VG barrel/piston/conrod query

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david.anderson
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Re: VG barrel/piston/conrod query

Postby david.anderson » Thu Jul 12, 2018 9:58 am

The parts books list the conrod spares no as 1100-35 and part no as A7/524 for both the VG and VB conrods, so they do use the same conrod.
The cylinder should measure 142mm from the bottom of the base flange to the top of the top flange (spigot). The cylinder base is 25.4mm thick. (I have checked 2 cylinders and a 3rd one in a bike also has the same height base flange)
It was common here in Australia to remove .125” from the base flange to raise the compression ratio. One of my cylinders had that treatment. However your cylinder may have material removed from the base flange and top flange as it is 4.5mm too short.
It is normally possible to have the piston crown altered a little (if necessary) to accommodate the shorter cylinder. there is no problem with valve clearance with a standard cam.
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Re: VG barrel/piston/conrod query

Postby DAVE.THOMAS » Thu Jul 12, 2018 10:13 am

Thanks David, that explains it then ! I’ll ask on here if anyone has a stock height barrel before I go further. A shame as I just rebored it

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Re: VG barrel/piston/conrod query

Postby paul.jameson » Sat Jul 14, 2018 10:26 pm

As you have already said Dave, one answer is to make a compression plate of suitable thickness to go under the cylinder, possibly with longer base studs to suit.
What has baffled me is why someone would shorten a cylinder like this but I now have a theory. Perhaps someone wanted to use short stroke flywheels (= 350 flywheels) and a VA piston. The VA pistons are vastly more common than actual VA bikes and are 81.8mm bore but have a much bigger distance from gudgeon pin to piston crown. So if a +0.030 VA piston is of any use to you, let me know. I only bought it for the rings and gudgeon pin as spares.
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), 54 4G Mk IV (project), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.

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Re: VG barrel/piston/conrod query

Postby nevhunter » Sun Jul 15, 2018 12:01 am

The stroke variation 350/500 is only 5 mm off the barrel For 10 mm of difference in stroke. The 350 rod is much less in length and would enable the VA piston to be used perhaps with some off the bottom of the skirt to clear the flywheels required. Any time you change the barrell length you must alter the pushrod and pushrod tubes length. Nev

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Re: VG barrel/piston/conrod query

Postby adrie.degraaff » Sun Jul 15, 2018 6:58 am

The easiest way is making a 4.5mm headgasket of aluminium or brass including the linerridge.

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Re: VG barrel/piston/conrod query

Postby david.anderson » Sun Jul 15, 2018 9:47 am

Dave
You can still use your cylinder. As I said I have one cylinder that is 1/8” shorter as the base flange has been reduced. I had a running 50VH that needed a rebore and as I had a spare cylinder I took that, had the broken fins repaired and had it bored for a new piston ready to fit into the bike. I then removed the head and cylinder from the bike and fitted the new cylinder, at which point I found the height difference. I initially cut a 1/8” aluminium compression plate under the cylinder and used that for a short period. Then I removed it beveled the top of the piston a little and I used the shorter cylinder for with no compression plate for years. There is no problem with valve to piston clearance with the 1/8” shorter cylinder. I did shorten the pushrods and pushrod tubes to suit and fitted the ends back with soft solder.
As your cylinder is 4.5mm short it would not be a problem to fit a 1/8” compression plate under the cylinder and not worry about the fact that you are still 1.5mm short on standard. 1.5mm will only marginally increase compression ratio with a 6.8 or 7.5 piston by half a point to just over half a point. And you will not have to alter pushrod tube length with such a small reduction.
What I found interesting was that when I went to measure a cylinder after reading your post, I first measured the height of a cast iron cylinder, and then I measured the alloy cylinder that I had recently removed from my 57 VH when I converted it to a short stroke. That cylinder was just over 1mm shorter. I then went looking for the second iron cylinder and it was the same as the first. I have run that alloy cylinder in my 57VH for the 46 years that I have owned it and not known until now. I knew that the original owner used my 57VH in competition work so it is fairly certain that he was responsible for that modification. Interestingly the Ariel factory HS tuned bikes also had 1mm removed from the base flange which bumped the 9 to 1 compression ratio up to 10 to 1 and the HP to 38
David

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Re: VG barrel/piston/conrod query

Postby DAVE.THOMAS » Sun Jul 15, 2018 1:50 pm

Thanks all..
Paul - my cylinder is +60 but thanks for the offer
David - I have some 2/3/4mm aluminium sheet so I’ll prob use 3 mm and make a spacer. The piston is a fairly flat top omega so I may bevel edge a little too


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