Building up a bantam engine

Morning all,

I have posted on here before about my project bantam, a B175 based, road legal cafe racer (cos I can't afford a goldie! )
I am about to start the engine rebuild in earnest, and I would like some guidance on getting the engine to as high a standard as possible.
The engine spec will be as follows:

D14 engine
B175 crank with T&G autos conrod kit, to be fitted by them
Suzuki piston to bring up to 185cc, again from T&G
Rex caunt stiffer clutch springs and iwis primary drive.
Rex Caunt C3 grade bearings

Current state of play is the above engine stripped to its component parts and laid out neatly. I have cleared a large area in the loft, and set up a bench to work on. This gives a clean, tidy and well lit work area, so I think I've at least got that bit sorted!

As I say, the bike is intended for road use. Therefore I don't want the engine revving like a turbine and needing a rebuild every 5 minutes. What I am asking you fine gents for is guidance on best practice, and any little tips to help boost performance and/or reliability. I want to get everything between the crank cases to as high a standard as possible, so I can (hopefully) put it all together once and never have to look at it again. If you are interested, I'll post my progress.
Many thanks
Phill

OK, first in a long line of possibly daft questions.

Here should hopefully be a link to a picture of my crank halves:



The mainshafts have some very light rust on them. This has left grey marks, and what looks like very, very light pitting if you look closely. You can't feel any difference though. Is this likely to be a problem? I am particularly concerned about the diameter where the seal is formed. Might it clear up with a bit of scotchbrite?



Other than that, how far should I go with preparing the two halves? I'm currently thinking of deburring and lightly polishing the flywheels, making sure the threads are OK and making the whole thing spotlessly clean. Does that sound about right?
Thanks again

Today I had a closer look at the seal diameter, and realised it is part of the flywheel, not the shaft. Told you I'd ask some daft questions! Anyway, I gave the flywheels a going over with fine emery to derust them, clean the crud off with alcohol and coated them in oil, ready for posting-I hope! Might try checking the runout between shafts and flywheels.

Next bit I want to look at is the barrel (that needs sending off for reboring with the crank, so I want those ready first). At the moment, I don't want to change the timings, just smooth out the ports. So, two more questions:

Inlet port: I am using a D14 barrel. The top of the inlet port on this has a funny bit in the middle. Is it better to leave this or remove it? Out of interest, what does it do?

Exhaust: There is a bit of a kink in the right handside of the tract (looking from the front), do I want it straighter or should I leave it, at least for now?

Ta again
Phill

Mr. Greenbat!
Do not remove the tonsil on the inlet as this stops the ends of the rings dropping into the inlet and smashing the rings on the standard pistons. If you use non standard pistons you may feel different about removing it but the difference in your power is likely to be minimal.
Smoothing the exhaust port on a standard engine is hardly worth the effort and you will not notice any improvements. Try lifting the exhaust port a couple of millimetres and leave it at that, with a slightly increased compression you will make the motor a bit more lively and still suited to the road.
Spend a lot of time getting the carb to work well, and fit some electronic ignition to make it all reliable. Then go out and wear the bike out!!

Thanks Alan, I'll keep it then!
Currently all I am doing is smoothing out any casting flaws in the ports, and matching the transfers to the cases. I won't polish the ports after a lesson at college in fluid dynamics. Basically, when fluid flows in a pipe (or port) without turbulence, it forms layers. The middle layer travels fastest, and as you go out from the centre they get slower until the outer layer is stationary. This effectively narrows the pipe. So, you leave a little bit of roughness to cause a small amount of turbulence round the walls of the pipe, to stop the fluid sticking to the walls.

I'm also thinking about the crankshaft bearing lubrication. I have a copy of George Todds article "boost up your bantam" where he suggests fitting an oil pump, and enlargening the oil holes, particularly on the non drive side. Does anyone have experience of this? I'm thinking of putting the necessary pipe fittings in the cases whilst I have them apart, but leave them blanked off until I can figure out the oil pump.

Hi Greenbat,
The oil pump is hardly worth the effort and will not really make a difference. Enlarge the holes slightly and make sure that they are clean and clear of swarf. It will be as good as it gets!
I agree with the surface roughness as you say.
Regards,
Alan

Greenbat,

Could you point me in the right direction for where I might find a copy of the "Boost your bantam" article?

Thanks,

Andy

1bantamcloser2divorce wrote:

Greenbat,

Could you point me in the right direction for where I might find a copy of the "Boost your bantam" article?

Thanks,

Andy

You have a PM.

Still working on matching the transfer ports at the crankcase, I'm getting there!

Thanks!

Greenbat wrote:

1bantamcloser2divorce wrote:

Greenbat,

Could you point me in the right direction for where I might find a copy of the "Boost your bantam" article?

Thanks,

Andy

You have a PM.

Still working on matching the transfer ports at the crankcase, I'm getting there!

Another update.

I have spent most of the last week smoothing out the ports in the barrel and matching the transfer ports on the barrel/crankcase join. I'm now looking at the crank alignment in relation to the barrel.
I am going by the haynes book on 2 stroke tuning by Alexander Bell (it's a follow up to his book on telephone tuning), which says that you need to check that the face the cylinder bolts to is parallel to the crank, I seem to remember within about 0,02mm, or a shade under 0.001"
To do this I have fitted the main bearings in place (on the drive side, just the big one) and bolted up the cases fully. I am using a surface plate, a 0.001"DTI and a clock stand.
I put the cylinder face on the surface plate, and position the DTI to read the height of the bearing inner race, moving it to find the lowest spot. I then measure the other side in the same way. This method gives me a 4 thou difference.
How crucial is it to get this face parallel? Is this a check everyone does, or am I being too fussy?

Hi Greenbat,
You ae being too fussy! wonderful to get it att spot on but there is enough slop everywhere fo the crank to work just fine. The conrod only see half of the error as it is in the centre of everything.
When the bearings are fitted the crank should spin freely in the cases, when the barrel is fitted for the first time leave off the rings and the crank should still spin freely. Otherwise look carefully for the real problem.
Cheers,
Alan

Greenbat - you are the man.

I also am building a road legal, (125), as I was deeply offended when I heard a local ACE Cafe guru being derogitory towards the old mist green, especially having just done a 'straight' restoration for my 12 yr old daughter. (She learned more about mechanical principles than most senior school kids doing it also.)

So, a 'Cafe' which can be 'clubbed' a the yearly oldtimers meet is the aim.

Cheers mate, and good luck... PS, I think I'm going to run a set of light girders on it as well.

South Oz Dave wrote:

Greenbat - you are the man.

I also am building a road legal, (125), as I was deeply offended when I heard a local ACE Cafe guru being derogitory towards the old mist green, especially having just done a 'straight' restoration for my 12 yr old daughter. (She learned more about mechanical principles than most senior school kids doing it also.)

So, a 'Cafe' which can be 'clubbed' a the yearly oldtimers meet is the aim.

Cheers mate, and good luck... PS, I think I'm going to run a set of light girders on it as well.

Good man, though girders on a cafe racer sounds unusual.
A few pics of mine here: http://www.flickr.com/photos/35741071@N03/sets/72157614315853615/
Give me a shout if you want help

I have three critical areas which are concerning me.

Firstly, I want to 'square the stroke', so I don't know which is easier, to either have a completely new set of flywheels made, or to have a rather nice original set pulled apart, welded up at the 'connecting rod big end' and then re machined about 2mm down towards the center thus squaring up the stroke - then having a couple of mm machinewd of the edge of the flywheel to compensate. I have had some very scary quote on both counts (about 5k Australian).

Secondly, aquiring the main and lay shafts for a 3 speed close ratio gear group. Bearing in mind that the traditional close ratio recommendations are for the track, not for road performance, (any thoughts here will be gratefully recieved).

Finally, decent barrels seem to be rare as close ratio lay and main shafts down here in the antipodes, so I may have to start casting around for an apprilia barrel for now, which will spoil the opportunity to play once yearly with the local vintage boys, but I suppose it at least will get me on the road.

Decisions decisions, anyway, any advice or opportunities will be greatfully recieved, so please let me know as I am stuck here in the rural backblocks and need all the back up I can get.

By the way, the tank, seat set up on your machine looks remarkably 'balanced', I love it. Which tank are you using?

South Oz Dave wrote:

By the way, the tank, seat set up on your machine looks remarkably 'balanced', I love it. Which tank are you using?

Standard D14 tank. They chromed the whole thing, then painted most of it. Strip it carefully and you'll find reasonable chrome, though not as good quality as the bits you can normally see. The seat I built myself.
Can't help on the other things, I'm using a D14 engine, B175 crank and a suzuki piston to get a 64mm overbore (187cc I think).

Now then, thinking about the head.
I've noticed mention of BTW barrels and heads. Can I machine a BTW head to fit a standard barrel? What does the casting cost, roughly?
Cheers again

Been a bit of a lull on this, as there hasn't been much I can do. The crank and barrel are due back early in the new year, and it has been very cold in the loft. I have given all the internals I still have here a good clean and oil ready to go back in, and filed out the wear in the clutch plates and basket.

I do have a pretty important question, which I'd like to try and solve before I put it back together. On my working D14, it jumps out of top gear a lot. Have a look on page B12, figure B12 here
In the bantam gearbox, the second gear on the mainshaft is on a toothed shaft, which it slides up and down (marked as 2nd gear). It has tabs on each side, which engage in the top and To engage top gear, the second gear slides to engage with the top gear to give a 1:1 ratio through the box. The tabs do look a bit battered, but I don't know if that is the cause of the gear jumping out or a consequence? Does anyone know? If it is the problem, I will replace it, but if it isn't there probably isn't much point.

Try some Motorcycle Mechanics articles. I've got a folder full of the articles I bought off Ebay with anything about Bantam tuning. I've got some snippets of 'How to build, tune and race a Bantam' By George Todd.

Tim

Bits arrived from T&G today! They look beautiful. I've fitted the main bearings (new C3 grade), a quick trial fit of the crank and everything is silky smooth. MMMMMMmmmm

I'm trying to think how best to measure the end float in the crank, so I can shim it up correctly. Current thinking is:

Bolt up the cases with crank fitted

wrap some tape around the alternator side mainshaft

Grip the tape wrapped shaft in my lathe chuck

Check the end play in the lathe spindle (!)

Using a DTI on a magnetic base, check the movement of the cases on the crank.

Fit shims as necessary, and repeat.

Should I try and put an even thickness of shims on each side, or doesn't it matter? (Sounds like an song from Oliver Twist. Shim-Shimery, shim-shimery, shim-shim-sheroo)

Finally, an idle thought on reed valves. Is it possible to fit reed valves to an unmodified D14 barrel, and is it worthwhile? I believe they give a better spread of power, which would be very nice on a road bike.

I figured out a better way for measuring end float. The DTI (clock) is bolted directly to the cases.
I put the sprocket on to give me a decent surface to measure from, and still allow me to tap both ends of the crank (it is a little tight in the bearings).
NOTE: My clock is one where the stylus rotates, rather than pushing in and out. These must be set up with the stylus as close to 90 degrees to the body of the clock as possible, and the clock as parallel to the crank as possible. If not, the reading will be inaccurate. You can check by slotting a feeler under the stylus to see what reading you get.



I get about 3.5 thou, which is a bit tight but I reckon it'll bed in. I emailed Rex Caunt last night, he recommends between 3 and 5 thou. I thoroughly recommend Rec Caunt Racing for anyone considering bantam ownership.

The cases now have all the seals and bearings fitted.



Another warning: On the D14 engine, the left hand gearbox seal (behind the sprocket) is the same as the crankcase seals. I discovered this after I fitted the gearbox seal Luckily I had a decent crank seal spare! When I first rebuilt this engine (using bits I found in a big box) I did fit new seals, but thought I'd better replace them again in case I damaged them on removal. I put the pre-used seal in the left hand case. I may have to strip the engine down to adjust the end play shims after the engine has ran in, which will not require the right hand case stripping down. If the seal does go I can sort it a bit easier.

Next job is put the gearbox back in, then bolt it all up properly

Seems a nice project you've got there Phil. I've just bought a D1 engine with ported head, piston, racing manual etc from an ex-racer Dave Brasier. It's all original. I'm going to change the internals alot though. RD250 flywheels, 86 or 87' RM125 piston since it fits the Bantam's standard stroke and TZ250 big end.
I'll update more once it and the parts has arrived!

Tim

Engine is finally together. I had a bit more fiddle with the end play, got it on 5 thou now. George Todd said that is right so it'll do me! Engine is now in the bike, just needs a piston and the timing setting up. I put some oil in to see if it leaks.

Looks really nice Phil! I saw a Bantam exactly like that at a show once. Even with the checker board down the middle of the tank! I think it was a D14/4. Pretty similar looking. Good luck for the future!

Tim

Timbantamracer wrote:

Looks really nice Phil! I saw a Bantam exactly like that at a show once. Even with the checker board down the middle of the tank! I think it was a D14/4. Pretty similar looking. Good luck for the future!

Tim

Wasn't Ashover festival of light was it?



The 400/4 is my main bike. Dad trailered the project there, and I won a rosette for best in class

Question I forgot to ask: What oil ratio in the petrol should I use for running in, and what procedure should I use for running in?

No, it was a red one. I have no photos of it unfortunately but I managed to find one in my BSA Bantam Bible so I'll scan it in and put it on.

Tim