78 bus with low compression after 35,000 miles

[Dominic]

Low compression (1-85 psi, 2-85 psi, 3-83psi, 4-80 psi). It appears as though the rings didn't seat properly. Noticed oil on the spark plug from #3 cylinder so I did a compression test. Everything is stock except the heads. (Hoffman built heads with larger valves) and a 72-74 exhaust.
My question is: which piston rings should I use, and what size? 1.5mm top, 2mm middle, 4mm oil? Is this standard size for a 94mm bore?

[Amskeptic]

Low compression (1-85 psi, 2-85 psi, 3-83psi, 4-80 psi). It appears as though the rings didn't seat properly. Noticed oil on the spark plug from #3 cylinder so I did a compression test. Everything is stock except the heads. (Hoffman built heads with larger valves) and a 72-74 exhaust.
My question is: which piston rings should I use, and what size? 1.5mm top, 2mm middle, 4mm oil? Is this standard size for a 94mm bore?

When you say "top end rebuild", are you saying that you just did the heads?
Did you also replace pistons and cylinders?

If you you have fresh pistons and cylinders or just re-ringed pistons, how many miles are them?
The mm thickness of the rings is irrelevant to putting the specific ring in its specific groove. You have compression ring #1, compression ring #2, and oil control ring #3 They have orientation marks or even a stamped "top"
Colin
(p.s. if rings are less than 100 miles, you can still save the ring seating by doing the ring-seating procedure)

[Dominic]

I rebuilt the entire engine 55,000 miles ago. New Pistons and cylinders, web cam and matching lifters. All reciprocating parts were dynamically balanced at Jake Raby's shop. I did a re-ring at 20,000 miles because a tiny screw fell from the carburetor into combustion chamber. Replaced slightly damaged piston with a new one, All Pistons were weighed and within the required specs. Installed new rings. cylinders were honed. AMC heads were new at the time of initial rebuild. HAM inc. Inspected and rebuilt to "camper special" specifications. Head temps and oil temps always remained well under dangerous levels.

My questions are: Is it advisable to just do a top end rebuild (re-ring), and if so, what brand rings do you recommend?

[Amskeptic]

I rebuilt engine 55,000 miles ago.
New Pistons and cylinders, web cam and matching lifters.
I did a re-ring at 20,000 miles because a tiny screw fell from the carburetor into combustion chamber. Replaced slightly damaged piston with a new one, All Pistons were weighed and within the required specs.

Installed new rings. cylinders were honed.

My questions are: Is it advisable to just do a top end rebuild (re-ring), and if so, what brand rings do you recommend?

Do you remember the rings you installed? Ring manufacturers have their own favorite hone grit/ surface finish. Typical cast iron Grants rings, for example, are happy enough with your typical AutoZone cylinder hone. Mahle rings would be destroyed.

If you are dealing with a recent rebuild, then of course "just a top-end" is called-for. There is no indication that you need to go into the case, right? There is no indication that you need to go into the heads, right . . . (except to do a check of valve-to-guide clearances since oil can get pumped into the cylinders through loose guides)?

The brand of rings to install is up to you and whatever recommendation may exist from the piston manufacturer. The rings and the piston work as a team. If your pistons have 2/2/4.75mm lands, you need to find the correct thicknesses for them. Maybe you have 1.5mm top land, you just need to measure, order your rings, check for the correct clearances, and end gaps in the cylinders.
Do NOT scratch your pistons in the ring land area. This is a very precise area of engine operation and you need to be uber-clean. If you hone your cylinders, you must wash cylinders in the kitchen sink with a scrub pad hot water and dish soap and rinse thoroughly and do it until your paper towel does not lift up any grey as you dry the cylinders.

I do not recommend chrome rings. Deves are fine, Hastings are fine.

Mahle with the molybdenum stripe on the top ring give the best longevity when equipped in genuine VW piston/cylinders (the cylinders are already honed to the very fine surface finish required for Mahle). I have never gotten less than 100,000 miles from Mahle (Cofap) piston/cylinder sets.
Colin

[Dominic]

The rings I used on the re-ring were Grant. I used brand new Mahle p/c on initial rebuild. There is no indication I need to split the case, or go into the heads. The heads just need to be inspected for valve to guide clearance, as well as valve seating.

[Amskeptic]

The rings I used on the re-ring were Grant. I used brand new Mahle p/c on initial rebuild. There is no indication I need to split the case, or go into the heads. The heads just need to be inspected for valve to guide clearance, as well as valve seating.

We need to be scientific as far as forensics here:

Low compression at 35,000 miles.

a) did you do a compression test just after the rebuild?
If so, what were the readings? This question follows the observation that all of your compression numbers are so evenly low that I doubt that you have a ring failure . . . .

b) do you have your deck height, combustion chamber cc's, etc? This question is the logical next question of you did not do a compression test earlier.

c) do you have "stroker piston pin location with a stock stroke? This is if your above numbers arrive at a 7.3:1 or greater compression ratio

d) do you have a chronic split vacuum hose in a dusty part of the country where you wrecked all of your cylinders equally from abrasive dust?

ColinDoYouTRUSTYourCompressionGaugeAnyway??

[Dominic]

Yes I did a compression test after rebuild. All 4 cylinders were right at 130 psi. Heads were CC'd and deck height was established. Although I do not have the CC's and deck height numbers, I do remember that the CR was almost 7.4/1. I did drive the bus 200+or- on national forest service roads. Lived on gravel road for 6 months. Only vacuum hoses are the brake booster attached to the base of the left carburetor, and vacuum hose attached from left carburetor to distributor.

[Dominic]

Can you explain item c. (Stroker piston pin location with stock stroke)?

[Amskeptic]

Can you explain item c. (Stroker piston pin location with stock stroke)?

Irrelevant with 130# readings.

Some pistons, Keith Black, place the wrist pin closer to the crown to allow longer rods with the longer strokes. This prevents too much rod angle, a good thing, but works the skirts pretty hard, a bad thing.
IF you had an aftermarket piston with the wrist pin hole up higher on a stock rod, you would have lower compression and I guess, the most generous deck height ever.
Colin

[Dominic]

Considering getting new P/C since the current ones have 60,000 miles on them. The looming question is, what is the reason for the low compression? I Am not equipped to do a leak down test in order to determine if it's the heads or the Pistons. I'm going to need to do a top end rebuild anyway, so inspection of the p/c, and heads will hopefully reveal some concluding information. ?

[Amskeptic]

Considering getting new P/C since the current ones have 60,000 miles on them. The looming question is, what is the reason for the low compression? I Am not equipped to do a leak down test in order to determine if it's the heads or the Pistons. I'm going to need to do a top end rebuild anyway, so inspection of the p/c, and heads will hopefully reveal some concluding information. ?

If the engine is still functional, you can do a wet test after you do a dry test. If the wet test is substantially higher than the dry test, it is rings. Make sure to add a generous several squirts of engine oil in each spark plug hole, spin the engine a couple seconds, then do each cylinder's compression.
Colin

[Dominic]

Engine is still functional. I will do the dry/wet test and report back. Does the engine need to be warmed up first, or should this procedure be done on a cold engine?

[Amskeptic]

Engine is still functional. I will do the dry/wet test and report back. Does the engine need to be warmed up first, or should this procedure be done on a cold engine?

Properly warmed up. People like Ratwell (think thorough thinking) will tell you to crack loose the plugs while engine is cold, snug them lightly, then start and warm up for the compression test. Then, when you remove the plugs with a warmed engine, there will not be any threat of galled spark plug threads.
Me? I have never had an issue with removing plugs from a warm engine. YMMV
Colin

[Dominic]

New developments: The first compression test gauge I used was a loaner from a FLAPS, this gauge produced low readings.
Found my gauge, performed compression test, and the results are as follows: #3-125 psi, #1-125psi, #4-123psi, #2-128psi. I am borrowing another tester this afternoon and will perform another test to establish valid and reliable results, and report back.

[Dominic]

Alright, did another compression test with a brand new tester. Test results were identical to the readings from my guage! #3-125psi, #1-125psi, #4-123, #2-128psi. Engine is just fine! I checked the valves (stone cold) and no adjustments were necessary. Oil pressure is 40 psi@ operating temperature running~2,800rpm's. (Original oil pump)
Thank you colin for leading a great investigation!
Lesson learned: never trust a compression tester that gets loaned out to the general public.