How much radial clearance is really needed around a valve (piston pockets)
Turbo Mini Forums

As we know, the K-valves are 23.0 and 26.5mm.

What would you guys consider the tightest radial clearance that should be used?

1.0mm radial clearance (i.e 25.0 and 28.5mm dia cutter)
1.5mm radial clearance (i.e 26.0 and 29.5mm dia cutter)
2.0mm radial clearance (i.e 27.0 and 30.5mm dia cutter)
2.5mm radial clearance (i.e 28.0 and 31.5mm dia cutter)
even more???

I just look at some of these cut-outs on the aftermarket pistons that you can purchase "ready to drop in" and they just seem overly large. Personally, I feel like 1.5 to 2.0mm radial is a good target, but I'm interested to see what others think. These pre-machined pistons also seem to have excessively deep pockets too... I guess I can understand that it's easier for them to err on the side of caution, than to have someone bend a valve... I think I'll go and actually measure the valve movement at the lift at TDC, and work out from there what I personally need...

Back to the diameters; I'm at the point that I've got my pistons dimpled using a spare head, and have started on a fixture to allow me to machine the pistons on the Bridgeport while they are still on the rods. I can get one of my spare 1-1/8" and 1-1/4" cutters ground down (and a corner radius added) at one of my suppliers for zero cost, to basically create a custom cutter specific for doing this job - just need to decide what size to make them...

It all depends on how well you measure/ mark the pistons to begin with. Me personally, with the way I built my engine, I simply copied the bmw pistons but increased the diameter by 1mm just for the sake of a little miss alignment. All pistons are the same. You can find some pics in my build threads. The cuts are tiny in comparison to the norm!

So, do you know what diameter the cut-outs in the beemer pistons are? You told me you increased the dia by 1mm lol, but I don't have any beemer pistons as a reference

I'd imagine piston rock and movement is a big factor.

In saying that though...have you actually measured how close the valve actually gets to the piston during the engine cycle in the first place ?

A lot of pistons can have cutouts generically, but the valve might still be nowhere near under normal operation.

If it was mine though, I'd probably want around 1mm clearance in every direction.

Better safe than sorry !

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Stevie
I've actually done a "dry build" without the rings on the pistons; as you say, there is a surprising amount of movement in the piston side to side, and the "rock" is also noticeable as an additional factor for "depth". I'll measure it all tonight and post the findings for others...

As far as the cut-outs, If I use 1-1/8" and 1-1/4" cutters as standard, it gives me 2.78mm and 2.63mm radial clearance. I'll probably get the cutters dressed a little so they are nice and sharp, which will leave me with around 2.5mm of clearance. Everything I've read suggests that 2mm is an ideal target.
Another factor - I also did the "dimpling" with the head mounted directly to the block (no gasket) which means they are already at the worse-case for clearance. I do still need to deck the block and head, but don't anticipate more than 10-thou of total lost material over the two surfaces. I believe a BK450 is no less than .028" compressed thickness, so I'm still good.

Edited by TurboDave16V on 7th Apr, 2015.

And I found this link too...

http://www.fordmuscleforums.com/engine-art...n-notching.html

I already knew about notching a spare valve to turn it into a cutter, brazing a HSS or carbide tip to a spare valve, but popping an emery disc onto a valve is one that is definately new to me!

Isky also make an interesting tool - although not for 6mm stems...
http://www.iskycams.com/pdfcatalog/2004-05/page17.pdf

Edited by TurboDave16V on 8th Apr, 2015.

Emery disc is a gentler option for valve pockets. The only snag is that when you need to replace the emery disc it might take some time... I used a notched valve to cut pockets to Rover V8 pistons.