Moving on with my project A15 engine build, i have gone for Mazda UC 77mm pistons.
These have a pin to top height of 31mm so i had the machine shop take off 1mm from the crown, this effectively makes them flat tops, there now a very small dish of around .2mm deep.. see picture.

I know i need to get back into the shed and measure it myself, but given a 1.2mm thick gasket and the valve lift with my Cam, am i likely to have the valves hitting the piston ?
Cam is a Camtech621, spec says .399 lift with 1.5 ratio rockers, so with the std 1.44 ones it should give .383 lift (9.73mm)
H89 head , unskimmed...

Attach file:

---

  mazda UC 77mm piston after machining off 1mm.jpg (59.04 KB)


  mazda UC piston.jpg (102.61 KB)

GX flattops have no valve-reliefs cut into them, and the GX also uses a thinner 0.6 head gasket. The GX cam is 8.4 mm lift. So yes, your combination is tighter. The clearance should be checked with a piece of clay.

To get a good quench, it is recommended to use performance gasket, such as the 79mm gasket from Nismo (Bprojects sells it).

Got some clay this morning and did a dry assembley..
See pictures.
I measure exhaust valve clearance to be aout 1.3mm and inlet to be 0.7mmm.

looks ok to me what do you think ?

Attach file:

---

  valve-piston clearance.jpg (130.31 KB)


  valve-piston clearance EX.jpg (79.30 KB)


  valve-piston clearance IN.jpg (94.26 KB)

when doing my custom pistons for my hillman Imp gt and later triumph herald both small L4 engines, I was recommended not to go below 2.25mm for the intakes and 3mm for exhaust valves especially going over 6500rpm yet stuffed up with my one tonner engine rebuild for rushing the build since it was my daily.

I would say you are not safe with this clearance and need more as rods stretch quite abit on missed gear changes and revving high with no load. During the dyno tuning of my 355 stroker 25 years back the tuner was able to hear the valves hitting the pistons which was just audible at 7000rpm and was a lot more work taking the pistons out, new valves and re assembly than where you are at now.
I later had a rod let go which always made me wonder if that one slug with the most damage should have been a reason to change the rod. Rods take good compression force but being stock ones with 300K Kms factory Hoes not checked for cracks it ended up ruining the crank and then licked my wounds and went the extra steps for my 383 and never looked back.

Yes, you could got as low as minimum of 2.0 intake, 2.5 mm exhaust with the gasket installed

Any way it need a little bit of notching. If you are careful you could do it by hand. The engine will need re-balancing afterwards so no worries

Neil dattoman10000 used these pistons on his hot A15 in his datto 1000 and his website may still be up with pics of them if not in his photo album

damn... ok i'll try and get some valve releif cut,, this project is working out more difficult than expected, but a good learning experiance...

DD, which balancing do you mean ? Cranks was polished but not machined so should still have the factory balance....

ok next question, i think i know the answer but just to check what you guys think....

See the picture, the Mazda piston has a cut out one one side, i assume this is for valve relief with the std mazda head.
if you install the pistons with the 'F' to the front (as i have done so far) then the cut out is on the side of the engine with the spark plugs, and with the H89 head where the Quench is.

Would it be better to install the pistons backwards? so the quench portion of the head is presented with a flat part of the piston and the cut out goes on the other side ?

That is my thinking.. the 'F' is just an aid to people rebuilding a std engine...

pretty important to decide before i start to cut the valve reliefs.....

EDIT... my bad, im getting confused with seeing the head upside down.. pistons should be fitted with the F to front as this aligns the flat part with the squish correctly.

Attach file:

---

  mazda UC piston cutout.jpg (98.90 KB)

Yes I run the same piston in mine
Never kissed a valve and planning on a fresh build using the same pistons again
I run 1.65:1 roller rockers too with this cam.

Check your valve to bore clearance if your running big valves... just to be sure... they were notched out alittle at the factory.
Might not be an issue but doesn't hurt to check.

Attach file:

---

  block.jpg (76.10 KB)


  cam card.jpg (50.25 KB)

Something to consider there pointed by Neil is the rocker geometry in case you change to roller rockers or similar higher ratio rocker, cam or other in the future. Doing it now leaves some scope for the future and safety margin.

Depending on the compression you can just mill them 1.5mm etc but notching for valves is not expensive.