Pistons pics.. Seca 2 vs FJ/XJj600

OK so I got some used pistons/rings for my 564 engine (550 bottom end 600 top end) project. I guessed the seca2s would be the same as the older fj/xjs but apparently not. I'm pretty sure I will run them anyway as they look nicer (better condition)and I have one piston with impossibly stuck rings from the xj/fj set - although the Seca 2 look like they will give a lower compression - I estimate 9.5:1 instead of 10:1 as there is clearly some lack of crown height. Thoughts ?

Seca 2's on the left..

 

Do lots of measuring and see if there is room to skim the head to get that compression back.

 

That's interesting, I always assumed they would be the same as well, what comp ratio is the Seca 2 compared to the FJ/XJ ?

 

Ha yes I checked this. On-line resources suggest they are both 10:1, compared to xj550 9.5:1. As well as a flatter crown the seca2
seems lighter built with a skirt 2-3mm shorter and no oil holes below the bottom ring (see earlier thread).

 

Could be a plan but I would have concerns about valve clearance.. If my estimate is correct ( I reckoned on an decreased piston crown volume of 1.5 x20 x25 mm very approx) I don't think I'll worry about it.

 

I used XJ600 pistons in one of my XJ550, the pistons looked identical except for the bigger diameter

 

Yeah I was planning to do the same but my XJ600 set were/are less than pristine and as I mentioned one has incredibly stuck rings - currently not sure if I should drill the Seca2/Diversion pistons with the oil holes - a signifiant opportuity for cock-up..

 

Whatever turns ya on

 

Further reading..

 

I got lucky and scored a set of NOS .5 oversize XJ600 pistons and rings for my next 550 build off a local auction site for about $200 NZD, last time I just bought a secondhand set off Ebay, but I'm looking for what I can use to take it out t0 600cc using the 550 crank

 

I think I figured Kwack GPZ305 piston (61mm) might work as an oversize in an XJ600 bore - that would give a shade over 600cc on a 550 crank I just calculated. Can you find four of them though!

 

I'm fairly certain that it's easier to find pistons for a 1916 Indian Powerplus. A shame really; I liked the 305.

 

I think when I was originally looking at this I was looking at CB 500/550/650 pistons as the stock CB650 was 59.88mm and there was a lot of aftermarket forged stuff available, have to have another look and check