deck height ?

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Ian.Taylor
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deck height ?

Postby Ian.Taylor » Mon Feb 06, 2017 6:37 pm

From the photo,is this normal? This isn't an Ariel Piston but some aftermarket purchase many years ago by one of the owners.
My 31 VF NOS piston sticks out like this as well and for the Life of I can't remember if my VHA does so or if it is flush.
This might explain the very thick base gasket they used.

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Re: deck height ?

Postby Simon.Gardiner » Mon Feb 06, 2017 9:30 pm

Ian
I don't think that's normal for an original piston in a VH but you don't have an original piston....
The issue will be if any part of the piston 'dome' hits the inside of the hemi in the cylinder head (or if the piston rings are almost out of the barrel at the top - or if you've got a ridge at the top of the bore and the rings now have to traverse it to reach the top of the stroke).
I don't think the compression ratio is anything to worry about.
(Dunno about a VF though.)
Hope that's of some use - I expect there will be some more informed comments along shortly!

SG
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

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Re: deck height ?

Postby Leejm » Mon Feb 06, 2017 9:34 pm

Ariel pistons have a small dome in the middle only with a 6.8 and a little larger one with 7.5 to 1. Your piston dome goes to the edge no flat part. Just what the compression rate will be is anyone's guess. I would check how many cc's the head and work the ratio out. I wouldn't go much over 8 to 1 with an iron engine. As long as the valve miss and the compression ratio is not too high then there will be no problem. Others may say different to me but that is what I would do Lee.
1948 NH, BSA D10 SPORTS. 1953 VHA, 1951 KH rigid project.

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Re: deck height ?

Postby Ian.Taylor » Tue Feb 07, 2017 1:49 am

If the Dome was Flat or concave the extrusion wouldn't bother me so much. Doesn't effect the VF any (Concave Piston )
Tonight after work I'll remove the barrel and compare height with my original 53 VHA Dished Piston just to have an idea how they compare.

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Re: deck height ?

Postby Ian.Taylor » Tue Feb 07, 2017 5:51 am

This photo shows the original VHA piston next to the aftermarket Piston in the VH.
Same height !
What I think may have happened and It maybe the same as my 31 VF ,my 31 had a new liner machined to fit back in the 80's and my NOS piston protrudes a bit .I think it maybe the same with this one. I would think since my 31 runs great this should be no different.

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Re: deck height ?

Postby adrie.degraaff » Tue Feb 07, 2017 6:48 am

You can maschine some of the piston top in that 20? degree angle and a 3-4mm allu plate under the barrel than you're good at 1 to 6.5.

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Re: deck height ?

Postby pappleton » Tue Feb 07, 2017 7:36 am

How do the pistons compare weight wise Ian ?
Paul Appleton '53 VH plunger, '53 VHA rigid - in many boxes, '58 H.D. pan/shovel rigid

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Re: deck height ?

Postby RTichard Woolnough » Tue Feb 07, 2017 8:05 am

When I purchased my MSS Velocette from the original owner it had been rebored to its maximum and at the time MSS max oversize pistons were not available, the builder therefore simply fitted a decompression plate under the barrel, a gasket each side thereof, to bring the compression down to MSS spec, I know it's not an Ariel but the principal applies. kind rergards

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Re: deck height ?

Postby Simon.Gardiner » Tue Feb 07, 2017 2:30 pm

The 'dome' on the VH piston (on the right) is very similar to an 'original' 7.5:1, but the edge of the 'dome' on my 7.5:1 is flush with the top of the bore.
(There's a photo of the new Club Omega 7.5:1 here:
viewtopic.php?f=4&t=326&hilit=what+pistons+fit+my+ariel&start=60)

SG
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

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Re: deck height ?

Postby Ian.Taylor » Tue Feb 07, 2017 6:29 pm

With both pistons in my Hand they seem to be about the same.Not scientific I know. I do have a 9.5:1 piston i had in my VHA back in the late 70's .It was heavy and the vibration was awful..lol but boy did it pull !

Whoever installed the liner ,I'm guessing they didn't Press it in to the correct height and that's why the top of the piston sticks out a bit..like I said my 31 is about the same..The VHA Piston measure 81.3mm and the aftermarket has 81.3 stamped on the top. .We had to have the cylinder honed some more as the clearance was only .003.

Edit
Just a thought..the cases are 47 NH with VH crank etc..would the cases be slightly different to cause the cylinder to sit lower than a VH case?
I always thought the cases etc were the same..


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