DickV wrote: ↑Tue Aug 25, 2020 7:06 am
Hi Knut, I am not familiar with all the Model numbers of the G80's . Mine has serial number G80/135275. The thing I was trying to say was, the rings from the Shovelhead will quite easily fit my piston which is stock 8.7-1 .
Dick, thank you for your advice on Shovelhead rings. You seem to have a roadster engine? If so, CR ratio is a modest 7.7:1. The figure you quoted is for the 1956-59 CS engine. Combustion chambers differ, so a CS piston will not fit a roadster engine without further modifications.
Cheers, Knut
Last edited by mdt-son on Wed Jul 12, 2023 1:04 am, edited 2 times in total.
G80CS piston - 1960 onward attached picture
I have had a quote for manufacturing , but quantity is 300 off pistons inc ring / pin/circlips , mixed sizes ie std , + 20 , + 40 , + 60 etc..
Unfortunately without the guaranteed demand I cant justify manufacturing
they would have been circ £80 each
I also looked , and they just are not available , despite wassell selling new Aluminium barrels (currently out of stock awaiting new supplies )
G80CS piston 1960 on.jpg
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Thank you g80csp11. A minimum quantity of 300 is excessive. I've heard of vendors offering custom pistons for a minimum quantity of 10. Maybe the high quantity number is due to gudgeon pin and rings being imperial size wrt. thickness, hence no commonality with existing pistons. Which company did you ask for a quotation?
Do you have a sample from which a proper drawing can be made?
Cheers, Knut
The problem is that I can get 12 made by Omega (min quantity) , but my previous quote for billet machines with such small quantities was nearer £230 each + VAT. NOTE 2021 price , so most likely >260 or more now !
Billet machined pistons in small quanties are very expensive
Yes I have samples , currently with my supplier of a new 12:1 to copy skirt profile , and used 8.7:1 for general shape
I have another supplier that im working with that I will make inquiries on likely cost
Forged pistons incure a tooling cost of approx 3500 , but then pistons are circ £150 each
Hi G80csp11,
I was lucky to find a stock 8.8 CR piston which will serve as a sample for a digital model.
I am thinking time is ripe for a redesign of cylinder, conrod and piston. Ken Dr Groome fitted a G50 conrod and shortened the barrel in one of his racing engines. I think he kept a stock piston. I believe a modern piston with a high location wrist pin is the way to go. Absaf kept the barrel height and fitted a longer conrod. I find a shortened barrel along with the stock conrod (7/8" wrist pin size) and a piston derived from the G50 is the way to go.
Conrod ratio is 174.6/85.5=2.04 which is a good value. Long rods typically provide more power from mid range to max rpm.it also promotes a longer dwell time at TDC which is beneficial for combustion efficiency.
- Knut
Last edited by mdt-son on Sun Jul 23, 2023 9:06 am, edited 3 times in total.
Ive got another quote for 24 pistons to the 1960 onward dimension at approx 50% of the Omega price , when ive got a firm price i can let you know .
The problem is that there is a order quantity Vs price relation ship , and if this is done on a commercial basis you have to add profit and VAT to the price
unless you can get enough members to get together to fund the exercise its not viable . not enough members realise that a set of rings from Cox and Turner will cost about £38 , which does not leave mush for the difficult part Piston , pin and circlips
1956 - 1959 CS piston in the UK (Feked) is £144.95 + VAT and delivery
pre 60 hepolite on ebay today Hepolite 13380 , bidding at £104
what will people pay for billet machined pistons using original Hepolite as a sample ?
Hi G80csp11,
The offer sounds interesting, but I fear you will face a hard time lining up 24 customers. Offering a STD and a +0.020" piston in one package may sweeten the deal and bring the required number of customers down to 12.
Is the core piston cast or forged? Value depends on quality and alloy composition. Are pistons available in low expansion (US) 4032 alloy? A good piston is worth about £200 excl. VAT I guess. That's the price of a custom cast 4032 piston made for the Norton twins.
I wonder how Grove Classic is able to sell new _complete_ Hepolite pistons to suit the Velo Venom and Thruxton at £129 + VAT ? (Provided prices are current.)
Circlips 000750 are still available from Russell Motors, assuming new piston grooves are identical to the BHB/Hepolite original, so no need to remake them. This may help bring the cost down. Cost of piston rings should also come down if 24 sets are ordered at the same time (expect a discount of 30%).
Please keep in touch by PM when final pricing is set.
As for weight, I came across a note by Don Madden, quote:
"I did not have late short stroke G80 type but did measure & weigh the 1960-up G80CS which is probably slightly heavier due to higher compression. These weighed in at 470.1 (gram) bare & 603.2 (gram) with rings, pin & clips."
Piston bare weight of 500 gram appears correct for the STD size (15710). Another sample in .020" O/S weighs 513 gram including rings and a bare weght of 476 gram. Adding 97 gram for the wrist pin brings total to 610 gram. Circlips account for a few grams only. Note: AE Piston 15710 is not the genuine fitment and may be a little on the heavy side, cf. Don's measurement of a (BHB presumambly) piston.
A NOS Robbins piston providing a CR of about 10:1 weighs in at 460 gram bare (std bore).
For comparison, an Omega forged piston for Velocette Venom and Thruxton weighs in at 541 gram complete, however, CR is somewhat lower at 8.5:1 . Same piston design as a cast sample weighs in at 600 gram. Forged pistons obviously allows piston weight to be reduced substantially.
A sample of a 90 mm bore Hepolite piston for the Seeley G50 (I think it reads no. 15815) weighs in at 512 gram complete. The latter is now an obsolete design of course, and the current design by Omega Pistons may serve as a better template. A revised low height squish band design for the 86mm bore using modern design and materials can be made to tip the scales at 450 gram, that's a whopping 25% weight reduction against the standard piston. Low reciprocating weight benefits longevity of the bottom end, reduces vibration considerably, and releases more torque because less energy is spent on deformation work. A modern piston along these lines is offered by KTT Services for the Velocette 500's. There are two ways to realize this design: Either by shortening the barrel, or by lengthening the conrod. KTT Services and Ken deGroome chose the former, ABSAF chose the latter. I am an proponent of the former approach, as it saves weight, making the bike a little less top heavy. Additionally, pushrods can be shortened, which benefits their buckling resistance. Aesthetically, I think the engine looks better having less height. Appearance could be improved further by a big fin barrel, which would improve thermal management and engine efficiency also.
I am trying to collect accurate data for the wrist pin, p/n 022394, which has an OD of 7/8" (0.875"). Length is 2.932" (74.5mm) and bore is tapered (thanks to Ken for the information). Estimated ID in the centre section is 43/64". Above weights suggest the wrist pin supplied by Hepolite, which has a plain bore, is 12 to 15 gram too heavy.
Wrist pins can be made considerably lighter by selecting a higher grade steel and by tapering the bore (less thickness at the ends where stresses are lower / almost nil at the extreme ends. This is the case for the G50 wrist pin, whose OD is 7/8" also. G50 and G80CS wrist pins appear to be of similar design.
Wrist pins used in Velocette Venom and Thruxton engines are of smaller diameter, namely 0.8236". This doesn't necessarily mean they are lighter.
Dimensions used in Hepolite pistons are 0.5" x 0.8236" x 2.992". Calculated weight is 130 gram (to be confirmed). Omega pistons may have different wrist pins.
I have passed a connection on to the club of a person who works in the Piston industry and as he is an avid Classic Trials lad he is helping people source pistons for classic bikes.
