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 Anyone convert these port specs to degrees? Please.

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Jimmie



Number of posts : 148
Registration date : 2011-07-25

PostSubject: Anyone convert these port specs to degrees? Please.   Fri Oct 27, 2017 8:01 am

I have obtained some Bantam engine bits including a barrel which has been partly tuned, ie, seems to be a work in progress which has not been completed. The barrel has stud spacing of 59mm and a pressed in liner with bore of around 57.35mm. The porting would appear to be of a high standard just needing the final finish.

As there is (as yet) no c/shaft I am at a loss for c/rod length to calculate the port timing. Anyone out there in Bantamland make sense of the following figures? All measurements are approximate and taken from the liner top edge.

Inlet port -    to top of port        - 57mm
                 to bottom of port     - 77mm

exhaust port - to top of port       - 28mm
                  to bottom of port    - 53mm

transfer port - to top of port       - 38mm
                    bottom of port      - 52mm

boost ports x 3 -   to top of port - 40mm
                     to bottom of port - 48mm

All help appreciated. Many thanks.  Jim

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Trevor Amos



Number of posts : 870
Registration date : 2010-08-13

PostSubject: Re: Anyone convert these port specs to degrees? Please.   Fri Oct 27, 2017 10:35 pm

Hello Jimmie,
You have certainly presented us with a conundrum here!
You don`t mention a stroke value is this because you have no information about that? As the data stands, timings are impossible to extrapolate for there are too many unknowns and the dimensions you list are themselves inconclusive. It is normal to find BDC and have the transfer port lower edge and the piston crown edge coincide with that point, traditionally the exhaust port lower edge is also at bdc. However, there seems to be no attempt on the designer`s part to establish this judging from the dimensions you provide. If we had a value for the stroke we could assume a con-rod length of twice that and come up with reasonably accurate timings. The big worry with these dimensions is the relative position of the transfer opening, being only 10mm from the exhaust port opening (blowdown) and so are 14mm high. That may be ok for a 58x125 set up where the port open duration is 130*, but for a shorter stroke and con-rod the duration becomes way off the mark. At the moment a 54 stroke would be my best guess?
If we assume that the stroke was such that tdc left say, 1mm for clearance at the cylinder head then all of the other dimension are subsequently affected. Without some details of the piston lengths inlet timings are also impossible to determine, do you have a piston?
Without some sort of datum we can`t go anywhere with the data, is there any paper work, sketches or conversational information that can help? I`m really struggling to understand the thinking behind this barrel`s porting arrangement!

Trevor
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john bass

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Localisation : Bensberg, Germany
Registration date : 2006-12-06

PostSubject: An approximation equation ....   Sat Oct 28, 2017 4:38 am

There's a neat** (approximation) equation for finding piston displacement (call it "S"...) from TDC which uses crank-angle from TDC as zero -- say "ä° = TDC ; con-rod length, "l" and "r"as crank throw which looks awful but does give numbers for S versus a° which I think is what you are after... but if you don't have postive values of r & l you'll have to assume dimensions for them.

It is:-

S = r{1 + L/r - cos.a° - square root of [(l/r)² - sin²a°]} . Sorry! I couldn't find the square root sign so I hope this understandable....

**neat when you can write it out by pen.

Cheers!

JayBee.
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Jimmie



Number of posts : 148
Registration date : 2011-07-25

PostSubject: Re: Anyone convert these port specs to degrees? Please.   Sat Oct 28, 2017 5:37 am

Thanks JayBee and Trevor for your input. Much appreciated. While I have being promised more bits and pieces nothing has appeared as yet. So I now need to wait for the complete c/shaft assembly, suitable piston  plus  c/cases with bearings and have the barrel bored to size before I am any wiser on port timing?

Give me a 4 stroke anytime!

Jim
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Trevor Amos



Number of posts : 870
Registration date : 2010-08-13

PostSubject: Re: Anyone convert these port specs to degrees? Please.   Sun Oct 29, 2017 12:28 am

Jimmie,
Welcome to the world of the mechanically simple two stroke engine with its few moving components. Trouble is, it looks simple and is anything but, and therein lies the real challenge, working with something that you just can`t see, differential pressures and resonant wave action. To a lot of us, the stuff of nightmares!

Cheers for now, Trevor

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Jimmie



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Registration date : 2011-07-25

PostSubject: Re: Anyone convert these port specs to degrees? Please.   Sun Oct 29, 2017 4:06 am

Trevor

If it were only a simple nightmare!


Jim
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Jimmie



Number of posts : 148
Registration date : 2011-07-25

PostSubject: Re: Anyone convert these port specs to degrees? Please.   Tue Nov 14, 2017 6:35 am

Update
I got some more parts – crank with rod, c/cases and Lumenition ignition system, some other bits and pieces but no piston.
Crank – not standard Bantam – consists of two 4” solid flywheels pressed together with conrod and main shafts. The timing shaft is drilled and tapped at the end. The drive shaft is of the Bantam type.  The outside of the f/wheel discs show marks of previous use although I can’t detect any b/e play.
The conrod, according to a box containing an identical (NOS) one, is Yamaha RD  400 (1A1).
According to Pro-X site the c/c of this rod of is 115mm.
http://www.pro-x.com/prox-connecting-rod-dimensions-brand/prox-connecting-rod-dimensions-yamaha/
After fitting the assembly in the c/cases and a lot of faffing around the stroke measured in the region of 48.60, so with a bore of 57.35, capacity is around the 125 mark.
With no piston I will assume the distance from piston crown to top of barrel to be 1mm (thanks Trevor). The Lambretta Port Timing calculator provided the following figures,
Ex duration    183 degrees
Trans             134 degrees
Inlet              140 degrees.
Do they make sense?
Someone has certainly put a lot of effort, expense and thought into this engine.
(I can hear the ‘real’ engineers in the background having a good laugh, and well they might at my befuddlement. Now I shall retire to a darkened room and have a long lie down before I read any reply. Please be kind to a poor, uneducated eejit who would now prefer a 4 stroke even more!)
Jim

Should have stated that the barrel has been shortened in height - 80mm against 89 on another barrel I had lying around.
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ptibbitt125

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PostSubject: Re: Anyone convert these port specs to degrees? Please.   Wed Nov 15, 2017 1:22 am

Hi Jimmie

If the timing side main-shaft is drilled and tapped, I suspect it may be for coil ignition. Is there a spigot upon which the contact breaker cam could be located?

If its fitted with an RD400 rod , the bore and stroke will probably be 56.00 x 50.60 mm. It probably originated from the South West Centre, of the BRC.

But I haven't a clue about the port timings......
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Trevor Amos



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PostSubject: Re: Anyone convert these port specs to degrees? Please.   Thu Nov 16, 2017 12:58 am

Discovering that the crank stroke is a mere 48.6mm, makes the barrel dimensions all the more confusing! As you sure that all of these disparate parts are actually meant to go together?
A stroke of 48.6mm for a single cylinder of 125cc makes no sense, be it a road or competition engine and by pairing that stroke with the barrel dimensions taken from your previous post make the logic behind the design ever more puzzling.
For example, if we take the bottom of the transfer port as bdc, 53mm from the barrel top, the full stroke travel will leave the piston short of the top by, 53-48.6=4.4 mm? Even assuming that there is a clearance of 1mm at tdc doesn`t really help very much.
The balance of port timings are also askew, the exhaust is on the money for a race engine, the transfers are too extreme for a Bantam at 134*, with the inlet being more suited to a road bike. Most worrying is the lack of blowdown for what will, potentially, be a high revving engine with a quite large stroke to bore ratio. At peak rpm there will be only a limited time frame to clear the cylinder of exhaust gas. If the intention was a roadster specification then the high exhaust port will not be helpful for a user friendly engine.
Sorry if all of this is distressing news but you have, what you have, and I come back to the question, were all of these parts ever intended to be a homogeneous whole?
Please don`t think that anyone is having a laugh at your expense Jimmie, we have all made many `howlers` in the past, including those of us engineers who should know better. It was by doing so that convinced me to make scale and fully dimensioned drawings of everything I design, it absorbs a lot of time and effort but the end is always justified by the means! It is far easier and cheaper to rub out a line on paper, or computer screen, than to have to remake a component.  In the case of your engine, a very basic line drawing would have revealed all possible dimensional anomalies before any work on the conversion from standard spec. took place. But then I guess that is easy to say with the benefit of hindsight.

Trevor
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Jimmie



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Registration date : 2011-07-25

PostSubject: Re: Anyone convert these port specs to degrees? Please.   Thu Nov 16, 2017 8:36 am

Many thanks to all for the recent insights on my query. Much progress has been made tonight as I found a piston with the same pin size as rod s/e. Assembled the lot together and, you were correct Peter, it is 50mm.  Your comment re South West Centre - in the old days did different centres follow  different theories on engine development?

I tried to post photos (with total failure) showing the additional bits listed below, what I assume is all I am due.

They are:
c/cases which are reinforced around the mouth
one GT head, sadly too damaged to be of any practical use
one of Ned’s c/head in good condition
two large dia carbs – on Mikuni, other Amal Concentric
selection of rear wheel ‘two part’ sprockets
two sets of 3 speed gears looking in excellent condition – sadly not c/r I think
Alui castings – I guess they are potential c/case packers
A collection of homemade (?) steel clutch plates
With the electronic ignition is what looks like a tab washer – possibly the ‘trigger’. component  for the end of the timing shaft?

Many thanks to all for your support which is much appreciate.  Please forgive my wasting of you time which was entirely due to my excessive stupidity.
It is beginning to look like an engine built to superstock spec might make more sense. Or, a four stroke Bantam!
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john bass

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PostSubject: Four Stroke Bantam   Thu Nov 16, 2017 3:13 pm

4 Stroke Bantam -- what a good idea, Jimmie  -- make it diesel as well.... i know a little bit about them 4 strokers....

I seem to have dreamt you asking something like, "what do the REAL (Auto?) engineer's think..."

My immediate reaction was that there are very few of those REAL engineers.... Sir Henry Riccardo were one and he made a mistake with the1927 Triumph 500 called "Model TT" where the inlet cam wore out quickly because the cam-follower was a pivoted lever thta did not "Trail as the exhaust cam follower did  
    But that is the important bit -- "He who made no mistakes made nought."
Well remembered is a cartoon on the Deutz Diesel Canada's cantine wall slagging-off its engineers. A scuffy, long haired, unshaved face with cigarrete dripping its ash say, "Lest wikk I cudden spill Ingeneer -- INow I are one.!"

Forgive me if I am on the wrong site ... but this was a favourite grouse of mine during my active functioning as a professional Automobile engineer. I argued that the big Auto companies boasted of their thousands of Engineers -- as a suggestion of their great amount of Intelligensia when these  unfortunate blokes were employed more as Technical Clerks than Enginners and a Component Test & Development engineer could spend 20 years working on one component and never -- no never -- ever  being allowed to interfere with the engineering of the whole bit and Tuning of same.....

As with Westminster's hounding of sexual harrassment criminals in among their politicians I am expecring to be arrested -- any time soon for murder and ABH  -- for having spouted how wonderful diesel engines WERE compared with petrol engines. That,  across the United States & Canada in the years 1981 to 1983 telling how wonderful Diesels were which must have contributed to thousands of deaths and disabilities attributable to soot-clogged lungs.
Well, that#s what the Media says...

Hmmm...!  maybe I'll delete this and go back to sleep.

Cheers!

John Boy -- coughing a bit, probably poetic justice.  

  .
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Trevor Amos



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PostSubject: Re: Anyone convert these port specs to degrees? Please.   Thu Nov 16, 2017 11:21 pm

Jimmie,
Why not try a Tiger Cub engine in a Bantam frame, half way house, a foot in both camps, or has that already been done?

Trevor
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Jimmie



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PostSubject: Re: Anyone convert these port specs to degrees? Please.   Fri Nov 17, 2017 12:44 am

What! A poorly designed 4 stroke in an excellent frame just an Triumph BSA did! Is my memory correct in thinking that BSA Regal got SeymourPowell Design (?) to develop a 4 stroke successor to the Bantam in the 80's? Seem to remember a TV prog about it.
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