View single post by Esprit2
 Posted: 11-28-2016 07:15 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
For an 8.4:1 compression 907, the cylinder pressures should be 110 - 130 psi cold, or 150 - 170 psi at full operating temperature. And if at all possible, it's much preferable/ more accurate to check the pressures hot. In either case, hold the throtle wide open, and crank the engine at 200 rpm or faster (ie, a good, fully charged battery, or use jumper cables connected to a running car). A closed throttle will result in very low pressure readings (you can't compress air if you don't let it in first).

Screenplay wrote:
Does it sound like rings?Presuming you did the compression test correctly, 85 psi is very low... as in "rebuild" low. Since the pressure readings improoved a bit when the cylinders were oiled, it's reasonable to presume the rings are a major contributor to the low readings. Without knowing any more than you reported, it's sounds like new rings are in order.

Can I get away with just new rings? What measurements would I need to determine that?It's not only a simple measurement, it's also about condition. It's likely that the iron rings corroded while sitting idle for many years, and the outer wear surface is no longer smooth. They may also be jammed/ frozen in the piston grooves due to rust.

It's equally possible that the iron liner is rusted, and may even have a corrosion 'grooves' rusted into it where the rings sat. That won't go away with a simple ring replacement, and any groove(s) will simply tear-up new rings as they run back and forth across it.

"IF" the liners are corroded/ grooved, then they need to be eithr bored oversized to clean up the surface, or replaced. If boring is required, then new equally-oversized pistons are also required in order to maintain the correct running clearance.

If you get lucky, and the liners are in serviceable condition, then give them a fresh hone-job to restore the cross-hatch pattern. Then check the running clearances. To do that, measure the bore ID and the piston OD, and subtract to get the running clearance. Measure the bore at 50mm down from the top, and across the thrust axis (left-right in the block). Measure the pistons at a point 15.1mm (0.594")up from the bottom edge of the skirt, and perpendicular to the wrist pin bore (left-right in the block).

If the pistons clean up nicely, the liners are in good condition after a light cross-hatch hone, and the clearances are all within spec, then replacing the rings will be adequate to address any ring-related low compression issues.

Having said all that, the valves and/or valve guides may also be corroded or worn, contributing to low compression. It's wishful thinking to presume that only one thing, the rings, is wrong. After years of storage, there could very well be a lot wrong. And you won't know that until you dismantle the engine and check.

Likewise, the bearings' suitability for continued use depends upon both condition and measured clearances. Use Plastigage to measure the clearances, both mains and rods. Use a micrometer to measure the crank's journal diameters. Even if everything measures correctly, corroded and/or scored parts are not acceptable. If the journals are corroded or scored, then the crank will have to be polished and/or reground undersize. The latter will require new bearing shells with equally undersize ID.

Good bearings will have a smooth, dull-gray wear surface. If it's scored, or worn thin such that the brass color is starting to show through, then new bearing shells are in order. If the dull gray has worn away to the point that patches of bare brass (or worse yet, bare steel) is showing, then the bearing shells are not only toast, the crank journal has probably been scored as well (maybe with brass, definitely with bare steel). You're back to grinding/ polishing.

It's unrealistically optimistic to think an engine that has been stored for many years, and that now has bad compression, can be revived with nothing more than a set of rings. If you get lucky, then great, I'm happy for you. But to plan on it is folly.
Screenplay wrote:
I need to see this one on the road, if for no other reason than to get some payoff for all my time, and maybe to show friends, family and neighbors that I just might know what I'm doing.Poor work cannot be done inexpensively enough to produce good results. And taking shortcuts to put an engine back together without all the work it needs will not demonstrate to anyone that you know what you're doing.

If you're not going to do it right, then you're much better off doing nothing with it for now, and putting it back into storage until you can do it right.

Good luck,
Tim Engel

Last edited on 11-28-2016 07:16 pm by Esprit2