RichardW's Velocette rebuild

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RichardW
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Re: RichardW's Velocette rebuild

Post by RichardW » 11 Sep 2019, 08:11

I tried a top end leak down test, and although I could get it to push the piston down, it wouldn't hold any appreciable pressure. I tried the bag over the carb inlet again, and it puffs out at some points, but there is no appreciable suck.

Took the barrel off, and re-measured. Piston is 63.8mm or thereabouts, which looks to be +40. Bore is around 64mm at the base, but it's not completely round! Ring gap is about 0.8mm at the base, but increases significantly towards the top of the bore (although it's very difficult to measure!) - up to about 1.3mm, although the ring is still holding the wall. When the piston is pushed up the bore it gets easier in the top inch or so, suggesting to me that the ring pressure is reduced - you can feel this effect on the kick starter in fact, and when the engine stops it always stops at about where this easier bit starts..

Hopefully it can be sleeved, otherwise it's going to get expensive....!!

moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau » 11 Sep 2019, 17:13

RichardW wrote:
11 Sep 2019, 08:11

Took the barrel off, and re-measured. Piston is 63.8mm or thereabouts, which looks to be +40. Bore is around 64mm at the base, but it's not completely round! Ring gap is about 0.8mm at the base, but increases significantly towards the top of the bore (although it's very difficult to measure!) - up to about 1.3mm, although the ring is still holding the wall. When the piston is pushed up the bore it gets easier in the top inch or so, suggesting to me that the ring pressure is reduced - you can feel this effect on the kick starter in fact, and when the engine stops it always stops at about where this easier bit starts..

Hopefully it can be sleeved, otherwise it's going to get expensive....!!
That'll be it then, from memory so may be wrong, about 0.2mm minimum gap going up to 0.7mm. 1.3mm is enormous. Is this a single of dual ring?
I bet if you could go back in time and have a chat he'd say "I'd kept it going by hook or by crook, in the end it would only start on a bump down a big hill, it was already on max oversize for the bore and another bike came up so it went in the shed"

Has the barrel got a liner? Regarding sleeves, I had 2 fitted in my 350lc (which had TZ350 barrels) Have a chat to someone who does race bike engines. It will also need to have the ports cleaned up afterwards. I think I paid about £150 for the 2 without porting, this is going back about 30 years!

moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau » 11 Sep 2019, 17:16

Just checked my Z1000 workshop manual (each cylinder being 250cc) and it states 0.3 to 0.5 with a service limit of 0.8mm.

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Mandrake
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Re: RichardW's Velocette rebuild

Post by Mandrake » 13 Sep 2019, 16:05

RichardW wrote:
08 Sep 2019, 21:31
Since it's a two stroke there's oil everywhere, so that's a pretty 'wet' reading!
I don't know a lot about 2 strokes, but surely you still cut off the fuel supply when doing a compression test like you would on a four stroke ?

If you were to leave the fuel supply active on a four stroke during a compression test the un-burnt fuel being injected will wash the cylinder walls clean and reduce the compression since it will wash away the oil film on the cylinder that is helping the rings seal.

In the case of a two stroke you may have the opposite problem - cutting off the fuel supply cuts off the oil supply to the cylinder as well...

So I would still be inclined to do two compression tests - one with the fuel supply cut off, and a second with the fuel supply cut off but after a squirt of oil has been injected into the cylinder. If the second reading is a lot higher than the first the rings / bore aren't sealing, if it doesn't change much it's something else leaking!

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Re: RichardW's Velocette rebuild

Post by RichardW » 14 Sep 2019, 13:46

I think the oil test is to try and figure out if it is rings or top end (head gasket or valves) - but since this engine has neither, if it's low on comp, it's the bore / rings. Have found These guys who say they can bore / sleeve it if required (and have the tools and knowledge to properly assess it!), so I will send it down and see what they say. It's not much use if it takes 2 hours to coax it into life every time I want to use it, as it would never get started or used..... I'm unsure if the piston is round enough to be re-used, and I have yet to definitively track a replacement one down (I've had confirmation that one place has a +30 for a GTP with a slightly different part number, which could well work).

Mean time I need to get on with painting the final bits and putting all the other stuff together, so it can then go down for exhausts when I get the engine back.

moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau » 14 Sep 2019, 14:26

That place looks good, exactly the kind of place I was thinking of, especially as they recon LC barrels. The price of an LC these days you can't afford to have a dodgy job done. They'll correct me if I'm wrong but if it has an iron bore you run chrome rings, if it has a chrome finish you use iron rings. See if you can go back to standard spec. Send them the piston assy at the same time to see what they can come up with.
All the best.

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Re: RichardW's Velocette rebuild

Post by RichardW » 15 Sep 2019, 21:03

Cylinder and piston now in the post, so we'll see what they say - the piston doesn't look that worn, and it measures almost round, so it may well be OK for re-use. I found one (of the two - the other broke getting it off) original rings, which is pretty much identical to the replacements I had made. No idea what the rings are made of - the bore is cast iron at present - by the looks of it their sleeves are cro-moly, given that this is not going to see mega miles, it probably won't matter too much.

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Re: RichardW's Velocette rebuild

Post by RichardW » 20 Sep 2019, 07:40

Hmmm, turns out they can't sleeve a blind bore, despite me making that point in the original enquiry.... back to the drawing board! I might see if I can get some slightly bigger rings to close the gap, and see if that will give enough compression to get it started.

moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau » Yesterday, 00:09

That's a real shame. So either find someone who can sort barrels with integral heads or get some rings made. I'll ask on a bike forum if you don't mind?

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Zelandeth
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Re: RichardW's Velocette rebuild

Post by Zelandeth » Yesterday, 00:20

I could ask one of two of my bus contacts if you like. Know some of the engines there (the Leyland 510 as used in the National for one) have integral heads...so someone there might have some useful contacts.

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Re: RichardW's Velocette rebuild

Post by RichardW » Yesterday, 08:24

Thanks both - any leads gratefully accepted! The bore is only 64mm so it's pretty tight. I had the rings made so he copied the original I think. Found somewhere that has lots of rings, so I'll see if they have something near.

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Re: RichardW's Velocette rebuild

Post by xantia_v6 » Yesterday, 09:58

Can you find someone who can measure the taper on the bore (or get yourself a suitable micrometer), so that you can decide if a rebore/sleeving operation is really necessary?

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Re: RichardW's Velocette rebuild

Post by RichardW » Yesterday, 18:23

xantia_v6 wrote:
Yesterday, 09:58
Can you find someone who can measure the taper on the bore (or get yourself a suitable micrometer), so that you can decide if a rebore/sleeving operation is really necessary?
What, using something like this...
micrometer.jpg
:rofl2:

Looked through the pile of measuring devices I inherited, and this was among them. I doubt I can get an accurate measure as I won't be able to turn the ring at the bottom, but I could probably get the feelers in down the side to get an idea of the bore change and any ovality.

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Re: RichardW's Velocette rebuild

Post by Gibbo2286 » Yesterday, 18:30

I had one of those, wonder what happened to it, probably nicked by one of my young mechanics but I obviously haven't Needed it.

I suppose if it's only a one off job you could use these:

https://www.mad4tools.com/silverline-i ... erline_New