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Flywheel Plates
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chickensoup
Bolton, Greater Manchester
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December 20, 2019 - 11:37 pm
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Hi Cocorico,

Just out of curiosity, why do the D1 and D3 cranks have these plates, are they for extra compression ?

The reason I ask is, I have a future engine restoration, a Lucas 125 engine. I have just purchased a lucas crank with a good big end, but one of the plates is missing. Would the engine run well without the plates.

Nick

1950 BSA D1 Plunger   242 UYW

1966 BSA D7  LBF 62 D

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SpacedMarine
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December 21, 2019 - 12:23 am
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chickensoup said
Hi Cocorico,

Just out of curiosity, why do the D1 and D3 cranks have these plates, are they for extra compression ?

The reason I ask is, I have a future engine restoration, a Lucas 125 engine. I have just purchased a lucas crank with a good big end, but one of the plates is missing. Would the engine run well without the plates.

Nick  

bantam-flywheeln.jpg

Sort of and yes is the answer 🤣🤣🤣 it's a bit of an ongoing debate.Not sure about a D1 as it probably needs all the help it can get but my d14 "flies" without them.

I'm using inverted commas as using the adjective "flying" and a bantam in the same sentence will no doubt give coco a fit of the giggles 🤣

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What's 7/16 in mm again?

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Stoo63
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December 21, 2019 - 12:34 am
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Where can one purchase one of these machines?? They seem to be a space-age superbike!! What amazing futuristic features which "ensure trouble-free reliability" laughlaugh Oh, brave New World.......

I'm going to be digging out some flywheel plates for an almost identical engine. Will post pics.

'52 D1 direct lighting plunger, '58 Square Four (project), '59 D1 direct lighting plunger,  '59 Tiger Cub, '60 5TA,  '76 FS1-E

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cocorico
Central France
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December 21, 2019 - 7:57 am
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chickensoup said
Hi Cocorico,

Just out of curiosity, why do the D1 and D3 cranks have these plates, are they for extra compression ?

Aargh! winkYou must have seen the debate sometime during your membership, Nick? ** Please log in to view ** for a few examples popcorn.  No one seems able to give a definitive answer, perhaps the best opinion being "why would BSA fit them if they were unnecessary?" Speaking for myself, having owned 2 Bantam engines (150 and 125), which between them had 2 plates attached and 2 detached, I found that the D3 starts and runs well (enough) without them, so that I am happy to reassemble the D1 without, too. Especially as they seem prone to becoming detached and later models don't have them. If I want more performance I can choose a different bike, luckily for me.

Stoo - The Advertising Standards Authority wasn't around at that time, so they slipped through... It would be nice to have a 1949 D1 Rigid though, an excellent year. rofl

1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine being rebuilt (slowly).

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chickensoup
Bolton, Greater Manchester
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December 22, 2019 - 9:27 pm
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Hi all, Ha ha ha, and sorry again for being me....

On an early strip down of my lucas engine ( once i had discovered how to remove the mag and keep it all together so as not to de-magnitise) i also found that these said plates were loose and spinny. same also on my new purchase crank, with one plate missing. After all they are only held in place by centre punching over the crank, not really held in place by much.

I think I will try plates removed first when I get round to it. Will keep you all posted in the future.

Thanks for your responses.... Nick

1950 BSA D1 Plunger   242 UYW

1966 BSA D7  LBF 62 D

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cocorico
Central France
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December 23, 2019 - 7:50 am
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I am worried now, though, that my 2 engines will have lost all the extra efficiency gained during the induction cycle, having lost their flywheel fairings.cant-look

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1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine being rebuilt (slowly).

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Sponge
Lancashire (A chip shop somewhere near Preston)
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December 26, 2019 - 11:24 am
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I have heard two credible theories about these crankcase side plates:

1.  RACER THEORY: In a two stroke engine the plates only become effective at very high engine revs. As the revs increase there comes a point where crankcase pressure drops to zero and you hit a power 'bench'.  These plates move the  zero pressure point and hence the 'bench' up the rev range by a few hundred RPM etc and hence make the bike a bit faster by sustaining power at high engine speeds. 

2.  GOOD ENGINEERING THEORY: In order to aid lubrication and minimise airflow drag...... In the same way that you might want to polish your conrod and your crank webs, the top of your piston and inside of your cylinder head etc then why not, also,  have a smooth polished surface to the sides of your crank.  This makes good (well published) engineering sense in that it minimises power wasted to air drag and airflow friction inside the engine. 

SYNOPSIS: Remove the side plates and the difference is marginal at normal Bantam road speeds - with a bit of power lost to internal efficiency and possibly a more tangible loss at the very high end of road speeds. 

MY OWN EXPERIENCE: Having owned a factory built B175, complete with crankcase side plates from almost brand new in the early 1970s,  I can vouch that it would easily run well above 60mph with the side plates in it.  At the first re-build (due to a dealer issue .....nuff said),  the side plates were left out and it then struggled to top 60mph but ran fine in all other respects.  It seemed to have lost a bit of top end revs.  After its next engine re-build at a reputable BSA Dealer using a BSA factory re-built exchange crank assembly it went back to running over 60 mph easily.    

MY THOUGHTS:  If you have a choice - then fit the plates.  If you don't have that choice - don't stress it - crack on without them .............      

As always very happy to discuss 

Sponge                 

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SpacedMarine
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December 26, 2019 - 1:48 pm
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Interesting sponge.My d14 currently has no plates and does an indicated 70 when wringing its neck.The crank however has been faced off and balanced for a lighter rod and piston.i do have my old crank which I'm building some new plates for and would be interesting to see how that performs as it seems to perform slightly differently without .All very seat of the pants.I d be more interested to see how it does with an expansion chamber and an ignition with retard at the top end.

Anyway getting off topic 🤣

What's 7/16 in mm again?

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cocorico
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December 26, 2019 - 2:51 pm
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That is probably the best post I've seen on the subject, Sponge. As with so many Bantam 'issues', it comes down to owner choice I guess. I am quite happy to be plateless as I don't run it flat out and it's one less thing to worry about - it's more or less a museum piece for me - besides which I've rebuilt it that way, so it will stay like that unless I  rebuild again. Plus it's always something to discuss. A bit like Ouroboros?

1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine being rebuilt (slowly).

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Stoo63
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December 26, 2019 - 5:54 pm
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EDIT:

Above posts moved here from Cocorico's D1 engine rebuild thread. ** Please log in to view **

More pics and context there:-)

'52 D1 direct lighting plunger, '58 Square Four (project), '59 D1 direct lighting plunger,  '59 Tiger Cub, '60 5TA,  '76 FS1-E

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sunny
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January 1, 2020 - 7:57 pm
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plates   on  the  crank   webb   have  had   many    names    sinc  the   erly   days of  m/cing  ,,,     thay  all do  the  same  thing   in  short       thay   cut  douwn  unnesascry   vapper  in the  crankcases   that   gos  to  the  bottom and  reduces   the volume     in  the cases

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JustinW
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January 7, 2020 - 7:09 pm
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I've just opened my engine and found one fixed plate and one spinning. If I take off the plate do I have to make up for the decrease in width with shims? Seems like a lot of width to make up. 

The plate seems a bit small in its recess I suppose with wear from spinning. Not sure I could punch it back on. I don't suppose theres any other method to attach it? Crank will be going to machine shop soon for new big end, perhaps they can reattach it?

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SpacedMarine
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January 7, 2020 - 7:28 pm
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JustinW said
I've just opened my engine and found one fixed plate and one spinning. If I take off the plate do I have to make up for the decrease in width with shims? Seems like a lot of width to make up. 

The plate seems a bit small in its recess I suppose with wear from spinning. Not sure I could punch it back on. I don't suppose theres any other method to attach it? Crank will be going to machine shop soon for new big end, perhaps they can reattach it?  

Depends what model. As Cocoricco discovered recently on earlier cranks they affect the effective width of the crank on later ones there is a boss that rests up against the main bearings .

There are various ways to reattach them.welding is the simplest.I dare say a decent machine shop would be able to sort it.

What's 7/16 in mm again?

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JustinW
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January 7, 2020 - 7:33 pm
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It's a 51 D1. Looks like the plates do indeed form part of the width. Boggles my mind that it had been running with a loose plate - like a giant washer I suppose.

I do have a mig welder which I dare say I could put a few little spots around. I could only do this after it comes back from shop - I wonder how crucial the balance is? A few ground down spots wouldn't at first thought be a problem - but maybe they would.

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cocorico
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January 7, 2020 - 7:35 pm
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Have a look at the link I placed on reply 4 above for the countless posts on the subject. As Spaced says - shims needed depends what plates were fitted.

On my D3, no extra shims needed, on my D1, extra shims needed - I covered it a couple of weeks ago in my D1 engine topic!

1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine being rebuilt (slowly).

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