ET: Engine (Twin) Pistons found when barrels removed from my Rapide

[Bogage]

I have had the machine for 7 years and ridden over 5 thousand miles possibly over 6 in that period. Started reliably with magneto, I never had her to a ton. When the head was removed to inspect the barrel after the compression appeared to indicate ring failure at some level, this was seen:

OK, so after the jugs were removed this was the photo I received.

So my questions are:

Why?
Would this damage the crank and or should the case be split and the crank inspected or should it be left alone (assuming it does not show any obvious failure upon manipulation)?
Would it be wise to confirm that the crank is properly balanced or would that be another reason to inspect the crank?

Any insight or observations are appreciated. The pistons are 7.3/1 and 8.4/1

I do not have a voluminous history. Only that it was delivered to Australia back in 51. It's been in the states and Canada for most of this century. There is a British Tax disc from the early aughts. I did find a unique chainwheel in the primary when the output shaft bushing failed. My crack mechanic pulled the chainwheel and I noticed that the hub appeared to have been re used and the sprockets appear to be brazed on. A home built fix from when parts where unobtainable?

Anyway, I thought this may provide a short burst of activity. Although, I will probably be rapidly embarrassed by the commonality of the situation and my lack of knowledge in things Vincent once again will be exposed to the masses.

Cheers

 

[david bowen]

Only a few weeks back I was talking to a guy in NSW Australia, he found the same with the pistons a engine he pulled down, I do not think it would make to much difference to the big end it may have been a mod that the speedway guys used ?

 

[david bowen]

check the size of the heads inlet ports Rapide 1 /1/16 Shadow 1 /1/18 this may be a clue ,

 

[Bogage]

I will ask about that David thanks very much.

 

[davidd]

Josh,

I have never done this, but years ago this was quite common. I believe that Gene Aucott favored this approach. The hypothesis was that due to the 50 degree cylinder angle you would want a piston with more bang to fire for the larger part of the rotation. However, I would have thought the pistons would be much closer in compression ratios, say one point apart.

It looks like those were the pistons available at the time of rebuild!

You might get better info from someone else, but I don't think it hurt the crank unless the crank is already hurt. That is, if your crank can pass the ineffective tests we normally do to avoid rebuilding flywheels, you are all set.

As for the balance, I was told that the flywheels in their normal state of balance would be fine for the range of pistons that the Factory normally offered. I can't say this is correct, or that this would have covered two mismatched pistons, but you mention that the riding was quite reasonable, so I expect that the engine took it mostly in stride by having much more blow by on the high compression piston.

David

 

[Bill Thomas]

Never seen anything like that, How would you set the ignition timing ,On which cylinder ?, Looks like it has had a few nip ups, I always find the conrods have too much end float, Which is when I change the bigend, I have only changed 1 bigend on a Vin' because of knocking. Good luck, Bill.

 

[Jim Richardson]

It would be interesting to know the weights of the pistons, when I was rebuilding my engine the 2 new pistons were about 12g different so I popped into the spares company and selected a pair, that were within 1g, using their digital scales.
At the time people were telling me that it wouldn't matter on a Vin, as a Triumph owner I am used to getting everything as close as possible.
Could you imagine the vibration in a vertical twin with that mismatch of pistons.

 

[timetraveller]

The difference in the pistons is the difference between what was fitted years ago and what is available now. The taller piston is one of the original ones. The shorter style tend to be low expansion ones. My guess is that at some stage a piston has had to be replaced and either they fitted a new style one only or someone fitted an old style one to the bike which had already got new style pistons.
The difference in compression ratios is larger than what as being done when I was racing. Generally, half a ratio was used e.g. 7.5 and 8

 

[Bogage]

All great information gentlemen. I imagine I’ll only be happier with a matched set and I was elated before so hopefully I’ll be enraptured after the rebuild. Rest assured the forum will remain in the loop.

Cheers and many thanks to you and this valuable resource!

 

[Cyborg]

All great information gentlemen. I imagine I’ll only be happier with a matched set and I was elated before so hopefully I’ll be enraptured after the rebuild. Rest assured the forum will remain in the loop.

Cheers and many thanks to you and this valuable resource!

If you decide to go with the period mod for nostalgia's sake.....