Top End or Complete Rebuild?
 

Hi All,

I stripped my '78 911 SC engine down to the short block. At least 85K on the car.. could be a lot more as speedo was replaced by PO. Keep in mind this is the very first engine I have stripped down, so my experience is minimal. I am debating whether I should stop at a top end or do a complete rebuild. Here's why:
*Oil pressure: 4-5 bar when cold. Then, when warm, will match the revs up to 3 bar. It maxes out at 3 bar regardless of higher RPMs. I did not check the pressure sender nor do a mechanical pressure test before pulling the engine. I checked the connecting rods and all look good. Some side-to-side play (which I read is normal) but no back and forth play. Cylinder 5 rod has some pitting on the rod where the piston wrist pin goes through. See photo.
*Knocking Noise: Seemed to be coming from cylinder 4, 5, or 6 valve train but I don't want to assume. My cam rocker shafts were beginning to walk out on the exhaust side. If it is a rod knock, would it be obvious upon inspection of the rods in the short block?
*Compression/Leakdown: 110-120 PSI on all cylinders (consistent). 75% leak on cyl. 1 (intake) and 5 (exhaust). 20% on 6 and 2 (rings). 4 and 3 had good leakdown numbers. Guessing this will be solved by the top end valve job and new rings.
*Chain: Looks good but has obvious wear/darker spots.

What would you do? Top end or full rebuild? Note: this is going to be a street car and not a track car.

Supporting photos:

Pitting on cylinder 5 connecting rod (where wrist pin sits). This is the only rod that has pitting:
http://forums.pelicanparts.com/uploa...1710740933.jpg
Rods on cylinder 4, 5 and 6 end: notice the gunk especially on #4 opening.
http://forums.pelicanparts.com/uploa...1710740954.jpg
1, 2, 3 side of the case. Notice how the case openings on #1 and #2 are a lot darker than #3. Is this a problem? What is the cause?
http://forums.pelicanparts.com/uploa...1710740971.jpg
Timing chain showing wear:
http://forums.pelicanparts.com/uploa...1710741000.jpg
Pistons and cylinder heads. 1,2,3 on the left and 4,5,6 on the right.
http://forums.pelicanparts.com/uploa...1710741017.jpg
Image showing carbon buildup on the piston. All pistons & rings seem to have a lot of carbon buildup.
http://forums.pelicanparts.com/uploa...1710741053.jpg

One thing you could do is check the big ends - split the bearing halves and plastigauge and visually inspect the big end bearings. You can do this without splitting the case.
The warm oil pressure pegging at 3 bar is a bit of a clue. A fresher bottom end will be higher.
It is probably nicely worn but not beaten up.
Alan

Same comment as Alan: Check the rod bearings either by measuring or by Plastigage. If they are good, the main bearings and crank almost certainly are good too. If you want to split the case, that will add several thousand dollars to your bill.

Darker color comes from longer intervals between oil changes , too long , way too long .
5,000 is just fine with Driven GP1. 11+ quarts , change your oil if it gets over 235-245 degrees , heat will change the color of the aluminum as well , dirty brown color .
The way your engine looks I would split the case , no big deal just take your time .
You have the best series of engines , 78 big port heads and the most simple CIS.
Put some M1 - Kermit cams it and it will go like stink.
AASE valve springs and Ti retainers , when you are done make sure you rev the ***** out of it
7,000 rpm every day at least 30-300 times , have fun with it .
Ian

Yes - I suspect the engine has had a lot more than 85k on it. And that oil pressure - while 'acceptable' is not what you would be looking for in an engine that has just had a refresh. It is only downhill from there. And less than 3 bar really would be unacceptable for even a medium term proposition. The bottom end will tell the final story.
Alan

We’ve never opened up an engine and thought it a mistake.

Full rebuild. 100%. No question. Replace, chains, guides, check sprockets for wear. Enjoy the process.

If you only do the top, you will always be thinking "I wish I had done the whole thing".

Thanks for the input. What are the biggest cost adders on the bottom end? I assume a replacement oil pump and IMS shaft are the big ones (there's $2k right there). I spoke to Walt at Competition Engineering this morning. He didn't think the 3 bar max was the end of the world, but he also did not see pictures of my case that I posted here.

What is the key cause of the oil pressure issue? I would assume lack of pressure being supplied by the pump is the main culprit on a "tired" engine?

Unfortunately, it ran a bit hot the last few times I drove it (upper white mark on gauge ~245 F). When it got to this point, I turned it off and let it cool down. Turns out that rodents made a large nest right on top of the oil cooler.

Ian, thanks for your suggestion. I sent William an e-mail asking for more info about these cams you mention.

Not sure about the pump being the issue. Wear in the crank and bearings. Machining and new bearings, and big end shells etc etc.
Alan

Gotcha, thanks Alan.

Just came back in from removing lower Dilavar head studs. Some came out with little effort and others required a lot of force. They're all out now minus two snapped ones on cyl. 1 & 2 that I have to remove the case nuts in order to slide my stud removal tool onto. That's for tomorrow night...

When I peeked into cylinder #6 again, I noticed this. WTF.. looks like a lobe off the crankshaft has broken off! :eek: This certainly isn't normal, is it?

http://forums.pelicanparts.com/uploa...1710831755.jpg

That does look a bit odd. You may find something in there :-)
Heat is your friend to get broken studs out. More the better. Normal propane gas is barely sufficient.
I use oxy and propane/butane. used to be acetylene. Don't break the short bit of the remaining studs - or you really will have problems. Use heat if they won't come. If they still aren't coming - more heat. They should come free easily if you have enough heat.
Alan

Like Alan said about the heat, really lay it onto the area around the stud and the inner register wall. Don’t be shy.

Yeah that crank web doesn't look right. I retract my earlier advice. You now have a good reason to split the case.

However, as you do, keep the bearing shells in order so that you can reuse them if they are good. Reuse bearings? Yes. journal bearings do not wear out in normal use. They wear out because of lack of oil or contamination. Measure and plasitage--if they are good, reuse them. That will save you $600 or so. Same for the rod big ends.

Your oil pressure is below factory specs. Why even consider a top end only build?

Cheers

I’ve decided to split the case after hearing your input and wanting to do this the right way. The kiddos slept early tonight so I got a good 3 hours in the garage. Got the hard oil pipe off (thanks pipe wrench), removed the 2 broken studs, and got most of the case bolts undone. Only a handful left. Will be splitting tomorrow or the next day. Gotta say those case bolts smelled very funky (kind of like formaldehyde).

The steel studs on the intake end appear ok. I recall Wayne mentioning that they can be left in the case. I may just do that unless you guys have heard of those snapping?

On another note, the Ultrasonic Cleaning shop tested my engine oil cooler and oil tank today. Both are leakers. External oil tank wouldn’t hold 2psi and oil cooler started leaking out at 10psi. Maybe that’s part of my pressure issue. Will keep you all posted on the progress.

Good work and likely a good decision. You will feel better when you put it all together with a fresh bottom end as well.
not too likely the cooler is the cause of low pressure. You would have to be loosing quite a lot of oil. You already have a loss system with the piston squirters and cam spray bars. You would need to be loosing more than those to make a difference. I suspect you will find the answers in the bottom.
Alan

Low oil pressure at rpm when hot can be an indication of excessive bearing clearance. Not just the rod big ends, but also the crank main bearings in the case. When the case is apart, measure your bore accurately.
I had a motor with simlar oil pressure and the main journals all measured between 3.5 and 4.5 thou. Those 7 sets of journal bearings on the large side, are like having 7 little oil leaks...

Mike speaks wise words.

The tank sees almost zero pressure..... Was it leaking? If not, testing it is a bit of a waste.

Did your oil cooler leak?? If not, it holds pressure. Maybe find a place to test it that knows a little bit more about what they are doing.

Cheers

Mike and Alan, thanks for the input.

Jeff, one of the shop’s specialties is testing oil coolers. While the oil cooler wasn’t actively dripping, the area certainly was covered in oil. I’ll stay on the safe side and try to source one in better condition. The oil tank’s leak was from the welded mount bracket area. Agreed that it doesn’t hold much pressure but it is still an entry point for oxygen. I’m talking to a couple of shops that may be able to braze/weld it.

Well I decided to pull all the steel head studs on the top of the case tonight. I just wasn’t comfortable leaving those on due to the age and the small amount of surface rust. A couple of them took a propane torch, but they’re all off. Got those handful of nuts off and now all the fasteners are off the case. Going to call it a night and get to case splitting first thing tomorrow evening.

I know the nasty smell from the case bolts. A very unique smell. I thought rotten eggs.