• Welcome to the website of the Vincent H.R.D. Owners Club.

    Should you have any questions relating to the Vincent H.R.D. Owners Club, or Vincent H.R.D. motorcycles in general, please contact Graham Smith, Hon. Editor and Webmaster by calling 07977 001 025 or please CLICK HERE.

    You are unrecognised, and therefore, only have VERY restricted access to the many features of this website.

    If you have previously registered to use this forum, you should log in now. CLICK HERE.

    If you have never registered to use this website before, please CLICK HERE.

ET: Engine (Twin) Piston choice.



oilyrag

Website User
VOC Member
#1
I have decided to check the bores as it's still on standard with way past 100,000 miles on the clock(s).

So what is the advice regarding the piston choice?
7.3 :1, or 8:1?
It's a Touring Rapide so performance not necessary.

Are the modern pistons the same weight, so I can I avoid having to rebalance the crank?

Anything else I need to know?

Thanks,

Chris.
 

greg brillus

Well Known and Active Website User
VOC Member
#2
The higher the compression piston the more efficiency and energy you extract from the fuel. Anything past about 8:1 and the engine will be inclined to detonate as the combustion chamber shape was better suited to slower burning leaded fuels from years ago. The available pistons either Omega's or the CP forged pistons are both excellent and the 7.3:1 pistons will work fine and be easy to start. The recommended piston to bore clearances are minimum, I live in a hot climate so I always give them at least one thou extra and never had a piston nip up..........Something to think about..........Cheers...........Greg.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
#5
I have decided to check the bores as it's still on standard with way past 100,000 miles on the clock(s).
So what is the advice regarding the piston choice?
7.3 :1, or 8:1?
It's a Touring Rapide so performance not necessary.
Are the modern pistons the same weight, so I can I avoid having to rebalance the crank?
Anything else I need to know?
Thanks,
Chris.
I am fitting Spares Co. 8:1, +0.020 Omega pistons. I weighed them at 470 grams each, approximately 20 grams lighter than the standard Omega 7:1, probably not enough difference to warrant a rebalance. The std pistons gave a balance factor with my crank of approximately 45%, yours may be different depending on what may have been done to your crank previously. e.g. Rebalanced to a different factor, different big-end may be a different weight, your std pistons a different weight, etc.
I have rebalanced my crank to a little over 50% as that seems to be the current thinking about balance factors. We will see what the Rapide is like when I get it back together. (Lethargy is winning at the moment and other things keep getting in the way.:rolleyes:)
 

Albervin

Well Known and Active Website User
VOC Member
#6
I am fitting Spares Co. 8:1, +0.020 Omega pistons. I weighed them at 470 grams each, approximately 20 grams lighter than the standard Omega 7:1, probably not enough difference to warrant a rebalance. The std pistons gave a balance factor with my crank of approximately 45%, yours may be different depending on what may have been done to your crank previously. e.g. Rebalanced to a different factor, different big-end may be a different weight, your std pistons a different weight, etc.
I have rebalanced my crank to a little over 50% as that seems to be the current thinking about balance factors. We will see what the Rapide is like when I get it back together. (Lethargy is winning at the moment and other things keep getting in the way.:rolleyes:)
I am interested in the 50% + balance. I am about to rebuild my Shadow with 8:1 CP pistons and was going for a static balance of 45%. I will probably need to rebuild the crank and maybe need new con-rods so a re-balance will need to be done anyway. Please explain the 50%. Thanks.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
#7
I am interested in the 50% + balance. I am about to rebuild my Shadow with 8:1 CP pistons and was going for a static balance of 45%. I will probably need to rebuild the crank and maybe need new con-rods so a re-balance will need to be done anyway. Please explain the 50%. Thanks.
Dear Alan,
I think that there has been lots of discussion about balance factors and what is right or wrong but I recently read this article by Neville Higgins in the "Resources" area 3.2. The article is about balancing a crankshaft but if you don't want to read all of it the last paragraph on page #4 talks about balance factors. I have put a link to the resources article below:
Resources link
 

Vincent Brake

Well Known and Active Website User
VOC Member
#8
Dear Alan,
I think that there has been lots of discussion about balance factors and what is right or wrong but I recently read this article by Neville Higgins in the "Resources" area 3.2. The article is about balancing a crankshaft but if you don't want to read all of it the last paragraph on page #4 talks about balance factors. I have put a link to the resources article below:
Resources link
Strd 46 makes its jump a bit up n down.
Going over 52- makes it shake a bit for- and back wards.
In a stndrd frame.

My experience
And indeed conrods can differ quite some yes
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
#10
The link I put in in post #7 above takes you to the relevant article but the paragraph referred to isn't on page #4 but earlier and says:
"A number of years ago I visited Bob Dunn, and when we talked about balancing he told me he had recently done a check on a number of engines, finding that those having a balance factor around 50 per cent were good, 45 per cent were OK, at 40 per cent they began to vibrate, and down to 30 per cent they were really bad. Some people running Norvins claim that a 60 per cent factor gives them a smooth engine. Since then I have aimed at 50 per cent for my engines, and Jindivik is currently running 50 per cent and The Heap 53 per cent. I cannot claim that they are noticeably better than when they were at 46 per cent but it seems that it is not very critical, so if your engine comes out at 46 per cent to 50 per cent you should be all right. "
 

Black Flash

Well Known and Active Website User
VOC Member
#11
I would always go for 8:1 pistons to boost economy and get some extra umph.
Iirc during the period the bikes were built they had to run on low octane pool petrol. If they had better fuel I am sure they would have been built with a higher cr in the first place.
If you are unhappy you can always fit decompression plates during the winter. It also gives you the chance to escape for a few hours for a couple of days,iyswim.
 

Bill Thomas

Well Known and Active Website User
VOC Member
#16
Funny the Bike with 36mm Dellortos, Starts OK, Because it has proper Chokes, But I don't have the Mk2 Amal
chokes sorted yet, For fast idle. Cheers Bill.
 

stu spalding

Well Known and Active Website User
VOC Member
#17
I have cracked three 7.2/1 comp pistons and I am convinced that obtaining the lower comp ratio by machining material from the crown on an 8/1 weakens it. I now run 8/1 pistons with compression plates to get the ratio down to 7.2/1. Cheers, Stu.
 

Glenliman

Well Known and Active Website User
VOC Member
#19
I have an 8 to one Rapide that works well. It's an extremely easy starter.
In a pinch it has run it on regular fuel without any problems. I normally run Chevron Supreme ethanol free , however when out touring sometimes regular fuel is the only type available.
55,000 miles , low e pistons , originally set up with 3.5 thou clearance .
Honda Chrome rings and no ridge, bore wear negligible.
Glen
 
Last edited:


Top