can you shave stock head?

[16bit]

machining heads has been done for ever but you must take proper advice on what to remove. Its also wise to actually work out what you expect to achieve and what are the risks in doing it. There is a $ cost in doing it and a $ cost running it because an increase in the compression ratio may result in the need for 98 fuel. Longevity may be affected. You may need a new headgasket etc. Paying for the work to be done - which most people do- is going to cost more than one might imagine

thanks.

i already run 98 fuel.

and yes head gasket etc would be needed but i don't think this is something i would do for now.

[PaulF]

your new compression ratio should be somewhere around 10.7CR.

Phwoar. No wonder it feels good. Just needs some cams and a tune...

[noobee]

not super high cmpression compared to many modern engines,should be somewhere between 10.55 and 10.75,given published figures.minus a little from cleaning up the combustion chamber sharp edges/hotspots.(which contribute to pinging)
its the extra torque that compression brings that you are enjoying,should be better on fuel economy too,so long as you are not enjoying it too much,lol.
Mick

[PaulF]

Mick, for the first time in months I can safely rev it past four grand. Let's at least give it a few weeks before talking about fuel economy.

[project.r.racing]

interesting that it has gone so high. wouldn't expect any mot than 0.2 extra compression.

i guess with 1mm removed the engine will not be non-interference anymore?

[16bit]

given the larger volume of the nb head in theory could you shave more from it? or is the na8 and nb heads similar?

i suppose your approach dann was best of keep on going until you get close to the valves.

[PaulF]

My engine is a B6 not a BP, so I wouldn't be using that as your primary source of info.

[16bit]

cool, i think if i do it doing the 2 thou per pass is the go.

[93_Clubman]

Conservative, but Tanner in Mazda Miata MX5 Performance Projects reckons "between 0.010 inch & 0.020 inch (0.25mm to 0.5mm) is a good range." He doesn't specify 1.6 or 1.8 in terms of shaving, but the article is on removal, cleaning up & refitting both the 1.6 & 1.8 heads.

Bit of an insight from CT & MB here:
viewtopic.php?f=29&t=35381&p=459594&hilit=40thou#p459594

Bit more from CT:
search.php?keywords=40thou&terms=all&author=ct&sc=1&sf=all&sk=t&sd=d&sr=posts&st=0&ch=300&t=0&submit=Search

[manga_blue]

The trouble with head shaving is that you need to know how much was shaved before you got it. Mazda specify a standard head thickness of 133.8-140.0mm, or 5.268-5.275", for B6 and NA8 BP heads and a maximum allowable shaving of 0.10mm, or 4 thou. It's generally recognised that you can take up to 70 thou from a BP head, which is 66 thou more than Mazda recommend, but 70 thou from what? Mine has had 45 thou taken off it with no problems BTW.

And yes, you can just take a head off, shave it and bolt it back on without having to dismantle anything else on the head.

Once you get past 50 thou you can run into problems with timing belt tensioning and valve timing. You might run out of adjustment on the intake side idler. Shaving the head also reduces the distance between the exhaust cam and the crankshaft, effectively retarding the valve timing. Both problems can be overcome by moving the exhaust side timing belt idler inboard a bit.

Also worth mentioning that you can find thinner head gaskets. I've measured stock ones at 38 thou. My current one came from MX5parts and is 25 thou, but I had to drill out a lot more holes for the water jackets around the exhaust ports before I was happy with it.