Author Topic: any piston ideas  (Read 1309 times)

Offline taroha10

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any piston ideas
« on: 06.01. 2013 20:20 »
Hi all, and a happy new year to you all.
Haven't been on the forum for a while but Super Rocket going well.
I know this has probably been asked many times but here goes.
I am not getting any younger and especially after getting some strange virus a couple of years ago which affected leg mussels etc , kicking the bike over is not an easy task. If I get it right ( which I often do ) ,all is fine , but if I fail a couple of times in a row, it is sometimes hard to muster up the energy to carry on.
I have no idea what pistons are in the bike, but it seems to have been put together very well about 20 years ago ( have some paperwork ).They do seem to be fairly serious though! My two previous A10s were iron head and really easy to kick over. So I have contemplated fitting some flat top pistons which i'm sure will cure the problem.
Now here's the big question .If I could be sure that I could safely buy some new pistons, I would be happy to take off the head,check what size pistons are in at the moment, and replace with same size.However, reading stories on the forum in the past makes me a little nervous about what I could end up with.What do I need to be carefull of,looking at new pistons, and has anyone had recent good dealings with a UK supplier.
I come from the school of " if it aint broke, don"t fix it " so with a bike that runs really well, I need to feel fairly sure all will be well before I get spanners out!!
Thank you in antisipation of any replies.
Cheers ,Mark   

Offline wilko

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Re: any piston ideas
« Reply #1 on: 06.01. 2013 22:29 »
Save yourself the trouble and eat more mussels!

Offline paulmbsa

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Re: any piston ideas
« Reply #2 on: 06.01. 2013 22:33 »
Go for Dished, can,t go wrong as only Hepolite Mkae the, also consider a commpresion plate to drop it

Online fringedweller

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Re: any piston ideas
« Reply #3 on: 06.01. 2013 22:39 »
You could do what I did, fit a Mcfarlane electric starter.
 I haven't kick started my SR in the last 1000k, makes life real easy.
 Apparently mine is the first fitted in Australia, but as I live out in the never never no-one else gets to see it.

Don

Offline muskrat

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Re: any piston ideas
« Reply #4 on: 07.01. 2013 09:15 »
 Get one of those race starting rollers then after a few miles of adrenalin you can kick start a jumbo. LOL
But seriously if it's not blowing smoke leave the barrels alone and get Lani at http://www.coppergaskets.us/ to make you a head gasket out of 92thou".
If its compression is 9:1 will drop to about 8:1, 8.5:1 = 7.5:1, 8:1 = 7:1. This will alter the rocker/valve geometry a tad but not enough to worry.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline taroha10

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Re: any piston ideas
« Reply #5 on: 07.01. 2013 21:13 »
Thanks for your replies guys.
I was expecting to be told to 'man up'  etc. Not a fish food fan either.
 I can't really justify an electric start even though it obviously would solve the problem. The copper gasket seems a good idea Muskrat and certainly the easiest option, but I have wondered about the valve geometry being altered and would hate to have a push rod jump out.There is probably a calculation which would show the angle difference.  I imagine with all your racing engines  the last thing you would want to do would be lower the compression! I've heard of compression plates but imagined these would go under the barrels. Possibility of the same prob though. I am a little nervous of both these mods for the same reason.Have you survived a similar change in the rocker gear? 
Again, thank you for the replies so far, things to consider.
Cheers. Mark

Offline muskrat

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Re: any piston ideas
« Reply #6 on: 08.01. 2013 07:54 »
 G'day Mark. A comp plate under the barrels would alter the geometry as well. The only way out is to make longer push rods by the same amount as the added comp plate or head gasket. Easy enough out of duralumin rod.
I used a 60thou" thick extra head gasket to drop the comp on my cafe from 10.5:1 to 9.25:1 (to stop pinging) with no ill affects. I'll get around to making longer push rods one day, and a thicker head gasket from Lani.
Cheers.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline Rocket Racer

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Re: any piston ideas
« Reply #7 on: 09.01. 2013 20:27 »
I agree with Muskrat, a decompression plate is a standard part for all beesa pre unit singles, albeit a simpler fitting going under the barrel above the cases. For an A10 I would go for a thicker head gasket for simplicity.

If you consider the A7 and A10 barrells are differing heights (1/2" ?) , such a minor change as dropping compression will be inconsequential to geometry, other than checking your pushrod lengths have enough length.

A lower compression will make it a nicer road motor.
Cheers
Tim
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
New Zealand

Offline taroha10

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Re: any piston ideas
« Reply #8 on: 09.01. 2013 23:37 »
Thanks again guys.I think I will give the thicker head gasket a go.It is also the least disturbance to the engine and should be a quick job.It will be interesting to see exactly what pistons are in there at the moment as well.I know its been discussed before ,but as it may also prove usefull to anyone new to the forum reading this thread, can someone remind how to work out the existing compression ratio, so I can then compare after mods?.
Cheers. mark.

Offline Rocket Racer

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Re: any piston ideas
« Reply #9 on: 09.01. 2013 23:50 »
post a picture of the piston crowns at TDC when you have the head off, that should tell what compression ratio the pistons are
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
New Zealand

Offline muskrat

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Re: any piston ideas
« Reply #10 on: 10.01. 2013 11:34 »
G'day Mark. Wind it up to TDC on comp stroke. Now lean bike over till spark plug hole is vertical. Go down to the local chemist (or shooting gallery) *eek* and get a 10ml syringe. Now measure how much oil it takes to fill the combustion chamber up to the bottom of the plug hole. Say it's 38cc + 325cc of swept volume = 363cc divided by 38 = 9.55:1 compression. Now put a thin tube on the syringe to suck out the oil. *smile*
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline taroha10

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Re: any piston ideas
« Reply #11 on: 10.01. 2013 17:51 »
Thanks Musky (and Rocket racer).I'll report back at a later date to let you know the outcome.

Offline Slide

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Re: any piston ideas
« Reply #12 on: 11.01. 2013 12:43 »
Hi Mark
If you did need a piston supplier in the UK (ref your opening questions), I used FW Thornton

 http://www.fwthornton.co.uk/

I found them excellent, very helpful with a good range of options. They exchanged a set of pistons I'd ordered with no problems when I found I needed new liners.
Lambretta LD150 1956 ~6 hp
A7 SS 1958 ~30 hp
Ducati 996 2001 ~ 128 hp

Offline taroha10

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Re: any piston ideas
« Reply #13 on: 12.01. 2013 00:04 »
Cheers Slide,
That answers the last of my questions.I will check them out.
Mark.