Substitute 7.3:1 scratched pistons by 8.1/1 or 8.3/1 Pistons

[Bill Thomas]

The Bike I was talking about was I think built by John Renwick, My L/ning rep' was smooth , For about one lap !! But i think it's about where it comes in, When I was young it didn't matter so much, But when the hands hurt, Not so good, Maybe it's time I built Soft bikes !!. I had a Modern !! 1983 BMW K100, The vibration was right on the speed limit !!After 10 years, It had to go !. Cheers Bill.

 

[greg brillus]

That's an interesting point about the two bikes Eddy, I think the answer to your last question may well be both issues, as in some way they are connected (Connecting rod.....) Perhaps Max's issue is not simply from a compression ratio point of view, but rather a mechanical complication of marrying up two engines. There are too many unknowns to provide a correct answer. I'm glad your comments back up how I described my racer Bill....I now feel a little more at ease knowing that the engine combination is not the result of something horrid in the building process. I can't say the same for the chap who built it, though.......

 

[Oldhaven]

While I do not know the balance factor it uses, the smoothest bike I have ever owned is my Ducati 750 GT. Rather mildly tuned at 60 hp, 8.5:1 compression, with bore and stroke of 80 x 74.4. It is a pleasure to ride at any speed and I could literally balance a quarter on the tank while it was idling on its center stand. My wife and I rode it from Maine to the Ohio North American Vincent rally and back in the 80's, and while the thing was a bit cramped for us and gear, I had no problem with hands or feet from vibrations. After it was taken off the road for a never completed rebuild, I bought a '75 860 GT. While it only increased the bore and kept the same stroke ( 86 x 74.4), and I believe it used the same frame geometry, it was quite a bit more buzzy, did not feel the same, and I sold it not much later. Sometimes a perfect combination of factors gives a great result, which for me was a nice touring machine. Slight changes can affect that combination of factors. I also owned an ST4 a few years ago, and did not like the way it vibrated, as it would put my fingers to sleep. (and try to change the battery!). My Vincent never bothered me, and while it vibrated a bit, it was of another lower order, and was about the same before and after I pushed the flywheels together myself after the drive side mainshaft got loose, and I'm sure I did a poor job of aligning them with my lead hammer. The original spec Rapide seems to be pretty forgiving.

Ron

 

[Bill Thomas]

Greg, Thats what happened to young Graham, Once I told him they all did that! He was happy, And rode the Wheels off it !! My Twins are OK when you are up in the 90s, But my ex L/ning rep' now is super at 70 ish, With the fishermans lead balls in the handle bars, Told by Big Sid. Cheers Bill.

 

[timetraveller]

If it is not a secret then perhaps Max could tell us what boost pressure he is using. His value of 6:1 would be equivalent to 12:1 if he is using two atmospheres boost. And why has his posting disappeared? My understanding is that he is allowed one per day provided that it is not too contentious and I could see nothing contentious about his now non existent posting.

 

[clevtrev]

Is there not a balance factor issue when changing to heavier/lighter pistons?

Peter B
Bristol, UK.
STILL WANTED: SERIES 'A' FRONT FRAME - any condition.

How did Dennis`s Twin feel after I had fitted the very heavy Alfa pistons, back in the early Eighties ?

 

[sidmadrid]

Them moderators are quick to delete.

 

[timetraveller]

Whenever I have set up a engine from scratch I have always used the original Vincent balance factor of 46% but this has always required drilling four more holes (two in each flywheel) near to the crankpin. I do know of two race bikes which are set up at 50%. This is not because they are in different frames from the original but because their owners believe that this is a better value for high revs. Any other peoples feelings on this ?

 

[A_HRD]

How did Dennis`s Twin feel after I had fitted the very heavy Alfa pistons, back in the early Eighties ?

Trev, As you know, it went like slick sh@t off a shovel! I don't remember either of us getting phased about any vibration; we just RODE the beasts in those days…

Peter B
Bristol, UK.
STILL WANTED: SERIES 'A' FRONT FRAME - any condition.

 

[vibrac]

Used 50% on Egli Racer on second rebuild I think it was smoother but it would be splitting hairs