Beaten up 50 Special

[tzsteve]

re the ring section and port width.
the norm is 65% of the bore for cast rings and 72% for the thinner drawn wire types bigger than this and the rings will get stuck in the port

these maximum sizes assume that the port shape has a radius at the side of the port ie. no big fat square ports

[potatopl]

Thanks tzsteve, lathe is exactly whe tool I want to use (have some pics of your work to learn from), I just meant that i need to sherten the cylinder, not the head (it's not tall enough).

Exhaust port is already 66%. Will make a portmap and show it to you guys.

But the main question was about crank balancing Any more advices? My mate is going to cut his crank for case reed use (flowed crankshaft), and we wanted to use those tips.

[tzsteve]

ther is this if you trust some of my work but if you do a piston swap and go for a better piston the crank is out of balance


there is also this bit of stuff but ignore the first little bit about the flywheel and coweling.  as that was idea that did not work out

[potatopl]

Thanks tzsteve! Will take your info into account when my mate finishes the balancing block and I get to crankshaft cutting.

Portmap of DR75F1.



Exhaust width is about 65%.

Boostport is about 0,5mm higher than transfer which is strange, usually it's the other way round. In blowdown calcculation i took transfers timing.

Measurements (accuracy 0,5mm) from the picture put into timing calculator (43mm stroke, 87mm conrod).



Still not sure which timings to go for. I know they all are quite similar and probably won't make much difference, but it's always hard to make a decision.

[tzsteve]

you will be mighty suprised how much difference 5 degrees makes i`ve not had much experiance of anything smaller than 125cc but at a guess the 1.6 gasket and exhaust up 1mm to 1.5mm would seem right to me

[henri]

for me its a toss up between the 2.4 gasket an steves option , with the 2.4 just edging ahead on transfer duration .H

[potatopl]

Engine work has been put aside for a moment as I'm rebuilding another one for my mate's wife's PK.

By the time I started to reassemble the chassis:

[leon]

Looking nice. Just the right level of scruffy/lived in look. How did you get the black off without taking the blue off? The indicators work well too.

[tzsteve]

looks like a lot of work gone into the frame

[henri]

a rag n thinners can take paint off a layer at a time , very slow n labour intensive . H

[leon]

I see, all the more impressive then. I love the result.

[potatopl]

It wasn't that much work. In fact I've chosen that way of resotration because it was the easiest and cheapest. The old paint was went off with lacquer remover and acetone. It was only one layer fortunately. Took me 3-4 "sessions" to remove it completely.

[potatopl]

Working on the crankshaft.

Inlet opened up from 110° BTDC->50°ATDC = 160° to 128° BTDC -> 58° ATDC = 186°.

Before:

After:


Tried to balance the crankshaft by opening the window in the counterweight but it didn't help much.

Before cutting:

After cutting (stock PK80 crankshaft on the right):

After opening the window:

[potatopl]

Getting there, halves are already together. Today will spend some time on fabricating base gaskets from aluminium plates. Still haven't decided yet what timing to go for.







Also done some tests of cylinder head turning on an old barrel. Results are satisfying.

[tzsteve]

novel way of holding the barrel

[potatopl]

Mate made this flange for cylinder heads a while ago. We couldn't think of better way to hold a head paralell to the turning plane. Takes some time to set it up with a micrometer but it works.

Since 75cc barrel fits inside we've decided to try it.

[kev]

Mar 2, 2016 15:57:35 GMT tzsteve said:

novel way of holding the barrel

That's what I thought ! I'd have put it in a 4 jaw chuck myself  

[tzsteve]

i`d have used a mandril

[kev]

Mar 2, 2016 19:34:52 GMT tzsteve said:

i`d have used a mandril

One of many ways to hold the head .

[potatopl]

Base gasket from 2mm alu plate:




With 2 additional paper gaskets 2,57mm:



Matching with cylinder foot:



Transfers full opened at BDC:



Edge:



Turning:




Squish:


Timing check:


- Inlet 186° ( 136° BTDC -> 55° ATDC )
- Tr 125° Ex 180° BD 27,5°
- Squish 1,35 mm