[Edited on 15/2/15 by Nickp]
Hi everyone. I'm at the stage where the engine is coming along and I've been checking valve clearances. I'm using kent cams dtec 35 cams, cosworth forged pistons with pockets. I did the clay/ blue tack test and I got 1mm clearance on the intake and 0.75mm on exhaust. Is this too close as I can't find any figures for what it should be
They're too tight IMO especially on the exhaust side, I reckon you should be aiming for upwards of 2mm. Kent cams should be able to advise you on
this though, but I'd be having the piston pocket machined (like I've just done on my Lancia twin cam). You may get away with it, but any
over rev of slightest of valve float and they'll be hitting
[Edited on 15/2/15 by Nickp]
Not many places will take on piston machining but there's a good place local to me that will and they do all Guy Crofts work, and he's a
perfectionist 
[Edited on 15/2/15 by Nickp]
If you don't mind me asking what sort of cost is involved with machining pistons?
Found this from wiseco.com
When assembling your 4-cycle engine, it is sometimes important to verify that you have adequate clearance from the piston to the valves. If you are
installing an aftermarket racing camshaft, this becomes even more important to measure. The usual rule of thumb for piston to valve clearance is .080”
intake side, and .100” exhaust side. These are relatively “safe” values that apply to most any engine. Some engine builders may cheat these values
down to .060” intake and .080” exhaust, and Wiseco recommends discussing these lesser values with your camshaft supplier.
Cheers
Ian
quote:
Originally posted by itsawindupuk
If you don't mind me asking what sort of cost is involved with machining pistons?
Or use the lowcost method I deployed 
Get hold of an old inlet valve and turn into a fly cutter.
Grind a cutting edge on the old valve and pop into the exhaust guide on no1
Turn the engine so no1 is at TDC and lock the engine.
Run grease around the edge of the piston, loosely bolt the head onto the engine.
Push the valve down so it contacts the piston and twiddly around until it is at it deepest point in and measure from valve stem top to guide top with
a vernier
Now carefully spin the 'valve cutter' down to the clearance you want, I used a little cordless drill going gently and measuring.
Pull out and repeat on the other three cylinders.
For the inlet weld a bit of steel on the outside of the inlet and grind into a cutter.
This gives you a precise port in the Pistons aligned to the valve guides as the head is the jig, with side clearance depending on how wide you make
you valve fly cutter.
It takes a couple of hours and saves you having to dismantle your engine.
The grease keeps the swarf from going down by the pistons, if you strap a vacumm cleaner to the port most of the cuttings will get sucked out as you
go.
Regards Mark
[Edited on 15/2/15 by mark chandler]
He may be reluctant to use that method on new forged Cosworth pistons
That's true, however what I have described will if done correctly give you very precise pockets that are exactly matched to the engine, this then
allows for piston/conrod piston length, crank throw and any misalignment with the valve positioning. 
Has the cam been timed in or are these valves open Pistons at TDC
Inlet should be closing although not fully closed at TDC exhaust could be opening but not fully at TDC
basically the cams are fully timed on the verniers. Looking at the head, it sits a whole 1mm lower than the timing cover!! looks like it was skimmed too, or maybe by previous owner or before that. So now I need another head grrrrrrr! Im running out of money! The cams have 11.3mm lift.
Is there a thicker, maybe turbo biased, head gasket you could use? That could get you most of a mm back, if they exist.
quote:
Originally posted by itsawindupuk
basically the cams are fully timed on the verniers. Looking at the head, it sits a whole 1mm lower than the timing cover!! looks like it was skimmed too, or maybe by previous owner or before that. So now I need another head grrrrrrr! Im running out of money! The cams have 11.3mm lift.