Brodie's Bastard BSA

Well...keep hunting, you'll settle on something eventually. The roller looks great!
 
So I bought a M21 crank case set with a better condition barrel that I had. The part lot also came with 1 and 3/4 "BSA Gearbox", turns out they are C11 boxes. Looks like I will clean them up and make my money back. Grabbed a few things off the other cases that were missing on mine, such as the exhaust tappet adjustment bolts and the studs for the magdyno.

I also had the liner pressed out of the barrel leaving me with a 88mm hole to be bored out and honed. This is an "ol' trick" I heard about from a few people and should bring the engine up to between 710 and 750cc depending on the size of the bore.
So yes. My mind has been made up, I will be building the flathead big bore engine with a bit of work to the head and a modern piston to raise the CR to a decent number. Stock is 5:1, I want to try for 7:1


Ohh and to chronicle my BSfield engine I will leave some info on what I have found to be the combo needed.

M20 lower end (due to 94mm stroke, allowing for a shorter engine.)
Enfield GT 87mm piston (Giving a close to square engine size)

This will create an engine with 560cc.

The reason I chose not to build this was because I do not want to have to cut and bend my frame to fit the oil pump into the frame. If I was to I would much rather trade the frame for a single frame, most of which are used as Goldie replicas.
 
So I have had the cylinder linder pressed out and taken the cylinder in to get new valve seats and valve train fitted. I just need to find a 92mm piston with a 20mm pin and a flat top before I get the cylinder bored.

Then I will have to get the small end bush bored out to fit the 20mm pin and rebuild the lower end and she should be set to go.

Gotta love the simplicity of a flathead engine.
 
http://www.ebay.com/itm/VW-Bug-Piston-1835cc-Type-A-Volkswagen-92mm-Piston-Type-1-Mahle-/322081358558?hash=item4afd8b82de:g:uy0AAOSwZ8ZW770a&vxp=mtr

Not sure as to the skirt and height measurements though...
 
VonYinzer said:
http://www.ebay.com/itm/VW-Bug-Piston-1835cc-Type-A-Volkswagen-92mm-Piston-Type-1-Mahle-/322081358558?hash=item4afd8b82de:g:uy0AAOSwZ8ZW770a&vxp=mtr

Not sure as to the skirt and height measurements though...

Skirt seems to be short enough
 
I was thinking the VW piston might be a bit heavy. Wonder if a klx400 piston would work. Might get higher compression with one.

But the VW does match my budget better.

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Those aftermarket pistons are plenty light. If you dog around a bit I'm sure the weights are listed online somewhere.
 
I cannot find any VW 92mm piston in Aus that aren't a part of a kit.

Does anyone know the wrist pin size of a '83 XT550? They are a 92mm standard bore and have a flat crown.

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Awesome. That must be the mysterious "Yamaha Piston" I was told would fit by one of the old blokes at a swap meet.
 
Thanks for the help Tune.

This is going to be one big "lawn mower" engine. Haha

I just dropped into the engine builders and asked them about the seats they are using. Some kind of steel alloy. He said it will work fine for unleaded. They still have not commenced work, but I did tell them not to rush it.
While I was there I asked about piston crown types and he recommended using either flat or slightly domed. So the XT550 piston is a go.
I came up with some stupid plan in my head to press out the bush on the lower end and fit a dished piston from a 2l subaru. I was thinking the dish would help the flame front push the piston down. I had hoped that a lower wristpin height would make up for the compression.

The engineer just said, "don't over complicate it, this is a 60 year old engine remember"

So there is that. 112 stroke 92 bore = 744.6cc.
Then do the usual round up and we have a 750cc single. Sounds good to me.
 
It has one stock. Lucky me. Not that I have the parts. It just lifts the exhaust valve off the seat.

I was thinking of using one in the head instead though. That way I can move the spark plug to a better location.
 
Better start hitting the gym, you're going to have to get that kicking leg in good shape! It ought to be able to pull down a barn! :D Cool build Brodie , keep it coming!!!!
 
Not sure if I could ever get my leg strong enough to pull down a barn, but I sure do hope the bike will haha.

My plan is to buy a few different primary sprockets and see where the engine needs to be for what I want it to do.

Ideally I want it to cruise at 100kph for a few hours and be able to pull me (6'1 110kg) up some mountains at a reasonable pace.
Big ask from an old flathead but that is the fun in it, getting all I can from such a simple engine.
 
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