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There are lots of places to get into trouble when building an engine. Some oil consumption is to be expected on a rebuild but not anything close to a case in 2000 miles more like a quart. Oil consumption can be related to 4 things, leakage, venting, or pulled down the guides or around the pistons.
1) Leakage you have, how bad I don’t know except for your description which doesn’t sound like all that much.
2) Venting usually is a problem with the PCV valve staying open when it shouldn’t allowing the intake to suck oil past the valve. Or there is so much blow by that the crankcase becomes pressurized and oil is blown out past gaskets and seals as well as past the piston rings, we’ll get into this with the piston discussion.
3) Oil pulled down the guides. There’s three principle elements to this:
a) First an simplest is guide seals, there’s “O” rings, not very good, umbrella
seals these are pretty good both O rings and umbrella work best with cast iron guides as these require more lubrication than does bronze and its alloys, the best for keeping oil out of the guide are the positive seal often made of Teflon and require machining of the upper guide boss. These can be a bit dry for cast iron guides but are great for bronzy types. They are easily damaged upon installation or from impact with the underside of the spring retainer. The intake is the most sensitive to seal as its stem and guide are exposed to manifold vacuum which tends to pull oil down this interface
b) The valve selection should always start with those models that feature a hard chrome overlay on the stem. This really adds to the wear properties of the stem and guide since the chrome requires much less oil for lubrication, therefore they get along real well with cast iron guides and Teflon seals, should you end up with that combination.
c) The guide is all important; there are several ways of refurbishing them. Stock from the factory with cast iron heads is simply a close tolerance reamed hole. Many rebuild shops refurbish these by running a knurling tool which rolls a thread like pattern into the guide wall. This squishes material together reducing the holes’ diameter but leaving and end to end screw thread in the guide, one can see oil traveling down the thread, this requires a really good stem seal to prevent. The knurled hole is then reamed back to the original size and stock stem diameter if not original valves are refitted. This is the least good “fix”. Next best, and close to factory, is to ream the guide oversize and replace the valves with those having a matching oversize stem. Done right this is as good as the factory’s original. Best, and of course most expensive, is to bore the guide boss to accept a replacement guide, these can be made of cast iron but most are some sort of a bronze based alloy. Guides are always refurbished or replaced before the valve seats are cut as they are used to locate the tool refurbishing the seat. If you do seats first then guides, in all probability, will be unable to seat the valve and the mismatch will quickly tear up the guide and stem from unequal side loading resulting from the valve being cocked on its seat.
4) Oh boy, what can go wrong with the piston, rings, and cylinder walls.
a) Jumping backwards I’ll start with the cylinder walls. Excessive over-boring like anything more than .030 inch without sonic checking for core shift can get you a wall that flexes with piston side loads and the pressures of compression and combustion. This will break the ring seal allowing oil into the combustion space and may lead to cylinder wall cracking in the extreme. Cylinder wall finish is important, this must be matched to the type of rings used. Moly coated rings want a smooth finish wall, a rough finish will strip the moly off the ring causing a loss of compression past the ring pack which results with oil consumption. Chrome rings need a rough finished wall to break into, if the finish is smooth, chrome rings skate over the wall and never get a seat, this pumps oil into the combustion space.
b) Keeping on with ring discussion; stroker small blocks especially when fitted with 5.7 or 6 inch rods force the pin upwards toward the ring pack in order to keep the top of the piston from protruding out the block at TDC. Some of these designs require a steel ring be placed under the oil control ring where it passes over the pin bore to provide the ring with structural support in this area. If that additional ring is missing, bent, or of incorrect thickness the oil ring will allow oil to remain on the cylinder wall where the compression rings will pick it up and pump it into the combustion space. Another common problem is spacers used, or not, on the upper compression rings. The width of the ring land must be compatible with the thickness of the ring. There are several sizes out there and measurement must be made at assembly to insure the ring not only end gaps properly but is the correct thickness for the land it’s going into. Needless to say the ring and piston must be the correct size for the bore diameter. I’ve seen standard bore pistons in .030 over bores. .030 over pistons in .030 over-bores with standard rings, you name it, from name brand makers as well as local shops I’ve seen it at one time or another. Ring end gaps need to be spec’d, too little and the rings butt together, bind, twist and often break, too little and oil and compression leak through the hole. Gaps need to be staggered around the piston as to keep gaps from lining up and presenting a slot for oil and compression to get around the piston. It’s also not unknown to discover the rings, especially the top two, were installed upside down, you don’t even want to go there with me. My face goes purple just thinking about that very expensive “don’t open” motor.
c) Certainly much of what I said about rings links into typical piston problems with incorrect sizes for the bore and inadequate support for the oil ring on engines running long rods and strokes. But pistons have problems all their own relating to incorrect shape both circular and lengthwise. Incorrect ring groove size both in width and depth, incorrect finish as well. The rings, except oil, flip between the top and bottom of their lands. They must have smooth and parallel surfaces upon which to reside or they oddly twist letting oil in and compression out. Pressure of combustion floods behind the upper ring causing it to “breath” on each cycle. If this can’t happen, it’s getting old isn’t it? It’s oil into the combustion space and compression out. Skirt cracking is not uncommon, nor is the pin bore not being centered and normal to the piston. This can result in the piston being twisted and cocked or both to the bore. In these cases the rings don’t stand a chance of keeping oil out and compression in.
Other issues that can develop unrelated to those above are too much oil passing thru the engine. High volume oil pumps can be a problem when combined with excessive bearing clearance, in that case gobs of oil gets around the crank bearings and overwhelms the rings. However, if clearances within the engine are nominal, the extra volume of a high volume pump becomes excessive pressure since it can’t get thru the bearings fast enough. Then the pressure relief spring opens and vents the excess oil back to the sump. However, if too much oil getting past the bearings is a problem, and this should only be a problem with a loosely set up race engine, scrapers and windage trays are used pull the oil off the crank and return it to the sump before it becomes a problem for the rings. Although the extra pressure will also push a lot of oil to the top end and if the engine does not use Teflon seals or has otherwise inadequate machining of the guides, the simpler types of stem seals will be overcome with oil. You don’t want so much oil in the top end that there is a standing puddle deeper than the top of the guides. But I don’t know about this one, the only engine’s I’ve ever seen that totally flooded the rocker box had drain back problems with the front and rear drains, so the rocker boxes filled up with oil till it ran back to the valley through the pushrod holes.
As you can see there’s no lack of possibilities and some of them are pretty serious.
Bogie
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