Big bore BSA B33

Hi everyone, I'm a new member from the UK with a 1948 BSA B33.
As you can see from the thread title it's a big bore conversion, yes i know!
We all know the b33 is an underpowered slugger but I'm just trying to add a little low end pep!
Why you ask?
Because the old girl dropped a valve, on strip down i found the barrel & piston was damaged and badly worn and unfortunately on its last over size, that's how it came about and tbh I wish I hadn't taken this route, but here I find myself with more questions than answers!
So I'm hoping some folk on here can help me with some advise, solving the problems that have seen the bike laid up and collecting dusk for several years now!
First problem, I've fitted gold star touring cams and the exhaust valve spring is coil bound on full lift, I've scoured the Internet for a replacement spring with similar dimentions /poundage but fewer coils, any suggestions? They are aftermarket valves with new hardened seats.
Cheers
Catweasle

Hi Catweasle, welcome to the forum.
I dont know the answer to you question, but someone else will chime in I am sure.

Cheers

Catweasle said:

Hi everyone, I'm a new member from the UK with a 1948 BSA B33.
As you can see from the thread title it's a big bore conversion, yes i know!
We all know the b33 is an underpowered slugger but I'm just trying to add a little low end pep!
Why you ask?
Because the old girl dropped a valve, on strip down i found the barrel & piston was damaged and badly worn and unfortunately on its last over size, that's how it came about and tbh I wish I hadn't taken this route, but here I find myself with more questions than answers!
So I'm hoping some folk on here can help me with some advise, solving the problems that have seen the bike laid up and collecting dusk for several years now!
First problem, I've fitted gold star touring cams and the exhaust valve spring is coil bound on full lift, I've scoured the Internet for a replacement spring with similar dimentions /poundage but fewer coils, any suggestions? They are aftermarket valves with new hardened seats.
Cheers
Catweasle

Click to expand...

Hi catweasle , I might be the one to help you out , I have a 54 B33 that's pretty modified, Mines came with a Harley shovelhead piston , but I've changed it for a evolution piston to lower the compression ratio which is a flat top , now my barrel looks as though it been bored from the bottom and had a short sleeve fitted and it has two caphead screws drilled and tapped to ensure its retained , then the barrel had been bored to Evo +.030 which is a little under 90mm , my crank is from a M20 with 94mm stroke fitted with the B33 timing side shaft , it has a 35mm intake port and 47mm intake valve and 45mm exhaust valve, Goldie type exhaust . I used terrys springs, terryble pressure stock ! But i fitted different inner spring for approx 86lb and 140lb over the nose , 36mm dellorto pumper Approx 592cc .

You might need to get some material removed from the top of the valve guides for retainer clearance , my engine has 5/16 valve stems , and I needed thicker lash caps to increase the clearance between the rocker and the retainer as the PO had it touching the retainer before the valve tip , I just bought a V8 set of comp cams lash caps with an increased thickness .080, I think . , where did you buy the touring cams ? . Another trick that may help your coil bind , alot of V8 retainers and keepers can be bought with + 060" or so i:e they moved the groove step down to raise the retainer if you know what I mean , those terrys spring have a fairly big diameter and it is really difficult to match some springs in there that don't coil bind , and like I have mentioned , moving the retainer up or having a longer valve , watch that rocker to retainer clearance its harder with a fixed rocker pivot . There's a company in the UK called PES engineering they have some BSA bits .https://pesltd.uk/

My mate had a B33 many years ago. In about 1960. We tried to buy race cams for it from a dealer In Melbourne who was racing a B34 Gold Star in scrambles. At the time there were two different race cams available - dealer would not sell the cams of the type he used in his own bikes - he only sold the tamer cams. We imported the better cams from the UK. The B33 was capable of showing 105 MPH on the speedo with nothing else done to it.
Replacing the cams usually gives better results than anything else other than fitting close ratio gears. However the RRT2 gearbox is a swine.
Big bore usually means heavier pistons. That B33 we had, almost immediately broke the crank pin.
If I was going to play with one, I would fit internals from a Jawa speedway bike.

Thanks for the advise, I never considered checking the valve guide clearance height, certainly worth looking at as they are newly fitted guides and reducing the retainer is worth noting.
I'm also runinng a harley piston, in my case it's a forged wiseco piston also intended for the shovel head.
I'm using the original crank but I've had it rebuilt with a bsa short rod and shortened the barrel, the piston at tdc is 20 thou from the top of the barrel. I did this to reduce compression or a bit of wiggle room either way as the piston is domed and I only want to slightly improve the engines compression from standard. That said, I don't know what's a normal compression reading on a standard B33!? and if a reliable reading can be obtained just using the kick-starter?
Other modifications I've done is obviously the GS touring cams, porting and a bigger carb. I've lighted the clumsy valve train and fitted a Suzuki GS Clutch.
I've had trouble with the worm drive powering the oil pump but I think I've conquered that one! more headaches on the way I'm sure, you never know what your going to encounter when you stray from the original design
Thanks for the link, I will be sure to check them out for any useful parts.
Cheers Catweasle

acotrel said:

My mate had a B33 many years ago. In about 1960. We tried to buy race cams for it from a dealer In Melbourne who was racing a B34 Gold Star in scrambles. At the time there were two different race cams available - dealer would not sell the cams of the type he used in his own bikes - he only sold the tamer cams. We imported the better cams from the UK. The B33 was capable of showing 105 MPH on the speedo with nothing else done to it.
Replacing the cams usually gives better results than anything else other than fitting close ratio gears. However the RRT2 gearbox is a swine.
Big bore usually means heavier pistons. That B33 we had, almost immediately broke the crank pin.
If I was going to play with one, I would fit internals from a Jawa speedway bike.

Click to expand...

Hi, thanks for the info, the GS touring cam I'm using is considered mild in comparison to others GS cam available and would just drop straight in, ive tried to be cautious, the aged b33 design isn't that strong! I would think anthing in access of 4500 rpm is a recipe for disaster
I've fitted a wiseco cutaway piston weighing in lighter than the standard B33 piston so hopefully heading in the right direction.

Jerry Doe said:

Hi Catweasle, welcome to the forum.
I dont know the answer to you question, but someone else will chime in I am sure.

Cheers

Click to expand...

Cheers

Catweasle said:

Hi, thanks for the info, the GS touring cam I'm using is considered mild in comparison to others GS cam available and would just drop straight in, ive tried to be cautious, the aged b33 design isn't that strong! I would think anthing in access of 4500 rpm is a recipe for disaster
I've fitted a wiseco cutaway piston weighing in lighter than the standard B33 piston so hopefully heading in the right direction.

Click to expand...

acotrel said:

My mate had a B33 many years ago. In about 1960. We tried to buy race cams for it from a dealer In Melbourne who was racing a B34 Gold Star in scrambles. At the time there were two different race cams available - dealer would not sell the cams of the type he used in his own bikes - he only sold the tamer cams. We imported the better cams from the UK. The B33 was capable of showing 105 MPH on the speedo with nothing else done to it.
Replacing the cams usually gives better results than anything else other than fitting close ratio gears. However the RRT2 gearbox is a swine.
Big bore usually means heavier pistons. That B33 we had, almost immediately broke the crank pin.
If I was going to play with one, I would fit internals from a Jawa speedway bike.

Click to expand...

acotrel said:

My mate had a B33 many years ago. In about 1960. We tried to buy race cams for it from a dealer In Melbourne who was racing a B34 Gold Star in scrambles. At the time there were two different race cams available - dealer would not sell the cams of the type he used in his own bikes - he only sold the tamer cams. We imported the better cams from the UK. The B33 was capable of showing 105 MPH on the speedo with nothing else done to it.
Replacing the cams usually gives better results than anything else other than fitting close ratio gears. However the RRT2 gearbox is a swine.
Big bore usually means heavier pistons. That B33 we had, almost immediately broke the crank pin.
If I was going to play with one, I would fit internals from a Jawa speedway bike.

Click to expand...

My B33 is bored and stroked so it has more swept volume , so 94mm stroke and the shovelhead piston and a stuck second ring gave me 190 psi , the evolution piston which is a flat top is + 2.0mm compression height which I fitted . , My PO said he thought the lower barrel face had been machined ? But I don't really know he didn't really tell much ! , it had a 5 mm and a 1.5mm now it has a steel 8mm , piston is down the bore a little , I wanted squish and I welded on two eyebrows 4047 12% and machined them to match a beveled edge in the chamber for .040" squish , also I have 170 compression now, which is approx 8.5 or 9.0 :1 .PES make a new big end bearing with more smaller rollers .cheers

Blimey I wouldn't have dreamt that was possible, raised compression means more heat surely? What fuel do you run her on? What's it like starting, bit like trying to kick start a battleship
I'm impressed

A mate of mine had a b31 he started off moderately with boring it so suit a t140 piston
Then it got bigger and bigger
He fitted a Pearson modified M20 crank with a special rod and a yam xt 500 piston
Then he had a special piston made and I can't remember what the cc ended up
But unfortunately it went bang in the end and he lost interest in it

baz said:

A mate of mine had a b31 he started off moderately with boring it so suit a t140 piston
Then it got bigger and bigger
He fitted a Pearson modified M20 crank with a special rod and a yam xt 500 piston
Then he had a special piston made and I can't remember what the cc ended up
But unfortunately it went bang in the end and he lost interest in it

Click to expand...

This was his bike

Catweasle said:

Blimey I wouldn't have dreamt that was possible, raised compression means more heat surely? What fuel do you run her on? What's it like starting, bit like trying to kick start a battleship
I'm impressed

Click to expand...

I run it on 95 octane , it starts first kick cold , ⅔ advance full choke and 3 pumps on the throttle,hot it's no choke one pump and it usually goes in one or two kicks . , checking that comp ratio with the burrett is tricky with the spigot step etc , and piston down the bore and deducting the Ccs etc for crown volume , is all a bit of work .cheers

Thanks for you're responses. I've checked valve guide to retainer clearance and there's plenty of room. So changing valve springs should resolve the binding issue. I'm stomped, I've not been able to find a replacement spring for the standard terry vs345, has anyone got an idea where I could find one in the UK?
Cheers
Catweasle

Tbh , how to end up with a favourable compression ratio eludes me, I have it in my mind, stroke, bore and finalising with compression gives you the desired ratio??
Some advise here would be very helpful...!
One reason I prefere to adhere to a lower compression is that I want to fit an electric starter as my knee isn't all that strong, thats another reason I thought it best to go with a short rod in the first place, a shorter kick to complete one engine cycle?
Cheers
Catweasle

baz said:

A mate of mine had a b31 he started off moderately with boring it so suit a t140 piston
Then it got bigger and bigger
He fitted a Pearson modified M20 crank with a special rod and a yam xt 500 piston
Then he had a special piston made and I can't remember what the cc ended up
But unfortunately it went bang in the end and he lost interest in it

Click to expand...

Bloody shame it blew up! Is it still around? Looking at it, it's a true café racer

Catweasle said:

Bloody shame it blew up! Is it still around? Looking at it, it's a true café racer

Click to expand...

No he sold it as a rolling chassis a couple of years ago
Maybe it'll turn up again with a different engine
Interestingly it had an early TLS brake plate fitted to a conical hub up front

Catweasle said:

Thanks for you're responses. I've checked valve guide to retainer clearance and there's plenty of room. So changing valve springs should resolve the binding issue. I'm stomped, I've not been able to find a replacement spring for the standard terry vs345, has anyone got an idea where I could find one in the UK?
Cheers
Catweasle

Click to expand...

I have my own valve spring tester , and I used to be an automachinist, I went to a place I used to work an asked if I could rummage through their old valve spring box , I just changed the inner springs it wasn't easy , it is a fairly common diameter but difficult to get a spring that isn't too strong , less coils so they don't coil bind . I machined up spacers etc because mine had been modified with valve guides and 5/16 stems.