OK, so I need some help from the "old injuns" in the crowd.

I'm working on an engine (4 cylinder) that I know has some ring wear. It is burning some oil, no question. I did a compression test and a leakdown test, both warm and cold.

Compression test numbers were dead even across the board at 85-90psi (cold engine). Numbers verified with 2 differwent gauges. Values actually went down about 5PSI when re-tested on a hot engine. This engine has a way radical cam, somewhere in the 300 duration range, so I'm not as concerned with the low values.

The part that has me scratching my head is the difference between hot and cold testing using a leakdown gauge.

COLD testing resulted in 30-35% leakdown on each cylinder. Eeek.

with the engine HOT, leakdown went to 7-10%. The gauge is the type that is calibrated at about 3%, so these numbers are better than they should be......

That's a HUGE difference, and I have my theories, but I thought perhaps some of you more experienced guys might have seen this before....

PS - on the cold test, leakage was definitely past the rings (audible leakage at the dipstick / oil filler cap) but surprisingly NOTHING, NADA, NIL, ZERO through the exhaust or intake, or via adjacent cylinders......

My WAG is you're seeing evidence of the various coefficients of expansion with alloy (pistons) and bore (cast iron, I presume). Couple that with rings being on the business end of the process and the results aren't that weird to me.

With the consistency of the numbers across the four holes it'd seem likely you haven't got broken bits, rather just normal wear-n-tear. And some good valve seats!!

Guide clearances and seal condition could be another matter, tho.

Has this beast been regularly driven, or sat for long periods in-between exercise? My idea is if it has sat static for periods you may give it some "Sea Foam" or Marvel in the crankcase over an oil change or two and see some improvement in oil use... Nothing amazing, but it could de-goop the oil rings some. Just a thought.


...stop scratchin', trust yer instruments.

Thanks, Doc.

I've been doing some reading, and it seems the older forged pistons were more susceptible to thermal expansion than cast pistons because of a lower silicone content. (this engine rebuild is roughly 15-20 years old) Seems high-silicone forged pistons have become more common in the last decade.

I'm also guessing that a racing piston would have a shorter skirt, and thus the piston skirts and tops of the cylinders might be more susceptible to wear @ TDC as the rod sweeps through changes in angle...... And since a leakdown test is done at TDC, this wear would show up especially during a cold test.

The seam foam idea is good. Absolutely yes - this thing is driven only a few times a year at best. Could probably use valve seals, too -- but I don't wanna.

I used to think that by the time I hit 40, I'd know everything. Seems the older I get, the more questions I have...... <sigh>

heheheeee!!!

It's an on-goin' process, y'know.

If the engine is burning oil too like you say it is, there could be oil in the cylinders affecting the warm leak down test. Like the difference between a wet and dry compression test. As the engine cools, the oil would drain back to the sump again. Could be an additional consideration along with the thermal expansion of the pistons.

'tiz a point to ponder, Shawn, but I'd think the sustained pressure of a leak-down test would push the oil down kinda quick... unless there was a LOT of oil gettin' in there.

Agreed, but with how "obvious" the burning oil was its a possibility. Especially if the suspected thermal expansion was reducing the gap for the oil to push through. I've seen wet versions of leak down tests as well.

If it is part of it, I figured it would be a smaller part of it, not the major reduction in loss.

We're engaged in pickin' nits! A real consideration, tho, I agree. Nothing overlooked.



BTW Shawn... you oughta plan a vacation to Florida sometime soon.

After the 12 inches of fluffy white stuff we just had come down, and the current predictions for this winter... a vacation in the sunshine state would be welcome. Its just not in the cards for bit. I'll let you know when I head that way again though.

87 today in West Palm Beach. Just passin' through...............

We're told today will be just as warm.

Thanks for the post. I agree - undoubtedly there is at least some ring wear, and an increased oil film on the walls and in the rings after the engine's been running.

When I asked the older guy at the local machine shop, he just kinda smiled and shrugged.... "Race motors are oil burners..." he said.

Kinda sums it up, I guess. Sounds like that rule of thumb doesn't apply to the newer forged pistons though.....