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 Building up a bantam engine

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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Building up a bantam engine   Thu Nov 19, 2009 8:53 pm

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! Laughing )
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
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Fri Nov 20, 2009 2:16 am

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
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Sat Nov 21, 2009 6:33 am

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
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alan
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Number of posts: 444
Age: 60
Localisation: Mexborough
Registration date: 2006-12-01

PostSubject: Re: Building up a bantam engine   Sun Nov 22, 2009 9:23 pm

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!!
Laughing
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Sun Nov 22, 2009 10:02 pm

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.
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alan
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Age: 60
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Registration date: 2006-12-01

PostSubject: Re: Building up a bantam engine   Sun Nov 22, 2009 10:58 pm

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
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1bantamcloser2divorce



Number of posts: 36
Localisation: Vancouver Island, CANADA
Registration date: 2009-10-06

PostSubject: Re: Building up a bantam engine   Mon Nov 23, 2009 4:04 pm

Greenbat,

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

Thanks,

Andy
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Mon Nov 23, 2009 7:39 pm

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!
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1bantamcloser2divorce



Number of posts: 36
Localisation: Vancouver Island, CANADA
Registration date: 2009-10-06

PostSubject: Re: Building up a bantam engine   Tue Nov 24, 2009 2:44 am

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!
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Sun Nov 29, 2009 8:52 am

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?
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alan
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Number of posts: 444
Age: 60
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Registration date: 2006-12-01

PostSubject: Re: Building up a bantam engine   Sun Nov 29, 2009 7:15 pm

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
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South Oz Dave



Number of posts: 9
Registration date: 2009-10-19

PostSubject: Kindred Soul   Thu Dec 10, 2009 7:12 pm

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.
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Sat Dec 12, 2009 9:19 am

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
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South Oz Dave



Number of posts: 9
Registration date: 2009-10-19

PostSubject: Re: Building up a bantam engine   Sun Dec 13, 2009 12:17 am

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?
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Greenbat



Number of posts: 47
Registration date: 2008-03-11

PostSubject: Re: Building up a bantam engine   Tue Dec 15, 2009 4:38 am

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).
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