Hello Members:
I want to find a head and barrel for a standard XB33 motor bottom end.
Finding ones with out broken fins can be difficult.
My question is, Would the 1953 and later "big fin" or "square fin" cast iron barrel and head fit the earlier crankcase, with out any modifications?
Is the carb and exhaust spigots in the same location as the earlier barrels?
Does the standard pushrod tube fit the later barrel?
I just want to build a stock engine.
Any advise at all is much appreciated.
Thanks Keith

email (option): keithatkinson@hotmail.com

Hi Keith...Nearly all the 'big fin' related parts are interchangeable with earlier motors....My own ZB33 (1951 rigid) has the 'big fin' parts fitted...

The only exception is the piston...

Big fin motors had the connecting rod shortened by 1/2", but to maintain overall engine height the distance from the centreline of the gudgeon pin to the crown of the piston was increased by the same amount...
So, the barrel will fit but you will need an early 'long rod piston to suit your connecting rod (assuming your XB bottom end is standard)...
Centre to centre distance from the big end to the small end is 7 3/8" for 'long rod' motors...6 7/8" for 'short rod' ones...

Unless you happen to have some particular parts in mind you won't find it any easier to get 'big fin' parts and they may well cost you more, as they are generally more in demand...Ian

email (option): ian@wright52.plus.com

Thanks for the advise.
It was just incase I was able to locate big fin items, now that I know they will fit.
They are available from time to time Part #65-1707 but are not cheap.
I will have to check the part number of the rod I have, which then will dictate what piston I require.
Cheers Keith

email (option): keithatkinson@hotmail.com

Hi Keith,dont know if its compatible but this expensive M24 barrel is on ebay
£1500.00 271264580153
cheers Rick
ps there is a B31 barrel 251324512929 dosn't look right but may bore out????

email (option): richardholt@rocketmail.com

I have a pretty good B31 barrel if you've got the low compression Bonnie piston to take it out to 410cc

email (option): davmax@ntlworld.com

Yes I think ebay item 251324512929 is for early B31.(Part#65-1701)
A tell tale is the thickness of the liner when viewed from underneath
Looks to be a good serviceable part though from the photo's.
No broken fins or damage. It should clean-up OK.
Early B31 parts seem to be more plentiful, than B33.
Ebay item 271264580153.
At that price, surely it must be the only one in the entire world.
AUD $2,600.00+ without shipping. Hmmm
Thanks Keith

email (option): keithatkinson@hotmail.com

I have a few B31 barrels somewhere.
I didn't know about the bonnie piston conversion. What specific model??? Very interesting.
Surely it would be a good alternative if a barrel is out past 0.080" or so!!
Does the bonnie part have the same size gudgeon pin, or is resizing of the
rod small end, and/or bush required??
Is the pin centre to crown height the same??
I usually just rebuild my engines to stock.
I don't normally have the confidence to "go out on a limb" with
non standard engine mods, although these B31's are great engines.
Can't wait to eventually get my B33 engine finished.
Thanks Keith

email (option): keithatkinson@hotmail.com

Hi Keith...You need the low compression piston made by L F Harris for the T140 Bonneville....This is 76mm instead of the B31s 71mm which will give a capacity of 400cc if a std size piston is used.
The Triumph gudgeon pin diameter is the same as the BSA so it is a very easy fit...
Although around 7.5-1 in the Bonneville the compression ratio is much higher when fitted to the BSA engine, with its shallower and smaller diameter combustion chamber.

Consequently, you will need to run a decompression plate under the barrel to achieve a sensible compression ratio for modern fuels and road use. I also machine the top of the piston to help with that...For the best results the 'decompression plate' should be a machined piece that fits between the barrel and the head..This gives a better combustion chamber shape (and squish area under the head) than just putting a plate under the barrel.....

This will give a very moderate power increase as a 'stand alone' modification and the engine won't make as much power or be as strong in the midrange as a standard 500 motor.

Ideally, the inlet port should be opened out and a matching, larger, carb fitted. Also a B33 inlet valve and a pair of 65-2446 Gold Star Scrambles cams to take advantage of the porting improvements.

Finally, a pair of lightened B31 flywheels will be needed...These engines will gain some midrange on standard flywheels but will not 'rev out'....lightening the crank is the key to performance throughout the range....(just take a look at a pair of Gold Star flywheels...)

If you are not keen on getting into the area of engine tuning I think your best plan is the one you mentioned....Get your 500 motor together....This will be superior in every way to a 350 that is just bored to 400 without any other mods....Ian

email (option): ian@wright52.plus.com

That barrel is not far off the right money for one of those, a head would be more & you need a head with it because of the built-in tunnel .. oh & then there's the different bore & stroke M24 is 82x94 (B33 85x88) ... & its from a "narrow crankcase" 38 engine but having said that all these stories about the wider 39 engines having the valve train moved out to give cooling behind the tunnel yet 38 & 39 M24 heads & barrels appear interchangeable from.pics I've seen..which surely means .... anyway Rick wasn't that serious i don't think & the M24 ins & outs are for another thread i think :D

Hi Fred..Only the sidevalve engines had the crankcase widened on the timing side...
Regarding the M24 barrel advertised...New M24 barrels and rocker boxes are currently in the process of being produced in the UK (in the midlands)...though as you say there is still the problem of sourcing an original head...With only 700 or so M24s ever made that is always going to be a problem...
I have been considering getting involved with the M24 repro parts, specifically the costs involved in getting the head pattern made (which is the stumbling block with that part)...
However...I would want to confirm that the complete top end can be fitted to later (available) crankcases to enable the construction of 'replica' motors first....
The barrel on e bay is being sold by George Wanda from Honiton....the chap who runs the M24 website...Ian

email (option): ian@wright52.plus.com

Ah only sidevalves that explains it, thanks.
Yes i knew it was George as i have dealings with him, nice chap & knows values, I've discussed these barrels with him before, the originals were produced evidently using "Aircraft industry technology" i also read somewhere & alloy is a very special type, a process where the liner was "spun" during production to eliminate air from between liner & the alloy was used i think i read, either way i am always sceptical about the remaking of parts like that actually ever materializing.
I was very lucky with my 38 barrel being good & on standard bore.

Hi Fred...The barrel patterns have been completed and the first 'test' castings have been produced. Patterns for the rocker box are also complete I understand...though I am unsure whether the first castings have been completed yet as I haven't had any contact with the chap who is doing it for a few months...I also haven't determined whether 38 or 39 type barrels have been made...My guess would be the latter type...Ian

email (option): ian@wright52.plus.com

Yes given the choice i think producing the 39 would be a good idea ... to keep the value of my 38 high :D
Here's a chaps 38 head on 39 barrel .. looks fine to an untrained eye.

I'd like the 39 top end...Then I could replicate the 1939 ISDT Army Gold Star...Ian

email (option): ian@wright52.plus.com

Wouldn't that involve covering up the top end with black paint though ... not sure i could do that : O