L28 Rebuild Dilema

[Zmanco]

I have to make a choice about rebuilding my engine and need advice.

 

Background:

It's a 73 with L28 that I rebuilt about 20k miles ago with flat top pistons (.030" over), shaved P90 head, stock intake, headers, Megasquirt, reground cam, oil cooler, etc. The engine runs great and is a ton of fun, but in the last few thousand miles has begun to burn a lot of oil. It consumes a quart roughly every 200 miles driven reasonably, and even more at the track. Compression checks out just fine, yet I can see the blue in the exhaust even at idle. I've also had to retard timing significantly in the 4k - 5k range which I believe is due to oil getting into the combustion chamber and detonating. My suspicion is that an oil ring is broken.

 

To make things more interesting, 2 summers ago the damper came lose and damaged the crank keyway and I fixed it using some Locktite products and it has held up well. But if I ever tear the engine down again, I'm going to replace the crank.

 

The issue is that it's time to fix the bottom end and here are the options I'm considering:

 

Option 1: find an old L28 short block and rebuild it using my current oversize flat top pistons. Total cost: estimate $500 - $600. Note: I'm not considering rebuilding the current block as I used a local machine shop that in hindsight probably didn't do a good job on the boring. When I had the head off recently I could see some cross hatching, but it was very uneven around the cylinders and between the cylinders as well.

 

Option 2: I've located an L28 short block that was rebuilt with dished pistons years ago by a guy who specialized in Datsuns. It has never been run and the top of the block is still taped/sealed. The price is very reasonable, and given all the other mods I've already done to the car, the thought is to swap this short block in and then go with a mild turbo build, not to exceed ~225-250 whp. Total cost: estimate $1500 - $2000.

 

I live at altitude in Colorado so the increase in HP would be even more significant since a NA engine loses aproximately 20% and the turbo can get that back and more.

 

The dilema is that this summer is really not the time for me to spend this extra money, but I have to do something with the bottom end and feel now is the time to make the switch to a lower compression engine if I ever want to go turbo.

 

I did have another thought which was to go with the dished piston short block for this summer and just run it NA. My CR would go from 10.0:1 down to 8.5:1. I imagine I'll lose some power, but wonder how much?

 

I'll stop here and wait for some feedback and thoughts.

[woldson]

Don't know how much will be lost, but you will notice it.

Then again your gas bill will be less;)

 

Can't you weld up the snout on the crank?

Also replace the rings? without tearing the top down? Just check the tolerances of the bores.

[Zmanco]

Don't know how much will be lost, but you will notice it.

Then again your gas bill will be less;)

Actually, I suspect my mileage would go down a little as a lower CR means a less-efficient engine all else being equal, doesn't it?

 

Can't you weld up the snout on the crank?

Also replace the rings? without tearing the top down? Just check the tolerances of the bores.

Yeah, I suppose that's possible, but given how cheap a crank is, I was thinking it made more sense just to replace it.

 

But your post has me thinking: assuming it is a broken oil ring, is it possible to just replace the bad ring and reassemble the engine with everything else left as it is? I mean, no honing the cylinder walls, replacing the rest of the rings, etc?

 

If that's possible, then the lowest cost solution would appear to be to leave everything else as it is and just replace the bad ring. That would be A LOT cheaper, although just as much work as rebuilding.

[Xnke]

Here's what I would do:

 

Replace all the rings, and hone the bores. No hone=glazed rings.

 

Replace all the bearings, and the crank, and crank rear seals.

 

Replace headgasket, oil pan gasket, and valve cover gasket, assuming I didn't tear anything up.

[woldson]

How many miles on that engine since last rebuild?

If you kept all bearings and same crank and just replaced the oil ring.....

Has to go back together the SAME way, all things have to be in place.

Were the rings oem type, or hardened?

If it is soft rings and last boring job is in good shape still,,,

There are others that can answer better then me, mabey Z-ya or Jhonc can chime in.

 

I know money is tight, if it were not I know what way you WOULD go, however......

[Zmanco]

Woldson, that's what I'm wondering: I know I can't replace compression rings without honing, but what about oil rings? The rebuild only has a little over 20k miles and except for the oil consumption, the compression is fine and it runs great. The rings came with the ITM pistons and are chrome/molly on the compression ring, and I believe cast iron for the rest. If I could just pull each piston, replace the broken oil ring (assuming of course that's the real problem), and reassemble, then it's just one long day and a set of rings.

 

Of course, you must know me because what I'd REALLY LIKE TO DO is go turbo and use this as the excuse But now really isn't the time

[woldson]

Pull everything from the bottom and inspect everything and post pics.

Worst case, you will have to do a full rebuild, and you will at least have most of the labor done.

 

I might want to do a leak down test, and also post up compression test numbers before disassembly. I've seen problems with cromolly rings not setting right.

Hopefully the molly ring has not set a lip in the cylinder yet.

The question of molly rings or not is above me and I would have to research more on the subject before I ran them my self.

 

One thing that I heard about a long time ago is the Detroit went hard rings, soft block, once the compression dropped the entire engine was garbage due to whored out clys. "planned obsolescent"

 

However, hard clys and hard rings, don't know.

 

With the pistons out, turn engine over and put some thin oil in, to insure valves are well sealed.