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Post by wannabe on Aug 29, 2019 10:11:02 GMT
An 80 or 100 thou head skim, port match and blend, port throttle body inlet,will give you gains for not much money. Already have 40 thou skim I don’t think any sort of port work is allowed AFAIK I is well confused...  I thought 40mm was about the max safe skim, at least if you wanted to retain the non-interference design? I am sure I've measured my head to be at about 50ish thou skim (from OEM head depth, using my Chinese measuring thing) and the valve seats are almost being eaten into at that depth  so I'm not sure I understand how a 100thou skim is even physically possible! |
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Post by dickie on Aug 29, 2019 15:39:26 GMT
My head has been skimmed by 80 thou, and it has piper 285 mechanical spec reground cams with around 11 mm lift with 10.5 to 1 pistons.
Nothing hits when its running !!! ( I was being conservative by only having 80 thou skimmed off, enough for another grind left if it gets warped )
I have read that a 100 thou skim is not uncommon across the pond. ( See if you can find Garage Quinn ITB Miata on the net )
If you look into cam timing you will find that at TDC there is not much lift so the the piston can be really close to the valve at TDC.
Think of the standard non interference engine with cam lift of 8 mm. this means that if the cambelt breaks the piston should not be able to hit a valve.
The other factor is the relative lobe position, with no cam belt the camshaft will never stop at full lift the valve springs force it into a position where most of the valves are partially open.
So if / when the belt breaks the valves will be about 4.75 mm open ( Reference Mk 2.5 1800 VVT head I have just measured )
The 4.75mm is measured at the valve stem angle, On the 2.5 engine the corner of the valve is above the head surface but the pistons are pocketed.
My engine is an 1800, the chamber design might be different on the 1600 although both are low compression as standard.
As far as keeping the engine non interference you are likely to be correct.
I you keep a decent cambelt on the engine there is no problem with going interference ! How many times have you heard of an Mx 5 breaking its cambelt ??
If it has been changed within 60000 miles or ten years, you should be fine as long as it has not been covered in oil.
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Post by Zed. on Aug 29, 2019 16:32:15 GMT
going slighty off on a (compression releated) tangent.... iirc, honda use 81mm & 83.5mm pistons in their type R engines that have 'intruder' crowns so an easy way to increase cr on an engine is to 'borrow' a set of pistons & use them in it  something I'm quietly looking into............................................................................... obviously added to the list of other things......  Rich. |
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so I'm not sure I understand how a 100thou skim is even physically possible!
le sigh