shaving BP-4W head

[StanTheMan]

so last time i did this to the B6 motor, the local machinist only took 0.7mm off the head. To Mazdas minimum recommended thickness.

I read that with these BP4W heads they take off 1.6mm(62 thou) which should get me to somewheer around 10.5:1 CR. or what ever the compressionb is.

now by doing that, assuming the head hasn't been touched before. does the BP4W head have that much extra meat or do they just go past that minimum thickness Mazda suggests?

once I get the motoir I will measure the head thickness & know where I'm at but I'm just curious.

[1600Dave]

Disclaimer : I have no experience with MX5 heads, below is just general experience from building other makes of rngine.

Often the issue isn't so much the thickness of the head but upsetting the geometry / timing / etc of the cam drive system. ie taking excessive amounts off the head may cause excessive "slack" in the timing chain / belt and can alter the cam timing. You would need to make sure the tensioner is able take up the extra slack, and adjustable timing gears may be needed.

[3gress]

I have had close to 2mm shaved off my head with no tensioner issues, nor problems with interference of the piston/valve relationship on a 9:1 stock bottom end and then 10:1 stock bottom end. It does retard the cam timing slightly ( head gets closer to block and alters cam/crank/belt specs) which meant the cam gears aren't only for bling purposes. The head guy did do a little bowl work for quench properties which i can not quantify though.

[StanTheMan]

2mm.... wow.
so has that gone below Mazdas minimum recommendations?

I've got adjustable cam sprockets. I've been offered some cams which are about 238 @ 0.050 10.4 mm lift. very high chance ill end up with those.


sounds like I need to find a different machine shop.
any recommendations for Sydney?

[3gress]

10.4mm lift is blooody 'uge mate. I would measure first before giving head a haircut.
I'm only using 9mm lift 256 toda cams and have some swing room but can't increase lift safely without machining deeper valve reliefs.
Most shops you go to would steer you away from shaving head for compression, citing Mazda machining limits amongst other things. However i posted to let you know that you can step past the conservative factory limits without probs.
Not for the faint hearted or your mate 'thumbs' down the road though.
Test assemble proposed setup and measure with putty on the pistons etc for sure.

Search for what that guy 'quinn' did on other related forums in respect to head shaving.
He went to 100 thou or so in the end and had to relive pistons for clearence.
You'd want to make sure valve float at decent rpm was catered for with heavier springs and such because things get close...and decent timing belt maintenance procedures.

[StanTheMan]

cheers
im familiar with Quinn. he does very nice work.

I still have putty from my last attempt.

May just leave it at 1.6mm. but will check clearance. I've made that mistake before on a 1.6 where my pistons ended up hitting the head over 3500 rpm after warning up..
also had some valve clearance issues but ground some material off the pistols.

that engine is now getting replaced.witj a NB8A I have 66lbs valves but need to check what their maximum compression is.