 LEO 25 EP ABCBy Malc Pinnock
(RCM&E January 2001)
Read the Instructions
On overhearing a couple of customers at a Show, I have looked into the above headline and found this is a big problem today. The modern A.B.C.-A.A.C. or hard plated soft liner piston set up raises a lot of questions and problems for some modellers. So, before we look at the Leo 25 I will try to explain what’s involved.
Throughout the trade, lots of these engines are returned to the importers or vendors with the comments “this engine won’t turn over”, “this graunches or squeaks at the top of the stroke” or engines with bent or broken conrods, big end damage or liner damage. What we have purchased is a precision instrument. Even “budget engines” are made to very close tolerances.
A.B.C. liners etc have a tapered bore and most pistons are either tapered or barrelled in shape. Only about the top 2mm of the piston is the sealing point at top dead centre and most pistons are either cut with a single or double oil groove. Hard plated brass or alloy liners with a high silicon alloy piston, usually 16%+ silicon alloy, can be run at much closer tolerances, due to a more equal expansion rate than steel liner, cast iron piston or steel liner alloy piston with iron ring set ups.
They are capable of producing more power due to closer fits and less friction due to the bore taper and nip where it matters fit. The cooling is also better, offering quicker, easier re-starts by hand when hot.
I am still a great believer in the properties of castor oil, especially for running in. Correct running in will give your engine a longer life, so follow the instructions. They are written by experts after hours and hours of tests. My way is by firmly bolting the engine in my R/C test bench (yes, 25 year old radios still have their uses). My usual fuel for running in is Flair Yellow Spot 20% castor oil, 75% methanol and 5% nitro using a prop of the correct diameter with a fine pitch (no more than 6” pitch so as not to overload the engine on the ground.)
Starting by hand to get a feel for the engine and rich running periods of 5 minutes at various throttle settings, allowing the engine to cool between runs (whilst sitting in the car holding the tranny and reading R.C.M.E.). After about ½ hour, start to lean the engine out on the main needle. If the engine starts to harden or slow, richen the mixture again. I have known some engines unable to hold full throttle after some 1½ hours but after running in, they are still healthy after 5 years good use. Be patient, it will reward you long term.
When the engine is capable of holding full throttle in its leaned mixture state, adjust the idle mixture for slow running. No 1 rule, careful running is the most important part of your new engine’s life. Also, allow your engine to warm up properly before use every time. Not the usual wind it up flat out and fly.
“Running in” on high nitro or synthetic fuels is not a good idea as synthetic oil burns. Castor will protect and allow the high spots to wear off. OK when your engine’s fully run in, then go to a synthetic mix but one containing some castor high nitro 16%+ is for very small or contest motors and is usually a waste of money for the average modeller with our cooking motors.
A few guide points for running in
1. Read the instructions... before you have a problem, not afterwards when it's often too late.
2. Make a point of knowing what’s in the fuel e.g. 20% castor, 75% methanol, 5% nitro. If it doesn’t state it on the can, don’t buy it.
3. At least 20% oil for running in.
4. Prime carefully finger over carb, turning engine over to suck fuel through. Connect glow clip, feel for “engine bite”, then a few smart hand flicks is usually enough on a squeaky new A.B.C. engine.
5. Don’t wind a new dry engine up with a leccy boot, this will cause excessive liner piston wear or worse. I have seen a con rod big end welded to the crank pin!
6. Excessive fuel on first time starts can cause “hydraulic locks”. With modern high torque leccy boots, this can easily destroy your engine. Snapped or bent conrods are the usual result. If you do use a starter and feel resistance, don’t press harder. Find out what’s causing it or get a feel for your engine. Use your finger.
7. Use a good after run oil. It protects against nitro and other chemical corrosion. If possible, store your model nose up so any castor or unburnt fuel drains away from bearings.
8. If you have problems or broken or damaged your engine, the girlie on the other end of your phone has probably forgotten more about engines than most of us know. Many of them, like Jenny Landels from Just Engines, have been dealing with engines for many years so treat them with the respect they deserve.
I know we live life flat out with no time to spare and expect things to run flat out straight from the box with some Clubs not permitting running in on the field but give these engines some sympathy from new, then they will reward you with a long lasting trouble free life.
Onward
The 25 size engine is just right for the one piece toy kept in the car boot, for jumping over the hedge for a quick play on the way home from work on those warm Summer evenings.
Leo 25’s fit the bill perfectly. Easy to tune and good starters. Also proving popular in Club 2000 pylon racing which is what I bought mine for (still practising this year). The engine shown stripped in the test is my own, with 10+ hours running under its belt, plus a few crash tests – one at 80+ mph. No damage, just two bent silencer bolts and skid marks on the silencer and two broken toys. Well built these Leo’s!
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 Crankcase and BackplateDie cast from alloy then heat treated, the case has five ull length webs running axially between the front bearing housing and the main bearing housing with another between the cylinder and carb stub and front bearing, making for a very stiff strong crankcase.
Internally the gas passages and exhaust port have been turned by approx 20o old O.S. F.S.R. style. The name Leo and size 25 are on a small bonded on polished plate on the opposite side from the exhaust, with the words and size in black. Moving forward, the carb stub is bored 13mm to accept the Leo carb which is retained by a black chemically finished pinch bolt. All machining, both internally and externally, is excellent showing no signs of pickup.
Looking down the crankshaft passage there is a small slot running forward from the intake which has a full circle groove just behind the front bearing which aids keeping the engine dry by sucking back excess fuel mixture when the engine is running. Still inside the engine, there are two very smooth good quality bearings with the front bearing being of the externally sealed type.
The crankcase has a very large exhaust of which the front portion has the internal transfer port passing through. It has been drilled for the 2 x M3 silencer mounting through bolts. Looking at the top of the cylinder which has been drilled and tapped for 6 x M2.5 black finished Allen bolts, there is also a small pressed in pin for correct liner port alignment. The back plate which again is from die cast alloy with an internally angled flat which directs gas to the rear transfer port. It is sealed by a paper gasket and held down by 4 x M2.5 chromed Allen bolts.
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CrankshaftMachined in one piece from steel, hardened and finished by grinding, with a fully counter balanced flywheel, the big end pin is of 5mm diameter with a bearing surface of 5.40mm. The mainshaft has an O.D. of 12mm stepping down to 9mm for the front bearing with two machined flats cut in for the prop driver which is machined from alloy barstock with a dirt shield for the front bearing.
There is a small steel shim of 0.95mm between the bearing and prop driver. Again, the shaft steps down to 6.35mm for the prop shaft which is threaded for a U.N.F. ¼ x 28 nut, with a prop washer made from steel and chrome plated. Rounding up the crankshaft, the gas passage is 8.50mm with an induction port measuring 12.65mm x 8.50mm.
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 Liner, Piston & Con Rod AssemblyOf the schnneurle port type, the liner is of brass and fully plated with hard chrome, then finished by grinding. It has a good sliding fit into the crankcase with a flat on top for clamping down and a small slot for location with the pressed in pin in the crankcase. The liner has a taper of 0.06mm top to bottom with a single unbridged exhaust port measuring 11.50mm x 4.80mm, with two transfer ports, one either side of the exhaust each measuring 8.20mm x 4.45mm with a single boost port at the rear opposite the exhaust of some 8.65 x 7.50mm. When fitted in the crankcase, the liner is turned by some 20o .
The piston is die cast from silicon alloy and lapped to fit the liner. It is slightly barrelled with an oil retention groove 1.35mm from the flat crown and has a very tight fit at the top of its stroke even after 10 hours running which will make for a very long life after careful running in. The piston skirt has been machined internally to a depth of 5.95mm with a wall thickness of 1.05mm. There are two cast in webs to support the fully floating gudgeon pin, which is made from steel and hollowed with a P.T.F.E. pad at each end for sealing. The con rod is from high grade alloy and bushed with bronze at both ends with two oil holes for the big end.
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