Post-Rebuild Pre-Start Compression?

[David Inc.]

Background: I recently finished a cam replacement that included pulling the heads for work. Not quite a rebuild (I had one of those 10,000 miles ago), the heads just needed to get pulled for a shop to deal with exhaust studs. I had the 964 grind on the cams from Dougherty and timed them to spec.

Now with everything buttoned up it seems like there's only the smallest of compression on most cylinders. There's air moving as I turn over the engine but nothing like what I would expect from previous experience when I could really hear the thing "breathing" and torque to turn it over was significantly higher. I had a squirt of oil in each cylinder through the spark plug holes but it didn't change things much.

So of course now I wake up at 2am wondering what I messed up, did I adjust all the valves? Am I sure I timed things right? Did I forget a cylinder head gasket ring? But more likely it's just that the engine was opened up, oil drained out everywhere, maybe some shmutz is caught in the valves, and nothing is really going to seal up until I do the startup procedure, right? .... right?

Someone please make me feel better, thanks in advance.

[icarp]

Sometimes the cams are installed on the wrong sides , also cam timing ?
Compression should be normal by hand cranking
Ian

[David Inc.]

Originally Posted by icarp

Sometimes the cams are installed on the wrong sides , also cam timing ?
Compression should be normal by hand cranking
Ian

The cams were wide spread on left, close on right per the manual so the exhaust opens first on each side. Timing was set with the Stomski tool to 1.31mm overlap each side, which is just a bit above the 1.26mm listed by Dougherty. I was planning to check the timing at about 1,000 miles.

There's compression, it just doesn't have near the same feeling as what I remember it was before I broke it down.

[snbush67]

Spark plugs in? If there is as you say some shmutz in the valves then you got to flush it out before you fire it up. Check the valve lash again, try setting them a bit loose to ensure they can seat fully.

[john walker's workshop]

Perhaps go over the valve adjustment again. Feel for clearance manually at least. Do you have a compression gauge or a leakdown tester?

[David Inc.]

Originally Posted by snbush67

Spark plugs in? If there is as you say some shmutz in the valves then you got to flush it out before you fire it up. Check the valve lash again, try setting them a bit loose to ensure they can seat fully.

The plugs are in, and valve lash was all set to 0.1mm with both the Kirk tool valve adjuster and double checked with the feeler gauge. I'm pretty sure I did every single one but of course that's what keeps me up at night. What do you mean by "flush it out"?

Originally Posted by john walker's workshop

Perhaps go over the valve adjustment again. Feel for clearance manually at least. Do you have a compression gauge or a leakdown tester?

I do have a compression gauge, maybe that's just what I need to do. Isn't it typical, though, for that to read low before first start or only when the rings have been changed out? Like I said I pulled the heads and reinstalled with new crush gaskets on the cylinders, but that was the extent of that--the pistons never left the cylinders.

If I haven't run the engine would the paper valve gaskets still be good or would I have to toss them?

[T77911S]

do a leak down
i had my heads done by someone on pelican and thank goodness i was curious what the leak down would be before finishing my rebuild. i had so much air going past the valves it wasnt funny.

you could be 180 out on your cam timing although not sure if you would hit vlaves if you were

they are fine but i would get the silicon reusable ones.
when you tighten them only SNUG the bolts. if the gasket squeezes out then they are too tight.
i still feel like they are not tight enough but you can always go back and snug it up a tad.
clean the threads very good so you have a good feel for when they nut stops.

[David Inc.]

I thought the silicon ones were only for the later style valve covers and didn't work with the 911SC?

[john walker's workshop]

The rubber ones get returned in a ziplock bag when I do valve adjusts.
You should see compression around 125 to 150 before it's been run. Do that first to ease your mind.

[David Inc.]

Wait... right now the engine is on an ATV jack waiting for the car to get back from body. How could I do a compression test? Don't I need oil supply and power to the starter for that?

[snbush67]

Yes, just reuse the valve cover gaskets if they were new.

By flushing out the shmutz (your word) I mean look into the intake and exhaust ports and verify that there isn’t anything in there that shouldn’t be. You can look in to see the valves open and close as you rotate the engine.

Did you have the valves checked when you sent the heads out or only the exhaust studs replaced?

[David Inc.]

Originally Posted by snbush67

Yes, just reuse the valve cover gaskets if they were new.

By flushing out the shmutz (your word) I mean look into the intake and exhaust ports and verify that there isn’t anything in there that shouldn’t be. You can look in to see the valves open and close as you rotate the engine.

Did you have the valves checked when you sent the heads out or only the exhaust studs replaced?

I misspelled it. Schmutz, meaning dirt and filth. I'm from New York, a lot of Yiddish words somehow made it into my vocabulary. Anyway.

Unfortunately the engine is already fully built up and ready to go back in the car, but popping the intake up wouldn't be too bad. That said I might go with leakdown first before trying disassembly.

The valves and heads were machined when I had the top end rebuild about 10,000 miles ago when my head studs gave out. Unfortunately this was when the shop misaligned the rocker shafts which chewed up the cams which lead to me doing another teardown. Anyway I didn't do anything with the heads and valves this time, just reassembly. Kind of why I'm thinking there might just be some oily dirt or carbon caught in the valve seats now that will blow out once I try to start it up.

[T77911S]

Originally Posted by David Inc.

Wait... right now the engine is on an ATV jack waiting for the car to get back from body. How could I do a compression test? Don't I need oil supply and power to the starter for that?

do a leak down.
listen for air leaks.

[David Inc.]

Leak down tester is on the way, guess we'll see. I'll try to report results later, thanks all.

[T77911S]

are you sure the cams are not timed 180 out.
well i guess it could only be one that is 180 out.

[David Inc.]

Positive. Here are the cams in time with the crank and under tension before I sealed it up. Unless a chain skipped a tooth or something after I sealed it there should be no issue.



Edit: Huh... pelican reduced the image size for some reason. Anyway, dots are pointing up on both cams, crank has Z1 at the case line. When adjusting the valves everything worked out fine with TDC at the expected 1-6-2-4-3-5.

Amazon tells me I have a leakdown tester waiting at home, we'll find out soon.

[David Inc.]

#4&6 read high and were happily blowing air out of the exhaust. Just them, though, the left bank and #5 were all fine. The exhaust valves for #4&6 were adjusted properly but the leak down values weren't steady as I turned it over, like they changed every full rotation of the cam, so I'm fairly certain at this point that those valves just have some crud in the seats--I must have gotten some thing in there during cleanup and assembly. Actually the last rotation #6 was as good as the others.

I'm thinking after I break the cams in I'll readjust those valves right away.

[David Inc.]

Final resolution in case any future person has an issue and searches.

After cam break-in both #4 and #6 were out of tolerance and needed adjusting, which I'll revisit for all cylinders at about a thousand miles or over winter storage, whichever comes first.

Otherwise the compression issue I think was my own mis-judgment and use of a breaker bar for turning it over instead of a shorter 1/2" drive ratchet I normally use. It's harder to turn over now for sure with everything well coated in oil and seated, and a respected air-cooled shop had it in for alignment and fuel mix and said it runs "like a scalded cat", so I'm happy.

New cams feel great, too, a lot more umph up at redline. I didn't feel like I needed to shift up on track as early, which is great.

[Jeff Alton]

Glad it worked out.

In future, if you are at all worried, do what T77911S said and perform a leakdown test while the engine is on the stand. Takes 10 mins, We do it on every motor we build. Not required, but piece of mind.

Cheers

[911 SLANT]

I was always told to do leak down with motor hot and after rings have seated. Is this not true?