ET: Engine (Twin) Piston Choice

Bill Thomas

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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

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:)
You thread burglars what about helping piston post #29? (and #1)
(I know he who casts the first stone....)
 

BigEd

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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

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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 Forum 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

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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

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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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