Bsa b40g engine tuning

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Bsa b40g engine tuning

Post by webby16 » Thu Apr 11, 2019 12:00 pm

Hi all,

I understand there are possibly other b40 tuning guides on the forum but with the search bar I cant search just "b40"

So after the formalities any advice on turning my b40g standard engine into a race motocross engine?

Thank David
Thanks David
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Re: Bsa b40g engine tuning

Post by minetymenace » Thu Apr 11, 2019 12:36 pm

Hi webbie16...The search needs at least 4 characters in a word to work. I think that includes the use of wildcards like ?, * etc.

All these engines are much the same, and it seems what is good for one is good for the other. Over time, the major improvements were sorted by the factory, with refinements by racers. So the tuning aspects should be similar it is just the weaknesses specific to the machine that need to be addressed first, and that requires an answer from someone that is familiar with B40s, that person is not me!!
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Re: Bsa b40g engine tuning

Post by kommando » Thu Apr 11, 2019 4:10 pm

The B40G has a low lift camshaft short duration cam as fitted to the C15 B40 standard models with sometimes the B40 valve head but later some seem to have got the B40 90SS larger valve head. The carb is also restrictive and it has a low compression piston.

You have a few options to pick from or you can do the lot and end up with a 90SS+ sitting in effectively B44 cases.

1. B25/B44 standard cam with flat followers, you will need to machine the underside of the timing crankcase cam follower tunnel to allow the extra lift, you may need to change the inner camshaft bush depending on the cam you get so check before buying.

2. Fit a B40 90SS head or an early B44 round barrel head which is the same but with bigger stud holes.

3. Fit a higher compression B40 piston, do not try the B44 piston as its height is all wrong and requires major surgery and a crank rebalance to use.

4. Fit a carb to match the head, 928 or 930 with some porting.

5. If this is a road bike and the engine has the B40G wide ratio gear cluster then sell it on ebay properly listed and buy a B25/B44 gear cluster to replace, you will have some cash left over to fund 1 to 4 as the Trials mod are desperate for the wide ratio cluster and its a rare beast ;)

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Re: Bsa b40g engine tuning

Post by webby16 » Tue Apr 16, 2019 9:26 am

Thanks for the reply Kommano,

My end goal is to build a pre68 350 motocross engine.

1. I plan on putting a complete b44 top end on the cases and shortening to match the stroke. taking into concideration the b44 piston height ect so its not a straight forward 20mm on the stroke.
i will need to get larger diameter studs made.

2. mx cam shaft with the later flat followers (engine is being delivered tomorrow so dont yet know if its the small diameter camshaft or not) either was machining the bush or cases to fit the later cam or even machine the cam down to fit the smaller diameter bush.

3. i heard the b50 needle roller big end is the better crank bearing not sure what this requires? also heard the original con rod isnt too bad strenght wise? are there any better alternatives?

4. balance and match flywheels

5. bigger carb and match head with porting

6. alloy clucth and check the wide ratio gearbox and sell as need to close ratios.

7. electronic ignition
Thanks David
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Re: Bsa b40g engine tuning

Post by kommando » Tue Apr 16, 2019 11:10 am

1. 20mm longer stroke means 10mm higher and then adjusted for any changes in piston height, B44 is 9.4:1 but B40 9:1 is available and the extra .4 can be adjusted in.

2. MX cam in a short stroke high revving engine may not be the right way to go, MX will give better low down grunt but not much up top in a B44 long stroke. Std B25/B44 cam may be better overall depending on the course in a B40.

3. Some B40G conrods had the B50 needle roller out of the factory, you may already have it.

4. Yes

5. B44 square barrel head breathes better if allowed in rules, porting needs to be done carefully so the charge does not go straight out the exhaust valve, counterintuitively a sharper turn downwards in the port will give better power than a smoothed out flat entry.

6. Alloy clutches do not last long.

7. Its a single so less advantage than fitting EI to a twin.

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Re: Bsa b40g engine tuning

Post by Jeff K » Tue Apr 16, 2019 5:50 pm

I have heard of people cutting down a Aluminum B44 cylinder, rebalancing the crank and using the later B44 square head before, carburetor mounting changes sides. Some were even known to trim the fins to look like the round head. The later B40 crank is the best of the B40 for bearings. Get the R. Ratio engine book for this information. Convert to the later B25/B44 gearbox set up also, it is better then the B40 set up. Even if you do not use the later gearbox, convert to the later "69" cam plate for better engagement, I have posted on how to do that some where on here.
I seem to recall that some one said it was best to press out the liner, cut a section out of the middle of the cylinder and then to trim the liner and press back in. Also if you have the distributer convert to the side points and ad EI.
Jeff

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Re: Bsa b40g engine tuning

Post by kommando » Tue Apr 16, 2019 6:05 pm

Its a B40G Jeff, so its B25/B44 crankcases slightly modified for the low lift cam, no distributor and other than wide ratio gearset the gearbox is already B25/B44. Not many in US but a lot in Europe as the Military used them. They were being built until 71 at least with all the changes except for the wide rear mount and and screw in oil pipes.

If you use the B44 alloy head and barrel but use a B40 piston with the same weight as your current piston no rebalance is required.

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Re: Bsa b40g engine tuning

Post by webby16 » Tue Apr 16, 2019 8:22 pm

Thanks for the help all.

My worry with the b44 barrel being shortened from the middle would be oil leaks and potentially skewing the barrel? I do understand that taking material from the bottom does weaken the base and the barrel is thinnest in the middle etc.

Any images or drawings or best way to do this?
Thanks David
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Re: Bsa b40g engine tuning

Post by Jeff K » Tue Apr 16, 2019 9:48 pm

Here are some older posts on this.
viewtopic.php?t=6361
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Re: Bsa b40g engine tuning

Post by kommando » Wed Apr 17, 2019 8:44 am

I do understand that taking material from the bottom does weaken the base and the barrel is thinnest in the middle etc.
The bottom is the strongest and the amount you will be taking off will not weaken it at all.
Last edited by kommando on Wed Apr 17, 2019 11:55 am, edited 1 time in total.

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Re: Bsa b40g engine tuning

Post by BSA_WM20 » Wed Apr 17, 2019 11:40 am

The B 50 needle roller needs to be treated with a lot of respect.
It is a very high speed roller bearing and if used under load at low revs will scuff the cage on the outer race till eventually the cage breaks up.
The broken bits of the cage will be hammer welded onto the outer race & the engine will become rather noisy.
Oddly enough is the build up is not thick enough to jamb the con rod the engine will run relatively well, for a while.
This was on low compression B40 GA's sevreral of them ( mine included ) ran for quite some time on said wrecked bearing till eventually the needles broke free .
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Re: Bsa b40g engine tuning

Post by webby16 » Tue May 14, 2019 7:41 am

Hi all,

Bit of progress on my plan/build;

picked up my square fin b44 top end where the barrel will be measured and machined down to suit the b40 stroke with JE B44 piston.

my new problem is what to do with the engine studs?

1- get a set of double diameter studs made up to suit?
2- sleeve the barrels in the holes
3- i heard rightly or wrong c15 studs work?
4- and this is my preference at the moment open the threads in the crank cases to b44 size and get studs made to length?


mild porting on the head smooth and polish open the exhuast out maybe to 1"1/2 and big valave rupert ratio suggests b50 collets and caps?

im going to machine and fit the later cam bush and mx camshaft and machine the camshaft follwer tunnel shorter by the difference in length from the round foot to flat foot followers.

big end doesnt require any work, almost no rock in the big end, gearbox is good with standard b40g gearbox but i will use later fork selectors and cam plate.

I will get some images up soon.
Thanks David
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Re: Bsa b40g engine tuning

Post by minetymenace » Tue May 14, 2019 7:54 am

webby16 wrote:machine the camshaft follwer tunnel shorter
why not just shorter push rods, or is it the thickness of the foot and the extra lift of the MX cam means there is not enough headroom underneath the tunnel?
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Re: Bsa b40g engine tuning

Post by webby16 » Tue May 14, 2019 9:04 am

The base of the cam follower has to be ground down to use the later (bigger) cam.

Just thought it would be better long term, if i need to ever replace the cam followers and more accurate as I know for cetain the followers will have enough clearance.

Seems like I would have to take away a good bit of material from the followers (I know its a common mod), since I am machining the cam bush housing anyway why not do the follow tunnel.
Thanks David
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Re: Bsa b40g engine tuning

Post by kommando » Tue May 14, 2019 10:51 am

Do not grind down the follower foot, the early ones are weak and BSA made them thicker to compensate, do not go back the way. Shorten the cam follower tunnel to the B44 size instead and check its enough before finishing the cut.
1- get a set of double diameter studs made up to suit?
2- sleeve the barrels in the holes
3- i heard rightly or wrong c15 studs work?
4- and this is my preference at the moment open the threads in the crank cases to b44 size and get studs made to length?
No need, the stud hole is common to B40 and B44, the B40 stud then reduces just above the thread whereas the B44 stays the same diameter.
So easiest route is to shorten B44 studs and rethread at the top as they screw into B40 cases anyway.

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Re: Bsa b40g engine tuning

Post by webby16 » Wed May 15, 2019 2:16 pm

Thanks Kommando,


That was the original plan to shorten the follow tunnel! and not go down the altering the followers route, for that exact reason!

Thats great news! To be honest i have completely cleaned and stripped the engine! The very thing I havent done is extract the studs from the cases, new to b40 engines, so I genuinely didnt think it was a reduced diameter stud, I thought the whole stud was small including the thread! :oops: :roll:

This has made my life a lot easier! :mrgreen:
Thanks David
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Re: Bsa b40g engine tuning

Post by webby16 » Mon Jun 03, 2019 9:53 am

Hi all,

I am getting the custom engine studs made up today but i have a query.
Anyone know what thread the b40 cylinder studs are exactly?

its 7/16 but it is;
    BSF 18tpi?
    or
      BSC 20 TPI?
      Thanks David
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      Re: Bsa b40g engine tuning

      Post by kommando » Mon Jun 03, 2019 10:42 am

      Assuming you are talking about the case end of the stud.

      Neither, 7/16 Whit, B44 started as 7/16 Whit and then went to 7/16 UNC which has the same TPI, so just a change of thread angle.

      But check first in case a DPO has messed about.

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      Re: Bsa b40g engine tuning

      Post by webby16 » Tue Jun 04, 2019 10:11 am

      Thanks Kommando,

      Always very quick in responding to queries, which I appreciate!

      My issue is the crank case end stud is 18-20 TPI and not the 14 TPI at 7/16 which would indicate whitworth.

      Plus being a B40g made around 66-68 I would expect whitworth?

      Going off BSA part numbers for the b40 cylinder stud 41-43 I'm getting BSF which is 18TPI. I didnt get chance yesterday to get out to the garage, but I will see if I can dig out some thread dies to double check!
      Thanks David
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      Re: Bsa b40g engine tuning

      Post by kommando » Tue Jun 04, 2019 2:34 pm

      My Bad, 7/16th BSF in B40G pre 69 cases as they do not have raised number stamp pad, 69 B44 cases 7/16th UNC.

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      Re: Bsa b40g engine tuning

      Post by webby16 » Wed Oct 23, 2019 10:36 pm

      &lt;069
      Hi all haven't been on for a while and haven't dont much to my b40/44 engine but now the race season is over I've got majority of parts required for my build!

      My question, does anyone have any suggestions for a replacement b40 conrod i was thinking of thunder engineering possibly alloy conrod keep the weight down?

      Thanks David
      Thanks David
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