now that I have my pistons clean, I decided to take some real measurments. good news! with a stock head, and stock block, with a stock headgasket and stock cams.. the engine becomes interference when a total of 1.47mm of material is removed from the block and head.
I used my vernier caliper. Granted, this is not the most accurate way to do things since I do not have the valve train installed and I don't have access to a dial indicator so to not take this information as gospel. If you are building your own engine MEASURE IT YOURSELF or have a professional do it. I hold no responsibility if you screw this up! It is however, a good ballpark to see what we're dealing with.
I examined the exhaust valve.
First, I used a flat metal plate measuring 2.41mm thick and measured the distance from the head to the plate/ head surface parallel to the valve, touching the bottom of the valve, when closed. This value was 20.10mm.
Then examining this profile of the stock 7mge camshafts, posted on Ed's from australia website.. pulled the valve out .300", simulating 7.62mm of lift. which is the max lift of the stock 7mge camshaft.
this means my caliper needed to read (20.10mm - 7.62mm) 12.48mm. Measuring from the same reference point (the bottom leading edge of the valve).
with the valve pulled out, I removed the metal bar I used as my datum. The thickness doesn't come into play since I used it to measure difference, not total protrusion.
with the bar removed, I used the leading edge of my caliper and measure that the exhaust valve protrudes 4.01mm from the head.
Therefore,
7MGE valve protusion from stock head, with stock cams = 4.01mm
I then used the same piece of metal to measure the valve cutouts for the exhaust side of the clean, original toyota pistons. I measured 7.03mm at it's deepest point; the leading edge. Then, subtracting the thickness of my plate (7.03mm-2.41mm) = 4.62mm
Therefore,
7MGE valve cutout on stock piston = 4.62mm
Finally, I needed to consider both the amount the piston protrudes from the block at TDC and the thickness of a stock headgasket. Unfortunatley my block is not assembled, but using values found on supramania....
0.508mm deck height
1.37mm compressed stock headgasket
4.62mm - 0.508mm = 4.112mm without headgasket.
including the headgasket,
4.112 + 1.37mm = 5.482mm
Therefore,
total clearance from surface of head to bottom of valve cutout on stock everything = 5.482mm
Finally, considering the 4.01mm of protrusion from the head surface...
5.482mm-4.01mm = 1.47mm
so, as said earlier, 1.47mm of material removal is the approximate critical point that a stock 7MGE will become an interference motor.
so, shaving of the 1mm of material I would like to do to bring the engine to aprox 10.5:1 compression ratio will still leave the engine a non-interference one. Granted, carbon build up and thermal expansion can quickly remove that 0.47mm of clearance that is available for the valve and cause an interference situation, but I do intend to keep my timing belt replaced regularly and not make the mistake of firing it up with the cams out of time.
I used my vernier caliper. Granted, this is not the most accurate way to do things since I do not have the valve train installed and I don't have access to a dial indicator so to not take this information as gospel. If you are building your own engine MEASURE IT YOURSELF or have a professional do it. I hold no responsibility if you screw this up! It is however, a good ballpark to see what we're dealing with.
I examined the exhaust valve.
First, I used a flat metal plate measuring 2.41mm thick and measured the distance from the head to the plate/ head surface parallel to the valve, touching the bottom of the valve, when closed. This value was 20.10mm.
Then examining this profile of the stock 7mge camshafts, posted on Ed's from australia website.. pulled the valve out .300", simulating 7.62mm of lift. which is the max lift of the stock 7mge camshaft.
this means my caliper needed to read (20.10mm - 7.62mm) 12.48mm. Measuring from the same reference point (the bottom leading edge of the valve).
with the valve pulled out, I removed the metal bar I used as my datum. The thickness doesn't come into play since I used it to measure difference, not total protrusion.
with the bar removed, I used the leading edge of my caliper and measure that the exhaust valve protrudes 4.01mm from the head.
Therefore,
7MGE valve protusion from stock head, with stock cams = 4.01mm
I then used the same piece of metal to measure the valve cutouts for the exhaust side of the clean, original toyota pistons. I measured 7.03mm at it's deepest point; the leading edge. Then, subtracting the thickness of my plate (7.03mm-2.41mm) = 4.62mm
Therefore,
7MGE valve cutout on stock piston = 4.62mm
Finally, I needed to consider both the amount the piston protrudes from the block at TDC and the thickness of a stock headgasket. Unfortunatley my block is not assembled, but using values found on supramania....
0.508mm deck height
1.37mm compressed stock headgasket
4.62mm - 0.508mm = 4.112mm without headgasket.
including the headgasket,
4.112 + 1.37mm = 5.482mm
Therefore,
total clearance from surface of head to bottom of valve cutout on stock everything = 5.482mm
Finally, considering the 4.01mm of protrusion from the head surface...
5.482mm-4.01mm = 1.47mm
so, as said earlier, 1.47mm of material removal is the approximate critical point that a stock 7MGE will become an interference motor.
so, shaving of the 1mm of material I would like to do to bring the engine to aprox 10.5:1 compression ratio will still leave the engine a non-interference one. Granted, carbon build up and thermal expansion can quickly remove that 0.47mm of clearance that is available for the valve and cause an interference situation, but I do intend to keep my timing belt replaced regularly and not make the mistake of firing it up with the cams out of time.
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