coletrain777 Posted August 9, 2009 Share Posted August 9, 2009 Soooo.... I found the reason for the low compression on #3 cylinder... a broken exhaust valve. The face of the valve had a 1/8 by 1/8 chip out of the edge, so of course the cylinder wouldn't have hardly any compression. The good news is that the cylinder walls of every cylinder look awesome (even #3) so wherever the piece went it didn't tear up the cylinder walls. So now the questions are: 1. What would have cause the valve to break like that? 2. I am just trying to get this together in an economical fashion so that the car it's going into will be able to move around and drive for a while (not a long distance DD or anything). With that being the case, would my best course of action be to just pick up a new exhaust valve, clean up the head and lightly lap the valves, put new valve stems in and call it good??? 3. All of my pistons have the numbers 3 and a little 4 except this number three cylinder. It has a 4 and a little 5. Whats the deal with the numbering on the pistons? It's not like aftermarket pistons that have the overbore sizing on them. Anybody know ??? Quote Link to comment Share on other sites More sharing options...
jasper Posted August 9, 2009 Share Posted August 9, 2009 From my experience..... # 1, probable lean mixture/exhaust leak. Check, replace injector/repair exhaust. #2 install new valve, etc, as stated in post. #3 Check factory manual...not sure. Quote Link to comment Share on other sites More sharing options...
MAG58 Posted August 9, 2009 Share Posted August 9, 2009 #3 should be a sizing by the factory for the correct piston. When the factory machined the block they bored it to 86mm..... ish. So after it was bored the bore was measured and a little number was stamped on the deck next to the piston, that is the size piston they put in from the factory. my F54/P79 had four 23's and two 54's. Quote Link to comment Share on other sites More sharing options...
coletrain777 Posted August 9, 2009 Author Share Posted August 9, 2009 So it's OK that my car has five "34's" and one "45". It seems odd that the factory would install different bore size pistons, even if the difference in size is extremely small. Also, do N42 and P97 heads share the same size valves both intake and exhaust??? I am just wondering because I have an N42 head that is partially disassembled, and I could rob an exhaust valve from it if they are the same. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted August 9, 2009 Administrators Share Posted August 9, 2009 (edited) Not a broken valve, but a burnt valve. For one reason or another the exhaust valve was not able to transfer its heat into the valve seat, (typical causes are chunk of carbon on valve seat, valve lash too tight, excessively lean combustion) which will act as blow torch across the valve head. Looks like knicks, splits, or chunks missing but really it was torched away. The exhaust valves are not brittle in that they will break, chip or shatter. They will however bend and deform, not chip or break. You will want to have that seat ground/cut at the very least. Should have the entire head overhauled. Edited August 9, 2009 by BRAAP Quote Link to comment Share on other sites More sharing options...
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