Mrmerlin

Thats a great idea get the Ladybug engine and swap it use the original for a coffee table

mark kibort

sounds like top end for sure... Gregs advice is good. go that route, but you can narrow down by leak down for sure. then, make the call. (to pull or just pull a head with it in the car) personally, i think pulling the engine is easy. and much easier for the head torquing, etc.

Tom928

After several months of attending to life events, I was finally able to get time to work on this.

Long story short, I removed the engine, pulled the head, took it to a new machine shop.
They found that the “tip height” was not set. The machinist called this a “rookie mistake” made by the previous shop.
Told me that is why the guy is out of business now. So this fixed the head issue.

Got the head back on the engine and it still fails leak testing. Other cylinders on the passenger bank pass with good leak times. But #4 has a gross leak.
One issue here could be, I am using the old head gasket for testing and only torquing to the first step.
I have a gross leak which sounds like it is coming from the water jacket hole next to the cylinder at the head gasket separation.

I checked #4 cylinder and there are 3 scratches. It looks like maybe I have a broken top compression ring. Below are photo's of the cylinder.
So I can keep going down this path, open up the bottom end, pull the piston, new rings...
OR
I have an 1984 S, it has a running 4.7L engine in it. The transmission will not shift out of 1st gear.
Do I pull the 84S engine (M28/20) and replace the 4.7L (M28/19) in the 81?

What to do, FIX or SWAP? Recommendations?

The Forgotten On

If you cannot catch your finger nail in any of the scratches I would say fix the engine. Rings are inexpensive in the 4.7.

Dave928S

What was the original leakdown of #4 at TDC result before pulling the head?
Lots of air out of intake (throttle wide open and no air cleaner) = open/damaged intake valve
Lots of air out exhaust = open/damaged exhaust valve
Lots of bubbles from the open coolant tank = failed head gasket
Massive blowby out of open oil fill = rings

If you don't have a conclusive original result, it's a bit hard to be sure now that you have the head only partially torqued on an old gasket, which could be giving you a false positive on head gasket.

Scratches could indicate a broken ring has been riding up and down for a while.

My guess is you you need to pull that piston. As you have the engine out, and it's easier, that's what I'd do ... it won't take long.

Mrmerlin

that bore still looks good, IE its not damaged yet.

pull the piston.
what does the HG sealing surface look like?

Imo000

Tip height = ?

The bores aren't bad enough to cause a huge leak, something else is going on. Put the new head gasket on it and tighten it down to the first step and do another leak down test. This way you can still re use it later.

Tom928

Leakdown values:
#1 = 4-5 sec
#2 = 6-7 sec
#3 = 5-6 sec
#4 has always been 0 maybe 1 sec.

The scratches are not enough to catch a fingernail on. They look like they are made by broken rings as I do not see these in other cylinders.
Below is a photo of the head sealing surface.

I don't as of yet have a new head gasket. I did put the old one back on and torqued to the 1 step. I get the same values on cylinders 1,2 and 3.
Since I hear a different sound when testing #4 like air coming out oil return holes, I plugged all but one of the oil return holes in the head.
On the open return hole I placed a small piece of paper towel just over the hole / head surface. When I pressurize the cylinder the paper towel is BLOWN off the hole!!!
This tells me I have a gross leak past the rings. Could both compression rings be broken?

Looks like I am pulling the piston. I've haven't pulled a piston on a 928.
Since this is new territory for me any tips would be appreciated.

Imo000

Time to pull that piston.

dr bob

No magic for 928 vs any other v8 engine. Make sure you have a ring compressor for reassembly.

Consider... Putting some ATF or maybe paint thinner in the cylinder and watching to see if it leaks down through the rings. You'll be changing the oil anyway.

Tom928

I have a ring compressor but my ring installer is gone.
I have the oil pan off, both rod nuts loose. I'm at the bottom of the stroke.

Here is what I've done before...
Tap upward on the rob bolts to separate. Remove the rod cap,
Then install rubber thread protectors, then rotate to the top of the stroke and remove the piston.

Tom928

OK we found the problem. 2 broken rings.
So I had bad tip height on the valve that was replaced and we had 2 broken rings all in the same cylinder.
No wonder there was no compression and leakdown failed so badly.

Mrmerlin

make sure to measure the ring end gap follow the WSM for specifics

Dave928S

Installed valve stem height .... has to be way out to hold a valve open, especially with hydraulic lifters at stationary TDC during a leakdown test. More of an issue dynamically.

Tom 928 ... you'll need to inspect that piston very carefully to make sure the ring lands aren't damaged from ring flutter.

Thinking about this issue, and that mine had #4 top ring go, during a run where I was wringing every last rpm out of it relentlessly. A phenomena that I've seen many times on race motors running on the limits, is that the cylinder which runs hottest is usually one at the rear of a V8 bank, and the rear of an inline 4 or 6, unless there's been some inventive re-sizing of coolant flow paths to even all cylinders out. I wonder if there might be an element of this, or cylinder A/F variation, on a 928, that causes individual cylinder (#4) detonation under extreme conditions, giving rise to ring flutter and breakage. Was there an event which you can remember when it suddenly just didn't seem to have the same power?

jeff spahn

Its funny. I put in a new piston and never checked this to the best of my recollection. That was almost 45,000 miles ago and at least 30,000 of those miles have been boosted.
Wonder what I did right?