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ET: Engine (Twin) Piston Choice


vibrac

Well Known and Active Website 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

Well Known and Active Website User
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

Active Website 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

Well Known and Active Website User
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

Well Known and Active Website User
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 Website 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 Website 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 Website 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 Website 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 Website User
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.
 

Bill Thomas

Well Known and Active Website User
VOC Member
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. "
Hello Eddy, Is there an easy way to work out the Balance Factor, I have read the Higgins bit and checked a few Web sites, But I am lost on the Maths side of it.
I want to do a Comet soon. Cheers Bill.
 

vibrac

Well Known and Active Website User
VOC Member
:)
You thread burglars what about helping piston post #29? (and #1)
(I know he who casts the first stone....)
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Hello Eddy, Is there an easy way to work out the Balance Factor, I have read the Higgins bit and checked a few Web sites, But I am lost on the Maths side of it.
I want to do a Comet soon. Cheers Bill.
Dear Bill,
Got visitors at the moment. I will sort something out after I've slapped that cheeky Vibrac's legs (;)) and the visitors have gone.
 

Little Honda

Well Known and Active Website User
VOC Member
Hello Eddy, Is there an easy way to work out the Balance Factor, I have read the Higgins bit and checked a few Web sites, But I am lost on the Maths side of it.
I want to do a Comet soon. Cheers Bill.
Hey, Bill,
the Prof. is talking abt his twin engines; you r looking for Comet values. Do not mix up twins and singles - they
are different also in balance factor. Best u can do, is to read, what Phil Irving wrote abt the subject in his book:
"Motorcycle Engineering". U may surely find someone, who copies or scans the relevant pages for u.
cheers,
 

Bill Thomas

Well Known and Active Website User
VOC Member
Hey, Bill,
the Prof. is talking abt his twin engines; you r looking for Comet values. Do not mix up twins and singles - they
are different also in balance factor. Best u can do, is to read, what Phil Irving wrote abt the subject in his book:
"Motorcycle Engineering". U may surely find someone, who copies or scans the relevant pages for u.
cheers,
Thanks I will go 60' but not good with sums cheers bill
 

Flo

Active Website User
VOC Member
Prof Higgins has stated some semi clever snippets in the link:
The four cylinder in-line engine which is the most common on our roads can be made fully in primary balance, but also has secondary out-of-balance forces which rotate at twice engine speed. These can be partially balanced by adding weights to the crank, and it is this and the reduction of main bearing loads which mainly concern the car engine designer. The in-line four can, of course, be completely balanced by using one or two balance shafts rotating at twice engine speed, and this technique is used by a number of modern manufacturers. However, our old fashioned single cylinder Comets generate major primary out of balance forces which rotate at engine speed, and we can only partially balance these by adding weights to the crank, so manufacturers chose to ignore secondary out-of-balance forces, which are only small in comparison to primary ones anyway. As we saw in a previous article on Breather Timing in MPH 608 the Twins, which have 50 degree out of phase pistons running on a common crankpin, are very similar to a big single, and can be treated in a similar way for balancing purposes, though
the ideal balance factor is different.


Mostly the confusion arises by mixing components of the proper and complete dynamics of the crank-slider mechanism - one of those is the balance of forces and another one the non uniformity of rotation!

F
 

Ken Tidswell

Well Known and Active Website User
VOC Member
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.
the 7.3 cr pistons are 8:1's with the piston crowns turned off i believe they fail after a large mileage with hairline cracks across the crowns i have seen these after the event in Bury. There is also an issue with the gudgeon pin circlips which move with the usual catastrophic consequences on the pin milling a groove in the bore . Mahle via the manufacturers in england issued a warning with pictures of such failures. and recommend that the circlips be fitted with the gap horizontally or vertically positioned. to prevent this happening . they accuse the user of over revving the motor for this failure . Sounds like a design fault to me. if the pistons are not scuffed and the ring lands are not worn you could treat it to some new rings
 

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