Bantam piston.

[scousebantam]

I have yet another question about my Bantam resto, this time regarding advice on pistons.
The engine was seized and I have managed to free it, when I got the barrel off everything looked good until I picked the barrel up and the
lower skirt was broken. I have taken it to a local engineer to see if he can weld it and grind flush. My question is, the number on the piston is;
90-1432 and below that is a number 1. In the spare parts book for D3 and D7 are;
D3 - 90-337, 338, 388, 789, 791, 868, 871 and 873  8 pistons in all.
D7 - 4 15 90 - 1311, 1312 and 1313  total of 3
Anyone with an idea on the number?
I can't do anything like buy new rings (because they may not match the D3 and D7 numbers above), and if the barrel is a write off I will have
to buy a new barrel, with maybe piston and rings to match.
 

[Allan G]

Don't weld the barrel, it's cast iron so it's a good chance that if it doesn't distort it will separate again.

[Spaceman]

It's hard to find this part number in one of spares catalogues although it seems to be a standard 61.45 mm, 2-ring Bantam piston. The only reason I can think of for it differing from the STD D5 piston (90-1305), is if it is slightly higher compression ratio.  However, it's a fair guess that both use the same rings.

In terms of the damaged skirt, if this is only a relatively chuck missing from the bottom, it may not make much difference to the way the engine runs and therefore does not need to be repaired (probably impossible) or replaced. Any chance of a pic?

[scousebantam]

Hi Guys,
Allan G, Yes I know it's cast iron that is why I didn't do it my self, I have taken it to a cast iron welding company (excuse the pun), they said that if they can do it they will get it ground so that it will work, if not it is a right off anyway.

Spaceman, it broke where the skirt and barrel meet so as I said above it was pretty much a right off. In reply to your other remarks, it's a D7 motor with two piston rings, no oil scraper ring.

The barrel is still in the welders and I never thought to take a picture, I initially thought that I had broken it as it was a seized motor but when I looked at the broken piece I noticed that either side of the broken piece it was black with about only 3/4 of an inch nice shiny cast, so it may have gone while I was riding it causing even more damage. Lucky in that respect I guess.
I agreed with the welding company in that it was a right off any way so nothing lost in trying to weld it.
I thought that it may be an original piston with having the number 1 stamped on it, but I am not an expert.           

[Spaceman]

The same comments apply to the D7 pistons. I believe the D5 and D7 essentially used the same pistons except the later D7 pistons changed to 3 rings. I am sure the piston rings were the same on all the 61.45 mm 2-ring pistons and may well have been the same as the compression rings in the 3-ring sets.

[scousebantam]

Spaceman that's useful information anyway, because if the barrel is repaired I will only have to get a couple of rings, thanks.

[Allan G]

Spaceman that's useful information anyway, because if the barrel is repaired I will only have to get a couple of rings, thanks.

Check it for clearance and have it honed or bored.

D5/D7 barrels are (or at least used to be) pence especially compared to D14 barrels (which had the larger transfer ports), I probably have a spare one blocking up my garage.. (amongst the other collection of Bantam and A65 bits I have collected over the years).

[scousebantam]

Allan G, that would be great, if you do have one let me know and I will PM you, cheers.

[scousebantam]

Allan G, don't let me stop you looking in your garage but, at a bike Auto Jumble at the week end I picked up a barrel for my Bantam.
There was a  piston in the barrel, the piston had the No 17952 embossed on the inside and .020  R8 stamped on the crown, I am guessing
that it has been re-bored.
The piston in the barrel that I bought has no rings on it, I tried to see if the rings off my original would fit and it doesn't, the thickness of
my original piston is 0.092" (2.33mm) and will not fit in the grooves of the piston that I picked up at the week end.
Can anyone advise please?   

[JulianS]

17952 is Hepolite number for D10/D14 piston.

These take rings 1/16 1nch wide.

The D7 piston takes rings 3/32 inch wide.

[scousebantam]

Thanks JulianS, that's good info.

[scousebantam]

Hi all, just had an after thought, I have been on to Heplolite about rings but, how can I be sure that the piston in the barrel is the right one, after all is was an Auto Bike Jumble.
Anyone know what the gap should be between the piston and the inside of the bore should be?

[scousebantam]

I forgot to say in the last 2 posts, got my cylinder barrel back from the welders, going to make a plant pot out of it for the wife.

[rhyatt]

From memory, the piston skirt to bore clearance is .004 to .005"

[scousebantam]

Many thanks rhyatt.