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By m1ks
#1021
First of all.


Gaaaaaaaaarrrgghhhh!

Got a set of new piston rings in the post with head and base gaskets etc when I was contemplating re-using the old piston.
Fitted them to the bore to check the end gaps, the end gaps were immense, I could measure them with the external prongs of the vernier calipers!
Thought, this just doesn't seem right, checked diameter of an old ring and the new with the gap closed up, the old ones are bigger.
Dug out the old first head strip pics and had another closer look at the piston crown, (I don't have internal micrometers and the only external I have is a narrow range so measuring said bore and piston accurately is not possible), on the crown under magnified inspection the bit that I thought said T040 looks like +040, (makes more sense), so it appears I already have a +0.40 oversize bore and piston already.
Will upload a pic in a mo, so I think I can say the head and barrel have been off before, the thing has been overbored and larger piston and rings fitted and tehn to add icing to the cake a previous owner got lairy with the black ATV silicone on every mating surface.
So bloody annoyed right now.
It's looking like a piston and barrel set is on the cards but i'm at the point where i'm seriously considering binning the lot as a needs rebuilding / project and looking for a 500 that's had someone with a modicum of mechanical sympathy caring for it in the past.
The thing with that is it'll then have defeated me but if I spend on the bits and get it up and running again i'll be disinclined to sell it having spent time and money and then be wondering what else may have been bodged!
By MadMike
#16355
OK so the barrel has been bored +040, simply get a set of +040 rings. Where are you based? If you are close enough to Leicestershire then I can measure the bores for you. If not can you get somebody to measure the barrel for you. That is a more relevant dimension, as the +040 piston and rings may no longer be OK, in which case can't you simply rebore +060 and fit new piston and rings? HTH.
By m1ks
#16356
I was under the impression +0.40 was the largest size overbore piston?
if you check my recent thread about the seizure you'll note there was scoring, which may hone out, but may not.
The piston really needs replacing as well as the rings so.
I'm a bit reluctant now to buy a +0.40 piston and rings and re-assemble everything to find it's still not right and have to go for a new barrel and piston anyway hence wondering whether to just get a new barrel / piston set.
Unfortunately i'm nowhere near leicestershire, i'm in the scottish highlands near Skye but thanks kindly for the offer.
Right now i'm feeling the need to do a Basil Fawlty and take a tree branch to it whilst yelling obscenities, (or preferably to the previous owner and his love of half arsed mechanics)
By Alan R
#16357
Hi matey !! Don't you just love 'em--- these 1/2-arsed "mechanics" ???. If you can obtain a used but ok Std. piston ---you could then have the barrel re-lined at STD size. You already now have a set of new, Std size rings and you will have everything back to original sizes. Get yourself a set of these }--------------http://www.ebay.co.uk/itm/Telescopic-Bo ... hash=item4 ------------ now you CAN measure your cyl.bore AND accurately too !! PS The shop has been trading for many a year and are OK.
By Les H
#16361
Here's another option mk1s. Get the barrel rebored to for a 535 piston...you get a slight rise in comp ratio (7.3:1) plus around 7% extra capacity probably given you something like 8% more power. This will also mean you are starting afresh with the rebore size. Just an idea.
By Les H
#16362
Also noticed the piston appears to be a JP one!...They have a reputation for seizing unless set up with a big clearance so this might well explain your seizure that has put you in this position. So not the engines fault.

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