Engine Rebuild – Low Compression

[JK73Super]

I have an AH ’73 Dual port 1600 34Pict3 standard engine with 80-90 psi compression. With oil added to the cylinders, 80-100 psi. I pulled the engine out of the vehicle and started tearing into it. Oh, by the way, I have never rebuilt and engine, but have done a lot of homework reading, and watching videos (Haynes, Bugme, youtube, etc).

I’m thinking that since I do not have knock in the engine, I will replace the cylinder and pistons, and if need be the heads. I’d rather not split the case. Any comments, suggestions to my approach? Is a piston/cylinder only kit available?

[FJCamper]

Hi JK73Super,

Everybody has to start somewhere. As a first timer, I understand your wanting to sort of try yourself out, get some experience, and get confidence for bigger jobs later.

And yes, you can get piston and cylinder kits. But in addition to that, you need either rebuilt heads or have yours reworked. Compression also escapes around valves and seats that need cleaning and regrinding.

FJC

[Marc]

With that small of a difference between the "wet" and "dry" compression readings it's for sure that the valves are the problem. "Loose heads" (compression loss between cylinders and heads, Beetle engines don't have head gaskets) would be a possibility but they're not frequently encountered on an AH engine. They'd also make a distinctive popping noise similar to a blown-out muffler.
Do you have any idea how many miles are on the engine? Knock or not, the bottom end could be tired and loose but understressed because of the low compression. A fresh top end would be the kiss of death.
If you're going to try and patch this up on a budget the heads will be your biggest expense. Avoid "rebuilts" which have been welded up when they should've stayed in the scrap barrel. Take your heads to a machinist who knows Volkswagens and see what it'll cost to do a valve job on them (expect them to need at least new exhaust valves and guides).
You can judge whether the pistons are worth re-using by seeing how thick a feeler gauge will slip between the top ring (when depressed) and the land. .006" or over and they don't have enough service life remaining to be worth bothering with.
I would never stop at this point, it is possible to replace the rod bearings without splitting the case and they aren't very expensive...but before you buy any parts, pull the pistons/cylinders and inspect the camshaft lobes for pitting and wear, it'd be pointless to invest in a fresh top end for a bottom end that'll need surgery long before the top end is worn.
Another thing to look for is a loose thrust bearing. Measure the crankshaft endplay. Even if you do end up only needing to freshen the top end you'll still want to replace the oil cooler seals and flywheel seal; if the endplay is .006" or less there's no cause for concern, but if it's any greater you need to find out if the thrust bearing is still snug in the case - if not, the repair requires a full teardown so the case can be machined for oversized main bearings.

viewtopic.php?f=11&t=79483&p=548846 (read my post there)

[JK73Super]

Well I pulled off the head and cylinders. There was no gasket between the cylinder and case or between the cylinder and head. According to information I’m looking at a gasket should have been installed between the case and cylinders and no gasket required between the head and cylinders.

Two of the cylinders on the head side had indication of blow-by (poor seating). Dark streaks on the head. The cylinder walls looked smooth and clean, no scratches.

I still have to take out the valves to check the condition of the seats.

The gap between the piston and the top piston ring was around 0.007” for all pistons. I understand that this is too much.

I’m thinking I should go all the way and open the case. Someone had said that 80 PSI +/- 10 PSI should have not caused idle problems. The Haynes manual specification indicates that the compression should have a minimum of 100 PSI.

Btw, I’ve installed a new Pertronix Single vacuum electronic distributor along with a new Euromax 34PICT3 carburetor and still could not get it to idle properly (valves were set).

[Marc]

This is a case where I'll agree with Haynes, it depends upon the accuracy of the gauge and cranking speed (and the cam) but ~100psi is pretty much the low end.
The factory installs paper gaskets at the cylinder bases. They're very thin (~.006") and not too effective on anything but a freshly machined surface anyway so many rebuilders omit them and use only sealant.
Simply prop the heads up and fill the chambers with solvent, if the valves are sealing the level won't drop overnight. Yours probably will, and you'll see where it dribbles out of the ports.
Depending upon the severity of the compression leaks, the head may need to be machined at the sealing surface. On a conventional head the term is "milling"; on a VW we usually call it "flycutting" since it more accurately describes the process used to reach into the hole and machine it flat.

[JK73Super]

So, I tried the solvent in the head and over an 8-10 hr period, one leaked out completely, one stay the same levlel and the other two were halfway down. I will remove the valves and check the seats.

The location where the head leaked against the cylinder (black spot) is actually small and minimal. I see there are copper cylinder head gaskets available staring at 40 thou thickness. What's everyone think about using copper gaskets?

I also checked crankshaft movement. Its 13-14 thou and I believe anything 6 thou or under is good. So, more reason to split the case and replace bearings.

[Marc]

IMO you're better off with a good gasket-less seal between the cylinders and heads for most engines, and should only use copper gaskets on turbo applications or for other special reasons (like a dragrace engine that might need to change bore or compression ratio because of altitude changes,etc.)
You can often use one of your old cylinders and some valve-lapping paste to "hand-lap" the seating surfaces on the heads if the leakage spots are minor.

[Steve Arndt]

Marc is right on with all his advice. If you see any compression leak between the head and cylinder that will only get worse in a hurry.

Also the chambers should hold solvent basically forever if they seal right. You take a shop air blow gun and blow backwards in the port at the valve seat inside the port. Look for bubbles.

[Jim Ed]

These links might help:
http://vw-resource.com/
http://www.speedyjim.net/
http://www.chirco.com/tech_articles/?p=153
http://www.justkampers.com/videos/
http://www.classicvwbugs.com/classic-vw-bug-videos/
http://www.geneberg.com/techtips.php
http://www.thebugshop.org/

stores
http://www.aircooled.net/
http://www.cbperformance.com/Default.asp
and sometimes
http://cip1.com/ but, they sell a lot of Chinese parts.

[Jim Ed]

Here is a clue as to whether or not the bottom end needs to be rebuilt.
With the fan belt removed, grab the crank pulley and try and pull it out and push it back in. If there is too much play you need to split the case and re do it too.
With the engine out it is a good time to replace the main seal, clutch, and the rear tranny mounts.
Many disagree on which oil to use.
Gene Berg said to use 10W30 oil year round while Rob Boardman(aussiebug) says to use 20W50 in hot weather and 10W30 in cold weather. I agree with aussiebug. I use an oil that still has ZDDP in it like
http://www.jegs.com/i/Valvoline+Racing/ ... 5/10002/-1
or
http://www.jegs.com/i/Valvoline+Racing/ ... 1/10002/-1
This oil makes the cam shaft last longer but, is bad for catalytic converters.