SHY_TOT_ENGINE_PREP - 1 - Bantam Tuning article written by Alan Hibbs in the 1970 s. Text in blue: my additions when retyping it. (A. Moulden) One of the first things that will become apparent when building and tuning from scratch is that the components will have to be assembled and then taken apart so many times that you ll be having nightmares about it. I ll take the engine section by section in something like the correct order. What you need for starters is a complete 3 speed BSA Bantam engine of either 125, 150 or 175 capacities. (D1, D3, D5 or D7) The major differences with these capacities is bore, stroke is common. If you are in the position to choose, a wide stud 125 motor should be used in preference to the narrow stud. The barrels for the 125 and 150 motors are the same casting with a different diameter bore but the 175 uses a different casting which will not fit onto the smaller motors due to the spigot at the cylinder base and holding down stud spacing being different. To clarify the last few lines the following table may help. Barrel Details Narrow stud 125 (D1) Wide Stud 125(D1) & 150(D3) 175 (D7 + D10) Bore 52 52 57 61.5 Holding down stud 52 55 59 centres Finning Small Normal Normal Carb. Mounting Stub Stub Flange Before you get at the engine with the spanners a good de Gunking is required and at the same time drain the gearbox oil. It s also advisable to make an engine stand so that the engine doesn t roll about all over the bench when you are working on it. This can be made from the bottom loop of and old frame, with a suitable cross piece to make it stand up. Right now for action. Strip the top half of the engine and remove the barrel. The work you do on this one piece is more important than any other. It is greatly responsible for power, revs, and reliability so don t skimp it. The standard port shapes and timings were designed to give a maximum in reliability and about 100 miles per gallon. After you have done your thing you will find those figures miles out. Exhaust and Transfer ports The exhaust and transfer port timings are controlled by their relative positions to the piston crown and in our case can be altered by cutting the ports.
SHY_TOT_ENGINE_PREP - 2 - With the piston at bottom dead centre (BDC) the base of the exhaust and transfer ports should be level with the top edge of the piston crown. Usually for this to be achieved a packer is required below the barrel to raise it to the required height. The ports then need raising and widening. The height of the port controls the timing but the cross section of it decides how much gas it will pass. Heights are not usually quoted, just angles of crank rotation which can be read off the scale below [chart has been omitted as PISTON1 program is a lot more accurate than chart and can cope with different stroke and rod length. Download PISTON1 from this site] (nowadays heights are quoted as it easier to compare engines) Note: when crankshaft has covered ¼ rev from TDC the piston has travelled more than ½ stroke down the bore. The point of opening for the exhaust port wants to be 80 to 85 before BDC and the width wants to be about 32mm with a decent radius in the corners. These radii help give the rings an easier time. The Transfer wants to be opening at about 60 to 64 Before BDC and be about 22mm wide again with radiused corners. The above statements are all well and good but don t give you much idea of how to arrive at the finished dimensions. The reshaping of the exhaust is done at the port window itself to allow the gas out at the correct time and inside the port to allow the best gas flow possible. The exit from the exhaust should blend into a 1 3/8 front pipe. To achieve this an adaptor ring should be screwed and locked to the existing stub. Barrel with Adaptor fitted. With the aid of an electric drill and a flexible drive working from the out side, open up the exit and smooth the dog leg in this port. You can also do a bit of work on the port window from this end with the drill and various rotary files and stones. To get to the edges and corners of the port window a variation of riffler files (diesinker files) will help. Reshaping the transfer ports is the trickiest job because without a dentist drill or something similar you can t get at them properly. Metal needs taking from both sidesof the port window and also from its top edge. The transfer passage needs opening out to allow more gas to pass. At the cylinder base the transfer wants opening out by about 3/32 on each of the three edges away from the bore. Its general shape
SHY_TOT_ENGINE_PREP - 3 - should be as sketch and as near as possible both windows and passages should be identical. This angle on first thoughts should Point the incoming gas more vertical So that less of the charge is lost through the exhaust port. But what is sometimes not so obvious is that the incoming charge cools the piston crown with a more vertical angle this cannot be achieved. (later theory says run streams flat across piston crown, when they collide they have to move up the piston is underneath them any charge lost down exhaust is pushed back by the exhaust system) The transfer ports should also feed into the upper cylinder in such a way as to not rush straight out of exhaust port, as in diagram below.
SHY_TOT_ENGINE_PREP - 4 - If you have used a 150 or 175 barrel it is obvious that the bore is too big and a liner will have to be fitted(the next bit also applies to 125 barrels) If you are struggling to cut the transfers due to lack of room to work try taking the metal out completely between port and bore then when you have finished cutting the ports a liner can be fitted which replaces the metal you have removed. Inlet Port The inlet port timing is controlled by its relative position to the piston skirt. This can be altered by cutting either the port or the piston skirt. If you are using a barrel of D1 or D3 origin the stub for the carb is neither of use or ornament. The O/D of this stub is smaller than the I/D of the carb you will be using which is about 1 1/16. This stub should be cut off as in sketch below. The fin above the stub has to be partly removed to allow access for brazing the new carb attachment. You now have a choice of doing 1 of three things. 1. You can braze on a flange to make your barrel similar to a D7 type. 2. You can braze on a new stub with the correct I/D and flange attached to give a pre-determined inlet tract length. 3. You can braze on a short stub and make a separate stub and flange which can then be connected to the barrel stub with rubber hose and jubilee clip.
SHY_TOT_ENGINE_PREP - 5 - The last method gives room for experiment with inlet tract length and also helps isolate carb from heat and vibration. When you have decided which method of carb attachment to use it should be brazed on as in diagram below. Work to all four edges of the old inlet port is required. The same rule as for other ports applies and that is timing controlled by port height and the amount of gas which can flow through it depends upon the cross section. The opening of the port wants to be about 75 to 80 before TDC. The best and possiblbly the safest way to go about this is to aim to cut port 5 less than the timing required then trim the piston skirt to give the finished timing. The top edge of the port can be cut till it is 1mm below the lower edges of the exhaust and transfer ports(racing use only). The lower edge can then be cut to give you the required timing and width increased each way to finish upat about 32mm radius top corners to help rings a bit bottom ones are not important as rings don t come that low. All this can be done from the outside.
SHY_TOT_ENGINE_PREP - 6 - D1 barrel with log inlet brazed on D1 barrel with brazed on flange Finned inlet spacer Some people finish the ports to a mirror finish which takes hours to achieve and may or may not be good. Really it is the icing on the cake and there are still quite a few things more important to do. Anyway shape is more important than finish.
SHY_TOT_ENGINE_PREP - 7 - Anyway that about covers port shaping. Its taken me a couple of hours to write but will take you a hell of a lot longer to achieve. It ll make your fingers sore your patience wilt and your temper rise. My suggestion is to do a bit at a time ½ a port a night for instance because if you try and do it in a couple of goes you will either make a cock of it or get that frustrated that you will sling it out of the garage and say sod it (a lot of you know exactly what I mean). To top it off a squish band head of a GT100 type (13:1 CR) should be used. Compression ratio depends on clearance between piston crown and squish band. This can be measured by assembling the top half og engine and placing a small blob of plasticene on the piston crown. Turn the engine over a rev then take engine apart again and measure how thick the plasticene has been squashed to. The required clearance is 25 30 thou (0.025 / 0.030 ). (A wipe of oil on head and piston allows plasicene to be peeled of and easily measured) GT 100 Head BTW head currently available and in use on most air-cooled racing Bantams You should use a racing type piston and the last thing to do when you are happy with the ports etc. is to have the bore (or liner) machined and honed to give about 4 thou (0.004 ) clearance at the piston skirt.
SHY_TOT_ENGINE_PREP - 8 -