Another curve thrown at me. Had to hone two inlet valve seats. New head gasket is genuine for 850 from MM (GUG702506HG). It measures .063 thick. The one I took out measured .032 obviously compressed. I can't see the new one getting compressed to .03ish so compression should go down. I had around 120 psig. Any guesses what it will be? I don't doubt it will run ok but read kinda bummed that I will lose some performance. It wasn't pinging before so would expect 120 psi shouldn't be a problem.
Thoughts?
Per minispares on that part number: 3.35cc compressed volume. If you give me your bore size, stroke, deck height, and head volume I can compute your compression ratio.
I'll bet it will compress down that small, unless the new one is a solid copper one?
Quote from: 94touring on June 22, 2020, 03:16:29 PM
Per minispares on that part number: 3.35cc compressed volume. If you give me your bore size, stroke, deck height, and head volume I can compute your compression ratio.
It's a standard 850 with standard bore /stroke 2.478 in(62.94 mm)/ 2.687(68.26) Deck is approx .014 with slight dished piston tops. MM has a note on this "Genuine Payen as fitted by Rover with a 2.8cc compressed volume." I haven't tried to do any computations but was going to try and estimate change in compression ratio per thousandths difference given a starting ratio.
I.e presuming 120 psi with .031 gasket add the difference .032 stroke to the computation. Do you have calculator/ spreadsheet?
Quote from: MiniDave on June 22, 2020, 03:24:01 PM
I'll bet it will compress down that small, unless the new one is a solid copper one?
New one is one side copper and composition. It has no identifying marks whereas the old one is marked AF470/TOP/FRONT on copper side and 3611 on composit side.
I wondered about getting compressed that much but have my doubts. I'll probably use it to get it back on the road and see what it gets.
I went back and found the order. It was a Gasket Kit GEG101-COPPER back in 2012 from MM.
I doubt the gasket thickness will make any meaningful difference on a 850, but it will be nice having all 4 cylinders making power again!
I have an app. So not knowing the piston dish CC here are some variables.
62.94mm bore
68.26mm stroke
0.014mm deck
21.5cc head chamber (stock 850 I believe)
3.35cc gasket
0cc piston dish= 9:5 compression
1cc piston dish= 9:2
2cc piston dish= 8:9
3cc piston dish= 8:6
4cc piston dish= 8:35
Thanks for the info. I didn't see a need to go to solid geometry to estimate the dish. I did go to the old thermodamnamics and estimated the change in pressure due to change in volume(stroke) and estimated a reduction of about 5% so I can expect 120 psi to go down about 5psi or 115. We'll see tommorrow. I think I overestimated compression drop.