Joined: Wed Aug 25 2010, 02:04PM
Location: Upstate NY
Posts: 78
Hey all, I wanted to touch base with everyone to bounce some compression figures off you from my 68 300 Convertible. The 440 was rebuilt a couple years ago and had to be bored so it's .030 over. New pistons, rings, cam, timing belt... pretty much all new guts minus crank and rods. New 906 heads stock but with hardened valve seats.
Joined: Wed Aug 11 2010, 10:15AM
Location: E WA
Posts: 1230
limabee wrote ...
Hey all, I wanted to touch base with everyone to bounce some compression figures off you from my 68 300 Convertible. The 440 was rebuilt a couple years ago and had to be bored so it's .030 over. New pistons, rings, cam, timing belt... pretty much all new guts minus crank and rods. New 906 heads stock but with hardened valve seats.
Car has about 1500 miles on it since rebuild. Runs okay but still a bit off.
I'm trying to figure out what's going on in 8 & 6 as well as 7?
I have not yet done the leak down test but will be doing that by the weekend.
Thoughts?
Thanks
Mike
Did you cc the heads? Wondering if some of the differences in compression could arise from factory variability in the chamber size in the heads.
Not sure that a leak down test will tell you much. None of the compression values are really low. It is more the variability between them and even that is not terrible.
Joined: Wed Aug 25 2010, 02:04PM
Location: Upstate NY
Posts: 78
When I first built this everything was coated in cam build lube, the deep red syrupy lube and I lathered all the cam bearings, lobes, lifters, rods, etc. I went through a normal break in running the car at varying speeds and did my first oil change and everything looked great. I did notice that the plug on 7 looked like it was a bit hot while most of the others looked normal. In the least I plan on pulling the heads, replacing the gaskets and having the heads inspected at good machine shop near me.
I have had a few heads from different machine shops sent back. Being that the short block has been rebuilt lately,most of the time the machine shops get the bores,centerline,deck height right, it takes a real good machinist to get heads right. So it's a good choice that you are going to send them out. Could be a few of the guides have more clearance than the others.I sent out a set of heads (70' 340 X heads) to a guy i met at a car show and he owns a shop here in Winnipeg called "Heads Up".....Heads Up alright. If i would have taken his word and just installed them and fired the engine. I probably would not have made it to the end of the "break in". 3 guides were so tight to the valve,it would have been gone,even though i got them to install brass guides. I don't think they honed them properly. I brought them to my other machinist that i generally use and he went over them and got them all correct, and instead of beating me up and cutting me down,he charged me $200 bucks to correct everything. I put about 15 hrs of grinding,cc'ing,etc. on these,so that was a bargain for $200. It is amazing how much power a stock head can give with the right tolerances,attention to detail,etc. I'm guessing 10 to 1 that the problem is in the valve guide. The engine will live fine with those compression losses,but if it gets any worse,it will be that time.
Joined: Wed Aug 25 2010, 02:04PM
Location: Upstate NY
Posts: 78
My thoughts exactly Glen. I am also at a point in my life where I don't believe in cutting any corners and if something makes me scratch my head (no pun intended), I want it looked at rather then continuing on to get to the point where something catastrophic happens.
The heads are coming out this weekend and I will be bringing them to a reputable machine shop that actually did all my block work and installed my cam bearings and rods to pistons. They have also done excellent head work at a reasonable price for my daughters car.
I'll share my findings with them and see what they can find and I'll post back my results to the group for information.
Joined: Wed Aug 25 2010, 02:04PM
Location: Upstate NY
Posts: 78
So the heads came back and even though they looked like anomalies, the machine shop assured me that they were within reason and that readings such as those are normal.
I went ahead and put everything back together very carefully with tons of build lube and the car is running superb now. I ditched the electronic ignition and went back to the petronix as well. Have a few hundred miles on the car now and it accelerates great and idles in traffic during the hottest weather without over heating (removed the electric fan I put in and went back to a new clutch and fan assembly).
Joined: Wed Aug 25 2010, 02:04PM
Location: Upstate NY
Posts: 78
So here we are, 4 years later, and I believe I have a burned valve on the LH side. I'll be starting with compression checks again this evening. I wonder if it's #7.
