B50MX cam in a B25 and Big bore ideas.

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Shawn_Mc
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B50MX cam in a B25 and Big bore ideas.

Post by Shawn_Mc »

Has anyone done this? (Post 71', 3/4" bushing type)

The base circle and lifts appear to be very similar, it's just a lot more duration.

And is it possible to get a sleeve in there that will carry a 78mm piston? I'm aware that this isn't a bolt on...Just wondering how much meat is in the jug. Could it carry a 3.187" diameter sleeve?

Even if the lower boss on the cylinder was machined completely away, it really doesn't matter. There's no thrust on that portion of the cylinder anyway.

I'd have to have a 78mm piston made, no big deal really. JE pistons is a 30 minute drive for me.

I dont have my motor apart at this point. Does anyone have the spec on the diameter of the sleeve clearance on the cases? How big is that hole in the cases where the sleeve drops in?
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johnu
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Re: B50MX cam in a B25 and Big bore ideas.

Post by johnu »

I don't know the answer to any of this but what cc would that end up being if it is possible. I'm pretty sure there is a 280 kit available.
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Shawn_Mc
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Re: B50MX cam in a B25 and Big bore ideas.

Post by Shawn_Mc »

johnu wrote: Fri May 01, 2020 10:40 pm I don't know the answer to any of this but what cc would that end up being if it is possible. I'm pretty sure there is a 280 kit available.
334.5cc

Big bore does a couple things for you. in addition to the displacement, it unshrouds the intake valve by moving the curtain away from the cylinder side.

The cam would also help it breathe.
Current Bikes
1971 BSA B25 Victor Trail
1983 H o n d a CB1100F
2003 CRF450R
I'm in Southern California
Shawn's Performance. Two and Four stroke engine and Suspension tuning.
Jeff K
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Re: B50MX cam in a B25 and Big bore ideas.

Post by Jeff K »

Call Ed at E&V engineering, 318cc piston in stock.
https://shop.shopevengineering.com/Pist ... ogId=-2087
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webby16
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Re: B50MX cam in a B25 and Big bore ideas.

Post by webby16 »

Ed V 280cc piston is very good and takes is up to 12:1 compression ratio.
I used an r&r rod and had the flywheel machines etc to match. Triumph t100r inlet valve and seat to match.

32mm amal mk1 carb, Alloy clutch, electric world ignition, b50 mx cam, mx first gear cast iron b50 oil pump.

Was well pleased with the results on pre 68 scrambles
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Thanks David
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BasilF
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Re: B50MX cam in a B25 and Big bore ideas.

Post by BasilF »

My B23 engine is on the bench at the moment. It looks like, without modification to the crankcase, you could machine the barrel out to take a 79mm od sleeve with a 75mm bore. giving about 313cc.
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SteveS
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Re: B50MX cam in a B25 and Big bore ideas.

Post by SteveS »

Hi

Remember the oif 71 B25T/SS crankcases are essentially the same casting as the B50 but with different machining carried out. So it is possible to convert a set of B25 crankcases to B50, although there is a fair amount of work required.

The crankcase throat of a oif b25 has enough meat to take a standard B50 liner (and even bigger), o/d of liner approx 92mm, std bore 84mm. The cylinder studs will need to be moved and you may(?) have to move the cam follower location.

EdV will know the limits using the standard barrel stud (and cam follower) locations
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Re: B50MX cam in a B25 and Big bore ideas.

Post by mlb50 »

The B50 crankcases are significantly different to the B25 in that the driveside cases are duplex roller bearings and have breather ports. Converting B25 to a B50 would as you mentioned require a significant amount of work with doubtful longevity.
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Shawn_Mc
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Re: B50MX cam in a B25 and Big bore ideas.

Post by Shawn_Mc »

SteveS wrote: Sun May 03, 2020 7:44 am Hi

Remember the oif 71 B25T/SS crankcases are essentially the same casting as the B50 but with different machining carried out. So it is possible to convert a set of B25 crankcases to B50, although there is a fair amount of work required.

The crankcase throat of a oif b25 has enough meat to take a standard B50 liner (and even bigger), o/d of liner approx 92mm, std bore 84mm. The cylinder studs will need to be moved and you may(?) have to move the cam follower location.

EdV will know the limits using the standard barrel stud (and cam follower) locations
I know the cases are big enough, I was wondering if there was enough meat in the cylinder casting to carry a 3.187 sleeve. Looking at the bottom of that cylinder with a 79mm sleeve (3.011ish), that lower boss is at least .060 thick still, Which would probably leave about .020 of the entire length of it. And that portion of the sleeve doesn't see any thrust. So even if the lower boss on the cylinder was gone entirely, the sleeve would be enough.

I guess I'll have to find a cylinder and figure it out.
Current Bikes
1971 BSA B25 Victor Trail
1983 H o n d a CB1100F
2003 CRF450R
I'm in Southern California
Shawn's Performance. Two and Four stroke engine and Suspension tuning.
Johnq1
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Re: B50MX cam in a B25 and Big bore ideas.

Post by Johnq1 »

Have a look at Micheal Waller on YouTube, he has a 280 conversion vid or try Roger at RTSRACINGPRODUCTS438@YAHOO.CO.UK. He does conversions.
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kommando
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Re: B50MX cam in a B25 and Big bore ideas.

Post by kommando »

mlb50 wrote: Mon May 04, 2020 1:39 am The B50 crankcases are significantly different to the B25 in that the driveside cases are duplex roller bearings and have breather ports. Converting B25 to a B50 would as you mentioned require a significant amount of work with doubtful longevity.
You don't need the extra ball bearing, it carries no load and only acts as a crank locator, duplex rollers can be shimmed to remove excessive end float instead so no need for the mods on the driveside other than the breathing. If you also pick the right bore size the camshaft does not need moving outwards. You might end up a bit short of 500cc but the 441 can go to 480 before the bore starts to be too thin.
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