Cosworth piston issue... photos inside
One more issue is the diameter of the cylinder hole in the gasket. The 'thickness' is based on exact cylinder measurement. If the gasket diameter is larger, this adds a little too.
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Evolving Member
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From: Evoland
Something does not sound right to me. You are telling me that the piston dish is 6.5cc. Ok.
The measurement of the deck height looks like 1.3mm. If that is true, that works out to be 7.4cc, but you've somehow measured 9.5cc. 7.4cc = 8.27:1
Are you sure your deck height measurement is accurate?
Are you sure the piston moves no higher in the bore?
Are you sure your 9.5cc figure is correct? How did you isolate the piston dish from the rest of the deck space?
I have not ruled out the possibility that there may be some inaccuracies that are clouding the real figure.
And of course, a 1.2mm gasket will compress somewhat to a reduced height when torqued.
The measurement of the deck height looks like 1.3mm. If that is true, that works out to be 7.4cc, but you've somehow measured 9.5cc. 7.4cc = 8.27:1
Are you sure your deck height measurement is accurate?
Are you sure the piston moves no higher in the bore?
Are you sure your 9.5cc figure is correct? How did you isolate the piston dish from the rest of the deck space?
I have not ruled out the possibility that there may be some inaccuracies that are clouding the real figure.
And of course, a 1.2mm gasket will compress somewhat to a reduced height when torqued.
The piston does not move higher.I am sure.
16cc was the Piston Dish + Deck Relief and 6.5cc was the piston dish it self so 9.5cc is the deck relief.
With the head gasket compressed and engine running, this is what I estimate, assuming the deck relief measurement is accurate and piston at exactly TDC:
V = 85mm Bore x 88mm Stroke = 499cc
C = Combustion Chamber = 48cc
P = Piston Dish + Deck Relief= 13.9cc (the piston WILL expand when hot)
G = 1.2mm Gasket for 85mm Bore (compressed to ~0.8mm) = 4.5cc
SCR = (V+C+P+G) / (C+P+G)
SCR = 8.51:1
Be absolutely certain the deck observations are correct.
V = 85mm Bore x 88mm Stroke = 499cc
C = Combustion Chamber = 48cc
P = Piston Dish + Deck Relief= 13.9cc (the piston WILL expand when hot)
G = 1.2mm Gasket for 85mm Bore (compressed to ~0.8mm) = 4.5cc
SCR = (V+C+P+G) / (C+P+G)
SCR = 8.51:1
Be absolutely certain the deck observations are correct.
My experience has been to bore/hone each one independently due to variations in the piston skirt measurements. Long time ago and I'm sure they're different/better now, but that was even on race pistons.
Thread Starter
Evolving Member
Joined: Nov 2007
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From: Evoland
With the head gasket compressed and engine running, this is what I estimate, assuming the deck relief measurement is accurate and piston at exactly TDC:
V = 85mm Bore x 88mm Stroke = 499cc
C = Combustion Chamber = 48cc
P = Piston Dish + Deck Relief= 13.9cc (the piston WILL expand when hot)
G = 1.2mm Gasket for 85mm Bore (compressed to ~0.8mm) = 4.5cc
SCR = (V+C+P+G) / (C+P+G)
SCR = 8.51:1
Be absolutely certain the deck observations are correct.
V = 85mm Bore x 88mm Stroke = 499cc
C = Combustion Chamber = 48cc
P = Piston Dish + Deck Relief= 13.9cc (the piston WILL expand when hot)
G = 1.2mm Gasket for 85mm Bore (compressed to ~0.8mm) = 4.5cc
SCR = (V+C+P+G) / (C+P+G)
SCR = 8.51:1
Be absolutely certain the deck observations are correct.
How much SCR is good?
How much SCR do you (everyone that you have a built engine) have?
clearance
1. how far down is the piston from the deck? it looks ~1.0 mm
from a power perspective, that isnt ideal.
deck the block & time the cams accordingly.
measure the piston pin height & rod c/c to figure out where the discrepency is, relative to the oem pieces.
i would feel safe running a piston to head clearance of 0.74mm if spinning to ~9k or so.
GL
from a power perspective, that isnt ideal.
deck the block & time the cams accordingly.
measure the piston pin height & rod c/c to figure out where the discrepency is, relative to the oem pieces.
i would feel safe running a piston to head clearance of 0.74mm if spinning to ~9k or so.
GL
Thread Starter
Evolving Member
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From: Evoland
I agree with Aby, .74 mm should be enough clearance between the piston and head. Thats where we shoot to be. I dont feel that you are getting an accurate cylinder volume due to the piston not sitting at the preferred height. I would check to make sure that rod length does not vary from stock. Also it is possible that the compression height is not correct in the piston. I almost wonder if Cosworth adds clearance due to so many people machining blocks.
Did you ever had the same issue with AMS pistons?
My mechanic has the same issue once more in the past with a set of JE pistons from USA in an Evo 8.He switch them with another brand and then everything was OK.
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Evolving Member
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From: Evoland
How much torque for ARP L19 EVO IX headstuds?
How much can I deck my block without having any problem?
What CR do you suggest?
Ill have to check the cosworth head volume, I dont have one here to measure. The L19 studs can be torqued in increments of 70-80-and final of 90
I dont like machining too much off of the block due to the cams moving closer to the crankshaft centerline. This changes your valve timing and will require you to degree your cams in for optimal performance.
We have upped all of our pistons to 8.9 to 1.
I dont like machining too much off of the block due to the cams moving closer to the crankshaft centerline. This changes your valve timing and will require you to degree your cams in for optimal performance.
We have upped all of our pistons to 8.9 to 1.
Thread Starter
Evolving Member
Joined: Nov 2007
Posts: 429
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From: Evoland
Ill have to check the cosworth head volume, I dont have one here to measure. The L19 studs can be torqued in increments of 70-80-and final of 90
I dont like machining too much off of the block due to the cams moving closer to the crankshaft centerline. This changes your valve timing and will require you to degree your cams in for optimal performance.
We have upped all of our pistons to 8.9 to 1.
I dont like machining too much off of the block due to the cams moving closer to the crankshaft centerline. This changes your valve timing and will require you to degree your cams in for optimal performance.
We have upped all of our pistons to 8.9 to 1.
What you suggest me to do now with my issue?
To get new pistons?