Holly Buddy Inline Twin

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Apr 3, 2017
Reaction score

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.

Screenshot 2022-02-28 at 19.11.42.png

Screenshot 2022-02-28 at 19.10.43.png

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


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


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.


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


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.

Last edited:
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.







  • IMG_2236.jpeg
    291.5 KB · Views: 63
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
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.












  • IMG_2458.jpeg
    88.4 KB · Views: 49
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
Last edited:
Oh and I think they have a 0.2mm radius at the tip.


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?
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.
OK so finally got a chance to move things forward. I was stalling on the tricky centre bearing and crankshaft parts as I had no idea of the right machining sequence. What then normally happens is that I procrastinate/prioritise other projects over getting in the workshop.

I took the plunge and got going again and I think it all might just work!.

here's where I got to.

IMG_2685 (1).jpeg

To make the centre bearing I followed this sequence.

- Drill/countersink/tap the bolt holes to hold the centre bearing together.
- Use a slitting saw to separate the two halves of the bearing
- Use emery on a sheet of glass to make the cut pieces have a smooth flat surface. This has to be airtight!
- Bolt together and drill ream the hole for the crank shaft 8mm.
- Superglue onto a precision 8mm silver steel bar that has been centre-drilled at both ends in a precision ER32 collet.
- Turn the outer down to the final size for a light press fit into the crank case.





Ill pop back later with details of how i made the centre crankshaft .



Here's how i made the centre crankshaft. Not sure it works yet but really tried to be as accurate as possible turning between centres and being mindful of chuck runout etc. The only things I think could be improved on from accuracy point of view were the use of a 'live' centre vs dead centre (better) and my method of getting 180 degree rotation on the crank pins as you'll see in the pictures.

So to make the centre crankshaft I followed the below sequence.

- Centre drill a piece of En24 high-tensile steel to +2mm on all dimensions. Drill the internal transfer/induction passage at the same time
- Mount blank between centres and skim to get it true to the centre holes.
- Mount in the mill and mark out the and drill the crank pin location and crank pin.
- Scribe a centre line (by just moving the x axis on the mill)
- Place stock in a V block and scribe a line through the centre line on the opposite side
- Drill the crank pin receiving hole on the other side
- Turn the crank pin in 4-jaw chuck
- Use the crank pin as the 'dog' to turn between centres. Note the centre on the driving end trued-up before use.

Turning the shaft was hard work using a parting blade. The material removal was OK but the surface finish was crappy. Lots of emery paper/polishing. I think i need to experiment with something better next time!.

Anyway, good result, nothing broken and the fits are lovely/most likely airtight enough for a dynamic system with loads of castor oil floating around.

Happy for now. The only things left to make are the pistons, contras and the NVAs so oily hands and a workshop full of fumes are in sight! One operation i'm still really dreading is the drilling of induction passages/FRV holes through the crank case and crankshaft. I'm sure its not going to be that bad but the stakes are so high after this many hours work that I'm scared!

I'll report back progress soon.




  • IMG_2681.jpeg
    82.2 KB · Views: 3
Welldone Patrick! I am liking this concept of yours a lot (might be the first one I think I comprehend fully :D). When done properly like you obviously did should be a winner for sure. I am again tempted by this on a lot! :D Thx for sharing and keep up the good work!
Welldone Patrick! I am liking this concept of yours a lot (might be the first one I think I comprehend fully :D). When done properly like you obviously did should be a winner for sure. I am again tempted by this on a lot! :D Thx for sharing and keep up the good work!
Thanks mate. Hope to have something to show for the effort soon!
Hi all. Happy New Year!

I finally had some time to continue with the twin build (in between mince pies and sherry etc) and can report some solid progress. I had a major setback in the autumn when I tried to assemble the crankcase and the two crankshafts in that they didn't fit together! I thought the worst and assumed something was misaligned axially/radially and I'd have to start pretty much from scratch and make a new crankcase and shafts.

I spent two months procrastinating and avoiding the shed altogether + was very downbeat on the hobby overall. I finally 'got back on the bike' a couple of weeks ago and I'm back on track. It turned out that I somehow I got the crankpin on the front crankshaft mislocated by 0.3mm ( I know.. sloppy!). I think next engine I'll try some new techniques to up the accuracy somewhat. After making a new centre crank with the hole moved to the correct place, everything fitted together nicely. A bit of a 'rub' felt while rotating the assembled crankshafts by hand was taken care of with a few strokes of a needle file and the assembled crankcase rotated nice and smoothly.

Anyways, she runs on the front cylinder!! Video attached showing the first low compression, rich run to let everything settle in. All running well and I am relieved that all the timing is OK even though the stroke is 0.3mm longer than it was supposed to be. The gas seal on the centre bearing and the intake ports are fine too. The only issue encountered on the front cylinder is that I made the gudgeon pin a bit too short and it floated out of one side of the piston. Hope I haven't ruined the bore or damaged the conrod. I will check everything tomorrow and give it a run with a new pin.

On to the back cylinder tomorrow and hope to have the engine firing on both soon. This is getting exciting!

Some pics below for some of the final operations.

Thanks for looking,

All parts after ultrasonic bath clean ready for assembly.


Locating the hole for the ports.


Holes drilled with an endmill first to give a flat surface, after this a centre drill followed by the jobber drill to. These are drilled with the centre bearing in place but not the crankshaft.


After removing the centre bearing and deburring.


The holes in the centre crankshaft were marked out by scoring through the port with a sharp needle after rotating. The angle was measured using a rotary encoder with 4000 steps per revolution. The encoder is connected to the computer via an Arduino Uno. I have used this setup so many times I might make a semi-permanenent version to keep in the workshop.


Port holes drilled and burrs filed away.


Produced piston and contra blanks knowing full-well ill ruin a few later on and hope to get 2 good ones.


Making the NVA. Ive had a 100% success rate on these so far so not changing the formula now!.

Completed NVA


  • IMG_3121.jpeg
    234.5 KB · Views: 3
She lives!! :) :) :) :) :cool::cool::cool::cool::p:p

Very happy.

It took a while to get used to operating a twin. To get started, I made sure each cylinder ran on its own without the other one attached. After I knew there were no issues with either, I put them both on and soon got it fired up. THIS THING IS LOUD. I mean REALLY. I wore ear defenders on the test runs and hoped the neighbors wouldn't have a sense of humour failure.
One issue encountered when running high revs with the smaller 8x4 prop (at 14k RPM ish) was insufficient lubrication in the front housing plain bearing causing it to seize. I took the engine apart expecting the worst but was able to fix the issue by running the reamer back through to remove the galled aluminium. I modified the front crankshaft to have a hole through it like an FRV to distribute more oil to the bearing. No issues since then. A lucky escape.

I will get to making a spinner and anodising the cylinder muffs when I get some time.

Massive thanks to @edholly for suggesting this build, tons of advice and some amazing fag-packet sketches. This is basically two of Ed's Holly Buddy's stuck together :cool:

Chuffed to bits and learned a lot from this project. Now finally having cleared the workbench I can ponder the next project (thinking either an ML Midge or a 4-stroke glow engine 🤔 )

Thanks for looking,






Last edited:
Stunning, stunning work, Patrick. Love the sound they make when both cylinders on song. A bit more temperamental than say a Taplin Twin as - as you say - it is really 2 engines liked together with a common shaft. Would love to see the plans on Outerzone one day.

Latest posts