how much to repair?

[new2bmw]

i have an m20 with low compression on cylinder #5, i was wondering what this may be caused by and what parts i may need to fix, trying to guess a cost here. i may swap it into my 84 318i if it doesnt seem like a lost cause. heres the results of test
300,000 km 89 325i
cyl 6 165
cyl 5 120
cyl 4 158
cyl 3 169
cyl 2 158
cyl 1 165
did a wet compression test on cyl # 5 and it went up to 135
I am hoping i can just repair cyl 5 and be done with it without too much trouble.

[jlevie]

First you need to determine whether the low compression of #5 is due to rings or valves. A wet compression test may tell, but a leak down test will almost certainly tell. Given the other cylinder numbers, there is a good chance that this is just a head problem which is pretty easy to fix.

[VinniE30]

I had no compression in 3 cylinders and it was from bent valves.