piston! pissed off!

[andy230]

Hello all,

I’m on a train!!  Amazing huh??!     Off to a wedding in Newcastle, and am gobbling up their free wireless tinterweb.  Fabulous.  Some technology I don’t hate (tho I did nearly throw a mates TomTom out the window the other night  ).  Still, electric start is still the “leg of satan” IMHO….

Anyway, I think all is good with our engine build.  Or maybe not “good”, but progressing…!

The oilway problem is solved (I’ve got an external line, and some little slugs to block up the internal oilway), a set of RD springs coming, and Boyds coming down in February to see if it will actually go in the frame.  Fingers crossed!  He looks like he’s been busy with the chassis:

www.team-thumper.blogspot.com

I was speaking to fellow supermono man dave bennet through the week.  He reckons a flat top piston (as mine is) is not a great idea.  I got this new wiseco piston (and barrel bored to suit) with the engine.

Apparently, he thinks it will suffer overburn in the middle of the piston.     This can be avoided by use of a domed hi comp piston.  He also reckoned I’m giving away useful bhp by running this lower CR.  I don’t doubt it.

According to Wiseco literature, this piston will give a CR of 10:1.

Whats the opinion on this?  Remember that as a 690 it blew the top end off.  I said this to dave, and he reckoned it could be prevented by using Ti barrel nuts and Ti head bolts (from slipstream).  And they’re not as expensive as you’d think…

I’ll probably build it for the start of the season with the existing piston.  If it holes, it holes, in that case I’ll replace it.     Maybe with an alternative

Any experience with hi comp pistons?  The 620 srx had an arias, but I don’t think it was “lumpy”… Steve, Bill, do you know what is in your motors?  And remember I still have the 102mm (690cc) hi comp wiseco to try towards the end of the season if I fancy, tho maybe that would just be a bit much…   

Cheers,

a

[guest146]

When you barrel parted from the crankcase before what gave? Studs or casting?
I would say you doing the right thing in trying it as it is.

Ken

[Steve H]

Apparently, he thinks it will suffer overburn in the middle of the piston.    

What is 'overburn' in the middle of the piston when its at home ?
There is an argument that domed pistons are bad as they tend to make a very convoluted flame path around the dome, as apposed to a flat topped which as a more open chamber and allows the flame to travel. The dome on higher CR pistons is also supposed to mask the valves to some extent,reducing signal between the inlet and exhaust. One of the Dutch guys was running standard pistons and machining the head to get a very tight squish and increasing the CR. Apparenly the bike was quite quick.
As for overburn in the middle, the point of the squish area is to force the mixture into the centre where burning starts (the plug).
The only down side (in terms of output) is a lower CR, but this has to be offset against a lower risk of detontation/pinking. That said with your BIG cam which probabaly has huge overlap the dynamic compression ratio is probabaly about the same as that with the  standard cam and piston.
If it was me I would just put it together, measure the CR and squish, then decide whether to raise the CR further by skimming the head. The limiting factor being the squish, and since you have a Carrillo rod you can probabaly run this fairly close without too many worries.

[Steve Lake]

You can have the bolts made from titanium, stellite, unobtanium, kryptonite.....or an alloy of all 4, but it's the threads in the barrel that give out in the end, there is an argument that says get the threads helicoiled as the steel thread insert has a larger surface area to purchase in the head, but i've pulled a helicoiled thread in my track bike, so i'm not sure, al i DO know is....the torque settings should be adhered to pretty closely

[andy230]

I'm on a train again!!!

When you barrel parted from the crankcase before what gave? Studs or casting?
I would say you doing the right thing in trying it as it is.

Hi Ken,  I'm not sure what happened first, but the barrel broke at the flange, and the studs were also broken when I got the motor.

And yeah, I've decided to (at the very least) try it with this piston.  And then get the rest of the bike sorted.  And maybe halfway thru the season I'll have a look at maybe changing it.  Funds pemitting!


SteveH

What is 'overburn' in the middle of the piston when its at home ?

Dunno......   I was assuming that it would be gettting too hot?  He said it risked holing the piston?

If it was me I would just put it together, measure the CR and squish, then decide whether to raise the CR further by skimming the head. The limiting factor being the squish, and since you have a Carrillo rod you can probabaly run this fairly close without too many worries.

Right.  Cheers for the pep talk guys!  Skimmming th ehead is also an option (later)

the torque settings should be adhered to pretty closely

Aye.  Agreed.

Looks like I can now get the piston on and into the bore, and bolt the barrel down.

I've decided to go for 4 new Yam valves, and I have some springs coming from the states. 

see you

andy

[guest146]

Hi Andy

What I was getting at was that if the studs pulled from the crankcase you could if there is enough metal tap  out the cases and go to the the next size up and drill the barrel holes.

Just a thought Ken