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#667731 - 09/15/16 8:27 pm BSA B31 cylinder head mods for 400cc conversion *****  
Joined: Dec 2015
Posts: 20
MikeF Offline
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MikeF  Offline
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Falkirk, Scotland
So I have a BB31 engine that I want to get a bit more performance out of. It's intended for road use & a bit of greenlaning. My other B31, a 1946 rigid, has the 399cc conversion carried out by a previous owner and I have the receipt for the work which includes porting & polishing, fitting an oversized inlet valve and replacing valve guides and springs. It's also got GS touring cams. I've researched as many posts as I can find on this subject but details of the modifications to the cylinder head don't seem to be covered. So does anyone know what inlet valve is used - also triumph T140? Which valve springs - I'm guessing GS ? And what diameter the inlet is ported to -32mm ? Looking forward to being enlightened by those wiser than myself !


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
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#667733 - 09/15/16 9:07 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
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quinten Online content
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#667834 - 09/16/16 6:35 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
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MikeF Offline
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Falkirk, Scotland
Thanks for that, I've read that thread - all about cylinder / piston combinations but nothing about cylinder head modifications. I'm interested in finding out whether anyone has increased their inlet valve size with the big-bore conversion and if so, what valve they used.


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#667838 - 09/16/16 6:59 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
Joined: Feb 2014
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chaterlea25 Online content
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Cork Ireland
Hi Mike,
A T120 inlet valve is a good fit especially if the valve seat is recessed, re cutting the seat will bring the valve back up to where it should be
The standard B31 cap and collets will fit
I'm sure an even bigger headed valve could be used,?
I had a box of odd valves to hand and the Triumph one offered an easy solution to the engine I was working on

Opening up the inlet too far and using too big a carb will ruin
drivability
I would go to 28mm to start off with
What cams are fitted or do you intend to fit??

John

#667840 - 09/16/16 7:03 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
Joined: Nov 2011
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B31 Ally Offline
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Northern Ireland
I did a conversion to 400 cc using the Triumph T140 piston but left the standard size valves in the head, I did enlarge the induction port slightly from the carb towards the valve and to make it match the carb better. I wasn't looking more speed, just a bit more grunt, that's why I didn't do anything else to the head as its a 1948 rigid frame B31, hard enough to sit on without going faster.

#667866 - 09/17/16 1:36 am Re: BSA B31 cylinder head mods for 400cc conversion [Re: chaterlea25]  
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MikeF Offline
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Falkirk, Scotland
Hi John, thanks for that information. I'm just at the stage of planning what I need to do so that's another piece of the jigsaw. I'm happy to start with fairly minor mods and work my way up if necessary. I'm thinking touring cams 2448/2450 at the moment as I believe scrambles cams might be a bit too aggressive for my needs. I do want the engine to last a while once rebuilt!


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#667867 - 09/17/16 1:40 am Re: BSA B31 cylinder head mods for 400cc conversion [Re: B31 Ally]  
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MikeF Offline
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Falkirk, Scotland
Thanks B31 Ally. That's where I might start. Just get the bigger piston in, tidy up the barrel/ head interface and see how it runs then work from there.


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#667868 - 09/17/16 1:59 am Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
Joined: Dec 2015
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MikeF Offline
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MikeF  Offline
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Falkirk, Scotland
Incidentally, did you fit a decompression plate to compensate for the higher compression ratio with the bigger bore? If not, how is it working out. Lots of discussion in previous threads about the need for one to reduce stress on the bottom end.


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#667942 - 09/17/16 6:12 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
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B31 Ally Offline
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Northern Ireland
Yes I made a 2mm thick aluminium plate for under the barrel, I was more worried about the piston hitting the head or a valve, the compression on it is still fierce, a bit more than a standard B31.

#668029 - 09/18/16 11:32 am Re: BSA B31 cylinder head mods for 400cc conversion [Re: B31 Ally]  
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GS DAVE Offline
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Would you not have to alter the squish on the head to get the maximum out of the conversion..

#668032 - 09/18/16 11:50 am Re: BSA B31 cylinder head mods for 400cc conversion [Re: GS DAVE]  
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B31 Ally Offline
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Well you might have to but the standard B31 piston is flat toped with virtually no squish at all and the T 140 piston is domed so I reckoned I was now getting some squish as the inside of the B31 head is domed. I left it at that and seems to be working all right, plug is a nice brown colour so I must be burning all the fuel nicely. It has a lot more grunt than a standard B31 so when something is working well for me I leave it at that.It may be possible to get it to go even better but im happy with my set up.

#668148 - 09/19/16 2:37 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: B31 Ally]  
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stevenick Offline
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east yorkshire uk
Hi , I have done the 400cc conversion getting info off this site and the bsa wd site. I used the triumph piston with a 3mm decompression plate and g/s touring cams . A big improvement over the standard performance. No head mods.
I did see some info on this site about using the b44 piston but not enough to follow it.
If anyone has done that (b44) what did you do ?
thanks.

#668281 - 09/20/16 5:53 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: B31 Ally]  
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MikeF Offline
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Falkirk, Scotland
What carb / jetting are you using to get the nice brown plugs? I'm hoping the AMAL 376 currently fitted (1 1/16") can be rejetted to suit.
I managed to pick up an original hepolite T140 piston (71-3676) and a set of Goetze rings (recommended by Roy Shearwood). Autocycle have touring cams 2448/50 in stock so happy days!


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#668292 - 09/20/16 7:10 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
Joined: Nov 2011
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B31 Ally Offline
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Im using an AMAL 276 and as far as I can remember a 150 main jet and the needle raised fully. With the standard bore it seemed to run slightly rich but since I got it bored to 400cc it seems to be running fine, plug is nice colour so im happy with it. Your lucky finding a original T140 piston, I fitted a new L.F.Harris piston into mine and so far so good. A friend of mine had 2438/2436 cams in his 400cc conversion but the inlet valve cotters was hitting the top of the valve guide at full lift, putting about 20 thou clearance in the pushrod valve clearance adjuster cured this but you cant run an engine with that much slack in the push rod. The guides were reproduced ones and may have been a tad long. Maybe this winter I will get him to take the head of and I will machine a bit off the valve guides and try the cams again as I would like to see how it went. Im running standard B31 cams at the moment in my bike.

Last edited by B31 Ally; 09/21/16 7:47 pm. Reason: cant spell !
#668428 - 09/21/16 7:00 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
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MikeF Offline
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MikeF  Offline
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Falkirk, Scotland
That's interesting, the 2438/2436 are Gold Star ZB cams with slightly less lift & duration than the 2448/50 DBD touring cams. I knew that they might cause the valve springs to become coil bound but if I'll make sure and check the clearances on the other components when I get to that stage.


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#668437 - 09/21/16 7:51 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
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B31 Ally Offline
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Northern Ireland
And I would have thought it was a case of standard cams out,2438/36 cams in, adjust clearances and away you go. But as with most things in life its just not as simple as that sometimes.

#669372 - 09/28/16 10:46 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
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MikeF Offline
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MikeF  Offline
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Falkirk, Scotland
So I've stripped the head and it's a bit tired so I've decided to do the valve mods. Not totally decided as yet but I've asked Ron Shearwood if he would do the necessary to make the most of my re-bore to get good bottom end torque with flexible mid range without going overboard. I've also agreed to an exhaust valve insert for unleaded. I've split the casings and opened Pandora's box !! The good news is that the crank pin looks good so no drama there, however the other bearings are another story. Timing side - the lipped bearing seems to have been installed the wrong way round. I've always understood that the lip was intended to stop the bearing from wandering out from the casing to contact the flywheel. My timing side bearing has the lip inboard - against the crankcase seat. The drive side has a home made washer instead of the oil flinger and the roller bearing, also lipped, is also installed with the lip against the crankcase - so when the shock absorber is tightened its forcing the inner race against the lipped outer race ! It also seems that the crank pin nut has been happily machining itself into the drive side crankcase as a result of this misunderstanding.

I have been known to get things wrong in the past but am I right in thinking that lipped bearings are supposed to keep the inner race contained therefore the lip should be innermost Ie closest to the flywheels.

With regard to my touring cams, I was advised to check that the tooth count from the timing mark to max lift was correct, apparently some aftermarket cams have been found to be less than accurate. I have compared them to my stock cams and they are within a reasonable margin. Worth checking though - could be a bugger otherwise.


Mike F
Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03
Last year's project - Yamaha XS400 Street Tracker
#669486 - 09/29/16 5:31 pm Re: BSA B31 cylinder head mods for 400cc conversion [Re: MikeF]  
Joined: Feb 2014
Posts: 363
chaterlea25 Online content
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chaterlea25  Online Content
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Posts: 363
Cork Ireland
Hi,
Quote:
My timing side bearing has the lip inboard - against the crankcase seat.


That is correct

The drive side bearing lip should be lip towards the case outer ball bearing. (as you describe)
the roller bearing should be a MRJA25
http://www.petersclassicbikeparts.nl/contents/en-us/d70.html

There should be a spacer sleeve between the drive side roller
and the ball race outer bearing (66-660)
This spacer is 1.000in long, if its worn the bearings will bind

The crank is located against the ball bearing
when the cushdrive nut is tightened.
It needs to be tightened to 65ft/lbs on final assembly

John


Moderated by  Allan Gill, Jon W. Whitley 


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