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you can buy one but I just took an old spark plug, knocked the porcelain out of it, stuck a tubless tire valve in the hole, I might have taken it to the grinder ot make it fit... you can probably buy one for 10 bucks..
if you screw that into the spark plug, Its going to try to push that piston to BDC but you can turn the engine to different ositions like TDC, at TDC it wont turn the engine and if you are at the firing position both valves should be closed. ( I think)
you can then hear air flow, past your rings or back into the intake or exhaust manifold. If you find that there is basically a hole it might be a burned valve that isn't closing. if you have a broken piston and bits of the ring exiting through the top of the piston or similar you might find you have a leak on one cylinder straight down to the sump. you cna lsten tot he air rushing trying to escape.
It might be possible to trap the valves closed by maintaining the air pressure, and then remove the valve springs and change the valve seals, if they are loose and worn then maybe replacing those could improve things with a lot less trouble and expense than a rebuild.
I once had a piece of crp old early 70's volvo , it had a badly burned valve and I tried sticking a valve from a 122 in it. the stem was bigger so I just used my closest drill but size, stuck the valve in lapped it to seal. put it together..
It used a quart to get to work and another to get home. no one tailgated me becaue it had this oily atomized spray. I could floor it and make big clouds.. I ran the thing till all my used oil was used up but it was basically a lesson that the valves need to fit closer in their stems..
it is sometimes possible not to change the valve stems , take it to a machien shop and they can knurl the guides. this leaves a pattern that oil wants to stay in and reduces the valve guide diameter by sort of rolling the metal, then they can ream them to fit properly.. even when I put new guides in and took my volvo head a knowledgeable race type machine shop they still knurled them anyway.. some clearance is ok maybe .002 to .004 " ? there is likely a porsche spec on that.
I actually did use a lot of that motor later, the head is in my 122 now.. rebuilt and in fine shape.
a compression check will give you numbers to compare, you can do it dry then add a bit of oil to each cylinder, then do it again wet. theory is that if you see a big difference then its bad rings as the oil may help seal it a little .. if no change then it may be a valve not closing likely a chunk burned off.. If one cylinder has little compression you are going to have to take the head off to find out more, If you pul the head you can lok better and maybe see if the pistons seem loose or inspect the bores, maybe you have a broken ring..
rebuit before really means nothing much unless you knew who did it, They could have made a mess or maybe did great work..
sometimes with old cast iron engine blocks people go ahead and replace rings, but the top ring will wear a bit of a ridge near the top of the bore. then they fit new rings and the ring can hit that ridge and break. there is atool called a ridge reamer to remove this ridge if you are re ringing but not reboring..
you might see a line up the side of the bore or similar.. but the porsche engine is not a cast iron block and others can explain the intricacies of a rebuild of the cylinder walls and pistons better than I can. I think there is a coating in the bores,, maybe you can replace the cylinder sleeves instead. I dont think you can rebore oversize and fit larger pistons like you can with a cast iron block.
if you pull the head and see things not looking good with the pistons and bores you are probably going to be doing a rebuild of the bottom end.
so if you think the head needs work you can pull the head then rebuild it all , increase your compression but then if the piston rings dont seal well now you have more blow by than you started with and a big disappointment as you wasted a lot of time and energy, or you do the engine with new pistons sleeves , bearing shells, whatever to make it right and maybe come out more broke and happier.
the other option is you decide its not so bad, lower your redline change your oil and baby the thign for another 100 K like it is. the money you spend on oil is going to be very little compared to a rebuild.
I think we'd all like to just "do it right and spend what it takes" and then you will be happy but you are in control of these financial decisions. best to analyze things a bit ,see what you find, think it through..
I remember driving my 122 sounding a bit rattly.. and it started making this psst psst psst noise , followed by a "Ting!" then ran ok. that was the sound of a chunk of the top piston ring that made its way up through the top of the piston. it was rattling up and down, it held the valve open for a few seconds as it made its way through the valve, then I could hear the "ting" as it went out my exhaust manifold.
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