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B31 using a T140 piston. #629311
12/08/15 10:53 am
12/08/15 10:53 am
Joined: Apr 2012
Posts: 253
England
British Ironwork Offline OP
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British Ironwork  Offline OP
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Posts: 253
England
A pal of mine in France wants to use a T140 piston in the B31 motor he is building, to up it to 400cc. He has heard that a barrel spacer is needed, but gets two answers when searching the web, some say 2mm whilst others say 3. Has anybody here had any experience in doing this mod?
Cheers.

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Re: B31 using a T140 piston. [Re: British Ironwork] #629351
12/08/15 3:29 pm
12/08/15 3:29 pm
Joined: Apr 2002
Posts: 962
North East England
beezageezauk Offline
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beezageezauk  Offline
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Posts: 962
North East England
I did this modification to my B31 back in 2000 and have covered many thousands of trouble free miles with it since. I've also converted quite a few engines for friends without any negative feedbacks.

I've always used a 2mm spacer (decompression plate) but must admit that I showed concern when I first fitted Goldie touring cams with the high compression T140 piston. However, there was no need for me to worry as nothing fouled and that engine has been running fine for quite some time now.

So, I would suggest that a 2mm spacer should be fine but once the job is done I would advise to turn the engine over very slowly before trying to fire it up, just in case!!

Beezageezauk.

Re: B31 using a T140 piston. [Re: British Ironwork] #629358
12/08/15 4:08 pm
12/08/15 4:08 pm
Joined: Apr 2012
Posts: 253
England
British Ironwork Offline OP
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British Ironwork  Offline OP
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Posts: 253
England
Thanks for that, I'll let him know.

Re: B31 using a T140 piston. [Re: British Ironwork] #629360
12/08/15 4:19 pm
12/08/15 4:19 pm
Joined: May 2013
Posts: 4,839
Rotherham - S. Yorkshire
Allan Gill Offline

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Allan Gill  Offline

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Posts: 4,839
Rotherham - S. Yorkshire
You can always put some strips of plasticine on the crown, the valves will cut an impression and you will know exactly how much clearance you have.

( I can vouch for Ray's bike)


beerchug
Re: B31 using a T140 piston. [Re: beezageezauk] #629367
12/08/15 5:35 pm
12/08/15 5:35 pm
Joined: May 2011
Posts: 432
england
G
GS DAVE Offline
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GS DAVE  Offline
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Joined: May 2011
Posts: 432
england
Did you use 2 base gaskets or just sealant ?

Re: B31 using a T140 piston. [Re: GS DAVE] #629544
12/10/15 6:21 am
12/10/15 6:21 am
Joined: Apr 2002
Posts: 962
North East England
beezageezauk Offline
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beezageezauk  Offline
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Joined: Apr 2002
Posts: 962
North East England
Originally Posted By GS DAVE
Did you use 2 base gaskets or just sealant ?


If I'm converting an engine for somebody else I ask if they have a preference and act according to their wishes.

I generally don't use gaskets in my own engines. Instead, I use "Granvilles Instant Gasket". This is a specially formulated silicon based substance that comes in a squeeze tube. It's available in various colours, including clear (my preference). Most of my riding is doing long distances and this stuff is ideal if I need to undertake repairs on the roadside or on a campsite miles away from anywhere. With no worries of damaging paper gaskets or trying to make replacements. I'm just more careful of using it around oilways.

I put a smear of this each side of the decompression plate.

Beezageezauk.

Re: B31 using a T140 piston. [Re: British Ironwork] #629580
12/10/15 2:03 pm
12/10/15 2:03 pm
Joined: Aug 2015
Posts: 1,123
Bolton Lancs UK
A
Andy Higham Online content
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Andy Higham  Online Content
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Joined: Aug 2015
Posts: 1,123
Bolton Lancs UK
Before you do anything decide what compression ratio you require.
Temporarily assemble barrel head and piston onto the engine, position with the spark plug hole uppermost.
Turn engine until piston is at TDC on compression stroke, or leave pushrods out
Using a graduated syringe fill the combustion chamber with thin oil until it reaches half way up spark plug hole and note exactly how much oil has been added
Your compression ratio is the cylinder capacity + the quantity of oil / the quantity of oil.


1955 BSA B31 500cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350
1967 Greeves 360 Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
Re: B31 using a T140 piston. [Re: British Ironwork] #629581
12/10/15 2:17 pm
12/10/15 2:17 pm
Joined: Dec 2004
Posts: 9,321
Scotland
kommando Offline
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kommando  Offline
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Posts: 9,321
Scotland
Andy, why 1/2 way up the spark plug hole, I would have thought it would be just as the oil entered the hole and touched the threads.

Re: B31 using a T140 piston. [Re: kommando] #629586
12/10/15 3:01 pm
12/10/15 3:01 pm
Joined: May 2011
Posts: 432
england
G
GS DAVE Offline
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GS DAVE  Offline
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G
Joined: May 2011
Posts: 432
england
Due to the void between the insulator and the threaded body.
Ie a near representation of volume when the plug is fitted.


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