 09 inserts. Worked really well and let a decent finish.

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Happy with the end result. So much time invested in this hunk of HE30 already i will cry if i ends up in the scrap bucket. Taking extra care from now on in.

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pat_pending

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Great stuff Patrik (and Ed). Going to be a monster of an engine. I am happy to follow this one 👍🏼
Hi Olli-Matti, looking forward to the challenge and hopefully have some plans to share at the end of it.

P
 

pat_pending

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Hi, bit of shed time today and managed to get the crank case done. Still some deburring to do etc but pretty happy with it.

This was my first time using a boring head in the mill which turned out to be a surprisingly pleasant experience / my fears were unfounded. It was a cheap and cheerful ebay Chinese one with the braise-on carbide tools that came with it. The surface finish turned out great.

Disaster almost struck on one of the manifold mounting holes where I grabbed a 3.5mm drill rather than the 2.4mm while on autopilot after drilling a ton of holes. I just stopped in time and was able to get 4mm of thread tapped which hopefully will be enough to tighten on the manifold. That'll learn me not to leave drill bits lying around!

There was also something a bit iffy about the aluminum I was using. Bought off eBay and the seller said it was HE30. In my hands it felt 'stickier' than normal. Tapping each M3 hole gave me the absolute fear (especially 10+ hours in!!!). In the end I tapped them in two stages with a swarf-clear in-between.

Anyways, happy to be underway. The crank case is normally the trickiest thing for me (although this build I have the centre bearing and twin crankshafts to figure out. I think I might go with the front housing and backplate next.

-Patrick


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a41capt

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Very nice! I know the feeling of trepidation as you near the final operations in a piece after countless hours of work. Stay the course, we’re all cheering for you!

John W
 

pat_pending

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Hi all, fast forward to the exciting bit. Over the last few weeks I have made almost all the parts that would be the same on a single cyl diesel so front, crankshaft, cylinder muffs, cylinder liners, conrods, venturi, compression screw, prop driver,.......... Now its on to the interesting bits :)

Next up: how to make the centre crankshaft and centre bearing. I am pinching some ideas from the modelenginenews article on the Sparey Twin here but think I will skip the o-rings as a close fit should hive me plenty of seal in the crank case. I hope to get some shed time this weekend so hopefully more progress to post tomorrow.

Thanks.
Patrick

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pat_pending

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Hi Oli-Matti. Thanks for the comment. I experimented with some different carbide inserts and these seem to be the business! That surface finish was straight from the lathe (ok the cylinder muffs had a bit of 800grip applied mainly to remove the burrs beween the fins). They were Sumitomo CCGT 060204 carbide inserts for non-ferrous metal. Also a bit of kerosene +T32 hydraulic oil as lube on the finishing cuts. I switched over to this insert for finishing it he high tensile EN24 for the crankshafts and, to my surprise, that worked great too (with really light cuts). Thanks, Patrick
 
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Oh and I think they have a 0.2mm radius at the tip.

P
Thanks for the confirmation Patrick. I was suspecting that you are using an aluminium insert judging by the appearance of the bit. I have setled on the cheap korloy ones from Ebay and like the ccgt 060202 and 060204 the most. ( the last digit of 02 or 04 is supposedly the nose radius to my knowing) They do very well on alu and brass and steel and you name it. Atleast on my table size lathe and on my Emco Super 11. What kind of rpms do you usually use with these inserts when working for example on the aluminium cooling head jacket or the steel crankshaft?
 

pat_pending

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I normally do by ‘ear’ or feel. I think normally 1200rpm for the finishing cuts or something? Probably 600-700 for hocking out big chips on the high tensile steel. For that I use the gold steel inserts and seems to go ok but a rough surface finish.
 

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