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deck height ?

Posted: Mon Feb 06, 2017 6:37 pm
by Ian.Taylor
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.

Image

Re: deck height ?

Posted: Mon Feb 06, 2017 9:30 pm
by Simon.Gardiner
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

Re: deck height ?

Posted: Mon Feb 06, 2017 9:34 pm
by Leejm
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.

Re: deck height ?

Posted: Tue Feb 07, 2017 1:49 am
by Ian.Taylor
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.

Re: deck height ?

Posted: Tue Feb 07, 2017 5:51 am
by Ian.Taylor
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.

Image

Re: deck height ?

Posted: Tue Feb 07, 2017 6:48 am
by adrie.degraaff
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.

Re: deck height ?

Posted: Tue Feb 07, 2017 7:36 am
by pappleton
How do the pistons compare weight wise Ian ?

Re: deck height ?

Posted: Tue Feb 07, 2017 8:05 am
by RTichard Woolnough
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

Re: deck height ?

Posted: Tue Feb 07, 2017 2:30 pm
by Simon.Gardiner
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

Re: deck height ?

Posted: Tue Feb 07, 2017 6:29 pm
by Ian.Taylor
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..