Pistons problemo!

Pylonracer said:
Did you know a chap called Dave Farmer who raced Mondial bikes in the late 50's/ early 60's. He told me that he was a works rider, he also came from the Southampton area.
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The name seems familiar, but I did not go to England until 1971.
 
tonyfoale said:
Be very careful with this much welding on a piston. The welding itself can be very difficult because cast aluminium items, particular of this vintage, can be a bit porous and years of sitting in oil tends to fill the pores with oil. Then no matter how well you clean them externally, they start weeping oil as you start to weld and then that's the end of it. Even with the right rods the piston will be softened unless heat treated after welding and heat treating can be difficult unless you know the alloy that was used originally.
I tend to think that your best bet is to look for new pistons that are close to what you want. There are a few piston makers in the UK, Omega for one who make specials. There are a whole stack of them in the US, probably a few in Germany too.
Have you tried asking around on classic bike forums? Try asking on this one https://www.facebook.com/groups/367147517553916/
it has nearly 50,000 members so maybe there is someone there who can help. Maybe there are groups specialised in Douglas?

PS. machining businesses with CNC machines often charge what may seem to be exorbinent prices but they have to often break down whatever they are set up for, then maybe make some tooling to hold a special job and then put their machine back to what it was. However, there are many Chinese companies that offer very low volume CNC machining at very good prices even with shipping. I have friends who have used these and the quality is good. I know that there is an Italian company that does the same I will try to find their details. There are two alloy grades that are used for billet pistons and I will try to remember to dig that out also.

PPS. Another option. Rather than welding the pistons I think a better option would be to machine the grooves a touch wider and get some rings made. I have seen adverts for making rings from centrifucally cast cylinder liners. Which is probably what Douglas did originally.

Here is a tip for holding pistons for machining in a lathe if you decide to machine what you have.
https://www.homemadetools.net/forum/work-holding-lathe-90665#post201593
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OK - so if I understand it right, one turns up a round base that will get gripped by the chuck. It gets fixed to the piston by pulling on a wrist pin via a threaded adapter. I see enough videos on YT of racing obsessed engine builders welding up holes and other damage on pistons to make me think the trick may be just about feasible.

I will, of course, be looking for something that "will do". I will have a search for WiseCo and also maybe check out the CNC options. I joined the London Douglas Motorcycle Club to get access to their spares stock.

The basic requirements are that the pistons have a 5/8" wrist pins, are made to suit a bore 60.8mm +0.020", with appropriate clearances to the piston, and has enough of a dome to get the compression ratio to 6.1 to 1, or so. It would not matter if that value went a bit higher.
 
graham-xrf said:
OK - so if I understand it right, one turns up a round base that will get gripped by the chuck. It gets fixed to the piston by pulling on a wrist pin via a threaded adapter. I see enough videos on YT of racing obsessed engine builders welding up holes and other damage on pistons to make me think the trick may be just about feasible.

I will, of course, be looking for something that "will do". I will have a search for WiseCo and also maybe check out the CNC options. I joined the London Douglas Motorcycle Club to get access to their spares stock.

The basic requirements are that the pistons have a 5/8" wrist pins, are made to suit a bore 60.8mm +0.020", with appropriate clearances to the piston, and has enough of a dome to get the compression ratio to 6.1 to 1, or so. It would not matter if that value went a bit higher.
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Yes, people do weld pistons. I have seen enough wrecked engines as a result.

16 mm is just a touch more than 5/8", so you could widen your search possibilities by including 16 mm and reaming out the bush in the conrod. The increase in size is only 0.125 mm (5 thou in old money).
 
Don't bother welding it! You are making this FAR harder than it needs to be! Have a local home shop machinist turn you out 2 dome-shaped parts, to sit right on top fo the piston, and install bolts from inside the piston to give you the dome shape you want. Even if you have to have cutouts for valve clearance, it's no big deal. Make sure you put lock-washers on the bolts inside the piston. The bolts don't even need to be that large. 1/4- 20 would be fine. You can have these done in a single evening's work, and ready to install in your engine.
 
SteamWilly said:
Don't bother welding it! You are making this FAR harder than it needs to be! Have a local home shop machinist turn you out 2 dome-shaped parts, to sit right on top fo the piston, and install bolts from inside the piston to give you the dome shape you want. Even if you have to have cutouts for valve clearance, it's no big deal. Make sure you put lock-washers on the bolts inside the piston. The bolts don't even need to be that large. 1/4- 20 would be fine. You can have these done in a single evening's work, and ready to install in your engine.
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It must be 1st April.
 
No, it's the 15th of June. Why do you think that this will not work? I have seen it done, it was perfectly successful, and was fast and cheap.
 
SteamWilly said:
No, it's the 15th of June. Why do you think that this will not work? I have seen it done, it was perfectly successful, and was fast and cheap.
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Firstly, it does nothing to address the problem which is corroded ring grooves.
Fast and cheap is usually the enemy of good.
 
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