470 Pro-circuit

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The bike was not complete when I bought it. The cylinder, piston and head were sitting on a set of cases that were hollow except the crank. The cylinder had a stock 430cc bore but was scored. So, I had to have it bored to fit a 86.42 piston.

When we assembled the crank into a good set of cases with a tranny, we noticed the the crank pit had been welded. We did not think much of it and just assumed the rod had been replaced at some point in the last 37 years the rod had been replaced.

It took a couple months for me to get a Wossner piston and have the cylinder bored. When we were assembling the top end, we noticed the piston skirt was hitting the top of the crank on the down stroke. At first I thought I had bought the wrong piston. Once we confirmed the new piston matched the original piston, we noticed the original piston had notches in the skirt to clear the crank. Then we measured the stroke to be ~81mm and the piston extended over the top of the cylinder at TDC. But the head was machined out to give clearance for the piston on the up stroke.

It also has a short thread spark plug B8HS instead of the stock B8ES. This is probably necessary as a result of the head being machined for piston clearance at TDC.

I assembled the engine without taking any pictures. But this is what I know:

Bore - Originally stock 430cc at 86.00mm
Stroke - ~81mm (measured with calipers)
Cylinder porting looks to be original.
Piston porting matched the replacement Wossner piston.
No spacer under the cylinder - head modified for piston clearance
Rod length - I don't know. (I may be able to calculate it based on the clearance needed for the piston skirt and the added 7mm stroke.)
Exhaust - I don't know, there was no pipe with the bike when I bought it.
Jetting - Again, there was not carb on the bike.

I haven't tried to start it yet. We have kicked it through and it feels good with a little more compression than the nephews stock '82 430 XC. But I haven't gotten a carb for it yet.

Bob

Ya, but the bikes are 37 years old! So let's post!

Hi Bob
Good to hear from you.
When I read again the publicity for the 470 PC kit it is written:
-This is done by modifying the crank, head and if desired a port job on the cylinder.


So when I read you I understand why that corresponds to the work done on the head.

I understand also that the inscription written on this cylinder: /81 MOD is linked with the stroke (81 mm) and not with the year (1981)

I've measured the distance between the ring ans the top of the piston and it is 3.3 mm.
Is there not a risk that the ring could go out the top of the sleeve or be damaged with a stroke with 3.5 mm more ?

Hi Bob
When you'll open your motor next time, can you take pics of the inside of the head ?
Can you also take pics of the used piston to see how the skirt has been rectified ?
If you can also measure the rod lenght it would be good.
Thanks
Michel

Picture of notches in skirt of the original piston...

Hello Bob
The machining in the thickness skirt piston is like on every 430 piston.
I thought you meant holes in the skirt to let the crankweights run.

My 430 piston skirt.

Sorry, I should have been more clear with my description. I circled the areas on the skirt in this attached image.

I did 85 of those stroked cranks sold a lot out of country. never stamped or marked them so who knows what was put on them after they left here. One way to tell is I spot-welded the sleeve to the crank wheels then the pin to sleeve sometimes pin to sleeve to crank if the welding pulled it a little. Found several when I moved can't remember which container they are in some day maybe will look.
Later George

Don't seem to have been machined. Looks like the piston skirt touch the crankweights when down.

I agree...

Me too ! I agree myself !

I removed the top end from my 470 for some maintenance. Here are a few pictures of the head and piston at TDC.

Hi Bob
Thanks for the pics.
So the stroke is longer than standard, the piston is higher.
The headstep's pic show us how it is done !
I need to check and compare with a standard 430 head.
No porting ?
Thanks a lot
Michel

I do not know if the cylinder has been ported. I do have another 430 cylinder to compare.

What is the size of the step ? 86 mm or a little bit more ?
Thanks