Dunstall 810 Pistons

Hi.

I thought 'd check out the pistons in my newly acquired 810. Charming eh!. I've never seem pieces added to pistons like this. Have you?
So, I need to buy some new 810 pistons to replace the ones shown. They are marked "Dunstall inside them.
If I cant procure any, can I use my alloy barrels which have the std 76mm bore for an 810 and sleeve them down to take a 750 + piston?.
I know you cant use 850 ones as the sleeves will not tolerate anything larger.

Thanks everyone

hillbone said:

Hi.

I thought 'd check out the pistons in my newly acquired 810. Charming eh!. I've never seem pieces added to pistons like this. Have you?
So, I need to buy some new 810 pistons to replace the ones shown. They are marked "Dunstall inside them.
If I cant procure any, can I use my alloy barrels which have the std 76mm bore for an 810 and sleeve them down to take a 750 + piston?.
I know you cant use 850 ones as the sleeves will not tolerate anything larger.

Thanks everyone

Click to expand...

What do you mean by "pieces added to them"
There's nothing added that I can see
It's the way the pistons are machined in a "slipper piston" style
Those 810 barrels were notorious for the liners moving
Are yours ok?

Will probably clean up with a fine (swiss) file . Check clearance and reasons for previous overheating , Timing , mixture etc.

baz said:

It's the way the pistons are machined in a "slipper piston" style

Click to expand...

An odd way to do it, compared to other slipper pistons the weight saving looks to be nil and so you only get the negligible friction reduction and extra piston slap noise.

It's an alternative to oval turning.

You certainly can replace the sleeves with 750 size sleeves. Dunstall sold their cylinders in both 810 and 750 configurations. I've converted 810 kits to 750 a couple of times in the past. In the US, you can buy replacements for Dunstall sleeves in either 750 or 810 bores from LA Sleeve Co. I've lost the part number for the 750 sleeves, but the part number for the 810 sleeves is FL-227. It would be easy to ask them to make liners identical to the FL-227, but with 73 mm bore instead of 76. Or you could just find a local shop that does sleeves, give them your cylinders, and ask them to fit new liners, but with 73 mm bores, and use standard Commando pistons. Or, if you want to keep some of the lost displacement, get a pair of 74.5 mm (+.060") pistons, standard or high compression, as you prefer, and have the cylinders lined for that bore size.

Ken

Thanks everyone. I had never seen pistons before like that. ( sheltered life) . I thought they were added. In the end a company in the USA is machining up new pistons to match the old.
I would follow your advice Icrken if they couldnt make any.
Now to try and figure out why they grabbed in the first place. Obviously oil problem . Time to check everything. Ive never started the thing. Came from Spain to UK to Australia.

I'd say it's not obviously an oiling problem, and guess at a heat problem, as it looks like a "four corner" seizure to me, based on one picture on the internet...

@norton bob mentioned to check timing, mixture, etc. I'd agree.

t ingermanson said:

I'd not say it's not obviously an oiling problem, and guess at a heat problem, as it looks like a "four corner" seizure to me, based on one picture on the internet...

@norton bob mentioned to check timing, mixture, etc. I'd agree.

Click to expand...

I would say it’s not obviously an oiling problem!

I’ve seen it caused just by going up a hill in too high a gear, or by running low on fuel in the tank, or insufficient fuel flow through taps, filters, float needle valves, or tank cap vent blocked.

Or, as said: timing, jetting, bore clearance.

If it wasn't for the carbon on the crowns I'd say it was a cold seizure on first start up
With insufficient bore clearance
Rather than an oiling issue

How many guys jet there carbs to settings recommended by somebody else ? All motors are different and most exhaust systems and cams have variations. A good main jet size is slightly too big. From that point on, it is up to the tuner to get it right. If I was a beginner, I would find somebody who has raced a lot, to tune my bike. That is my advice, but I was never smart enough to follow it.