Getting BBK right on Derbi Predator

[pinkscoot]

Today I pulled the engine on my Predator to get the ports right and a better squish and higher compression. The problem is that my piston at BDC covers the Transfer ports by 2mm on a 10mm port I'm losing 20%. My squish right now is .95mm not bad but it would be almost 3 mm. I propose taking 2mm off of the top of the cylinder and using 2mm of base gasket to make it up. I know this is going to be some work. The only problem I see is that the lip at the top of the cylinder will be thin. Here is a picture, it measures 3mm:



Another question is while I'm here should I open the case up for the larger ports on this cylinder? Here is a shot of the case with the gasket that fits the cylinder really well:



It's not much but its something. I have no problem splitting the cases and will probably put new bearings in while I'm there. The piston ports match up well to the cylinder so I'll probably leave that alone, unless someone thinks I should open them too.




The final question is should I do a spacer or just build up gasket material? If Gasket material can any one suggest a thick gasket material for this. 

Thank you in advance on your advise I know you guys are the smartest in the room.

[190mech]

Have you put a degree wheel on it to get the port durations?I wouldnt deck the jug as the deck will be too thin to seal..An aluminum base spacer is preferred as gasket stacking settles and head torque goes away= blown head gasket..

If the durations look good with the transfer port bottoms flush with the piston at BDC,then your only good option is to step cut the head,yep your O-ring groove goes away then...Gotta love aftermarket kits!!

[pinkscoot]

This is why I haven't started yet. I'll do the durations as my next step, I did them with it as is and got 180/ 126. How would I seal the head if I went that way? Maybe do a mix of some off the cylinder and some off the head? I have also been looking for a Metrakit, the only other kit made for the Derbi, I found a couple in Europe. I could also go back to the 50cc because this isn't that much larger and as is isn't really doing anything for me.

[190mech]

The 'as is' durations are OK for an around town ride,,190Ex and 130Tr are good for sport riding..Yes,some could be cut from the jug and the rest from the head,twice the set up though..I think the machined surfaces seal the combustion pressure and the O-rings keep the water in the right place..Ive seen MX guys use ultra copper rtv as an LC head sealant,but Ive never tried it...

[pinkscoot]

Thank you for your help John. I'm thinking I'll go back to the 50cc cylinder. I'll do a port map and get durations on it. In doing some research yesterday I discovered that I think the reed block is in backwards. I swear when I replaced the reed I put it in the way I found it. But I found a picture in the manual that shows it the other way. Its a single reed and I have it facing the back of the case away from the cylinder. It would make more sense for it to face the cylinder I would think. I'll bet that straightened out when I put the 50cc cylinder on. Not that I won't in the future go back to the larger cylinder but it only adds 16cc to the engine. let me see what the ports and duration look like on the stock cylinder. I'll take this build back to original thread.