ET: Engine (Twin) Piston Choice

vibrac

Well Known and Active Forum User
VOC Member
Perhaps someone can help me I am looking for a low compression piston for one of my old 90 bore racing engines It has a high comp job at the moment I have tried packing it up but the angle gets to wide for happy push rod tubes ( screw up racing type) I could get one made but I wonder if there is something out there that will sit in the 7:1 region and give me a real beefy easy starter. I did think about turning the crown but I don't like the idea and am not certain what a safe crown thickness is.
I have found some info back in 2016 here but it all seemed to point to a new one from Venolia I just wondered if there was any change I know that Bob Culvers can be turned down but they are still not flat top
 
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Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Work out what volume increase you want and I will make a screw in canister for a plug hole with an additional plug hole so one can use it as an isolated combustion chamber and so easy to revert back to H.C.
 

Flo

Well Known and Active Forum User
VOC Member
Marcus, have you consdered what effect the throttling orifice between cylinder and canister might have and how the combustion will be?
 

vibrac

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VOC Member
I have discussed this with Marcus before we are talking about small amounts say 10-15 cc for a whole ratio I think it would remain unburnt and I think the chances it might cause pre-ignition is small and it might help the piston in later stages of decent and extraction of the burnt gas however it is lost charge and thus lost power.
Of course the clever way would be to open the chamber for kick starting and close it for riding a bit like a valve lifter but a bit less go or stop
All in all a making a new piston in the absence of a existing one seems the best way
 

b'knighted

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VOC Member
A neighbour had a tractor, possibly New Holland, which he had fitted with an engine from the same make of crawler. This vehicle had a diesel/petrol engine. On one side was the petrol pump, distributor and carb while the other side had a fuel pump, injector pump and injectors. The change of compression ratio was achieved by having the extra combustion chamber opened or closed by a poppet valve so with the valve open the engine was fired up as a low compression petrol engine then once warm the diesel fuel was fed to the injectors and the valve was closed as the sparks and petrol were turned off. It would seem likely that the spark plug was in the extra chamber.
 

b'knighted

Well Known and Active Forum User
VOC Member
Not New Holland - International Harvester.
From Wikipedia:-
In 1932, IH produced their first diesel engine, in the McCormick-Deering TD-40 crawler. This engine started on gasoline, then switched over to diesel fuel. Diesel engines of this era were difficult to start in cold weather, and using gasoline allowed the engine to start easily and thoroughly warm up before making the switch to diesel in all weather conditions. In 1935, this engine was put in the International Harvester WD-40, becoming the first diesel tractor on wheels in North America.
My neighbour had the American crawler for use during WW2 and put the engine into the tractor years later.
 

Somer

Well Known and Active Forum User
VOC Member
My Rapide had 6.8 pistons with plates. It was probably 6.3:1. Extremely docile. Ran great and would still break the ton.
 

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vibrac

Well Known and Active Forum User
VOC Member
My Rapide had 6.8 pistons with plates. It was probably 6.3:1. Extremely docile. Ran great and would still break the ton.
Yes that's my feeling age and speed enforcement will take their toll and I can't see the VOC holding anymore speed trials at Cadwell any time soon (mores the pity).
 
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vibrac

Well Known and Active Forum User
VOC Member
So I can confirm Venolia are not making custom pistons and the possible alternatives want multiple pistons.
( I forgot to say I have the racing type pushrod tubes that do not accommodate lots of barrel packing.)
So without spending on a custom piston what I really need is any pointers to a existing piston from something that is 90 bore flat top with a gudgeon pin (wrist pin) configuration that is compatible.
 

Little Honda

Well Known and Active Forum User
Non-VOC Member
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
Alternative literature: PEI, "Motorcycle Engineering", Chapter: "Torque and Balance". No questions after that
has been understood.
 
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