97 GTI low compression brand new top end

andy.262

andy.262

Active Member
I have a 97 GTI with the 720 that I just put a brand new top end in (SBT cylinder exchange) and it won't start at all. Just cranks away, not even a pop or anything. I literally just finished assembling the new top end, its never been started with this new top end before, and I checked the compression and got 150psi PTO and 130psi mag. Obviously way too big of a difference between the two. What could I have possibly done wrong while assembling that could have caused that 20psi difference? How screwed am I, did I ruin my brand new top end before I even started it?
 
You are going to have to pull it and check.
Could have been something you did or could have been bad machine work from the shop.
Either way you should not run it as is.
 
I would thinks it’s ok I would check to make sure you didn’t leave a chunk of old base gasket on the jugs or something. Also your o rings on the head. Your gaskets should be fine to take the top end off and put back together. Check the piston and rings if all looks good then put back together and try again
 
Took it all apart again, everything looked ok to me as I was taking it apart. O rings all present and seated correctly, base gasket intact (although I did rip it when i took the cylinders out, I saw it happen so I know it wasnt like that before), piston rings oriented correctly with the notches, pistons facing the correct direction with the arrow pointing towards the exhaust.

At what point do I start to suspect the machine work? I have to get a new base gasket now before I can put it back together and try again, but once I get one is it worth trying again without changing anything, or do I need to start the warranty process with SBT...

One thing I did notice was that the PTO cylinder and piston (which had 150psi) were noticeably more wet than the mag side (which had 130psi) - is that just telling me what I already know? That the PTO had more compression than the mag?
 
You are the 2nd person with a low compression 720 with a fresh SBT topend this week. Coincidence, I think not. SBC is sending them out with incorrect tolerances or something..
 
mikidymac said:
You are the 2nd person with a low compression 720 with a fresh SBT topend this week. Coincidence, I think not. SBC is sending them out with incorrect tolerances or something..
Click to expand...
That is very interesting... I do have the measurement tools required to check the cylinder wall to piston clearance, I will measure after work and report back. Maybe they just got a new guy or something that's still getting the hang of it. Thankfully their warranty process has been pretty good in my experience.
 
From their website:
1725470469239.png

Although I could have sworn I didn't pay for shipping when I warrantied a whole engine last time. Maybe I'm just remembering that wrong or maybe they changed it recently.
 
Now I'm no machinist, so my measurements may not be absolutely perfect, but here were my results:

✅ Cylinder taper: 0.002" difference between top and bottom measurement, within spec
✅ Cylinder/Piston wall clearance: 0.0025" difference between OD of piston 1" from bottom of skirt, and ID of cylinder 5/8" from top per the manual, in spec
✅ Ring gap: my feeler gauge only goes to .024" at the largest, but even that wouldn't fit (upper limit is 0.039"), in spec

So, as near as I can tell, this cylinder and piston are within spec...
I guess when I get the new base gasket I'll just put it back together and try again? I didn't really see anything wrong when I took it apart, but maybe the base gasket had a really small leak or something?
 
What thickness of base gasket did you take out originally and what one did you put in, I could be wrong but I think thinner base gaskets raise compression. A squish test would be in order though.
 
Ok yeha put it back together and make sure all your o rings go together nice and try again every things seem good
 
burtshaver2021 said:
What thickness of base gasket did you take out originally and what one did you put in, I could be wrong but I think thinner base gaskets raise compression. A squish test would be in order though.
Click to expand...
The base gasket would not cause the 20psi difference between cylinders. They should both be exactly the same on a fresh bore.
 
mikidymac said:
The base gasket would not cause the 20psi difference between cylinders. They should both be exactly the same on a fresh bore.
Click to expand...
I missed that, was thinking it was low compression on both sides. The wet side that’s getting 150 pounds could be a false high reading due to excessive rebuild lube or a crank seal leaking into that cylinder.
 
I did put liberal amounts of assembly lube on both new cylinders/pistons, I always thought that you could never have too much lube with this kind of thing, is that wrong?

Before I took apart the original top end that failed I did pressure test the engine and found that it held 10psi without leaking for several minutes, which I took as an indication that the crank seals were OK.
 
andy.262 said:
I did put liberal amounts of assembly lube on both new cylinders/pistons, I always thought that you could never have too much lube with this kind of thing, is that wrong?

Before I took apart the original top end that failed I did pressure test the engine and found that it held 10psi without leaking for several minutes, which I took as an indication that the crank seals were OK.
Click to expand...
You’re doing things right
 
Well, depends on what engine builder you speak to.
Bearings and cranks should get 2-stroke oil. My trusted engine builder and endurance ski race builder says pistons/rings should go in a dry cylinder. His claim is the rings have a teflon coating and need a dry cylinder to seat at first.
I have never been able to bring myself to do that so just a light schmere of oil on the cylinder walls. Too much oil is not good for ring sealing.

Break In Seizures – The most common break in related seizure is usually caused by the ring not the piston. Most new piston rings come with a gray or green Teflon coating on their outer sealing surfaces. This coating seals to the cylinder wall in just a few operating minutes, which provides better power during the break in period. As the engine is breaking in, the Teflon eventually wears away and lets the hard surface of the ring come in from behind to provide the long term seal. The down side of this Teflon coating is that it makes for a dangerously small end gap during the first hours of operation. If the engine is run too hard too soon, the heat will cause the ring to expand in diameter which will drive the ring ends together and drive the ring surface hard against the cylinder wall. A piston ring that is being overheated in this fashion will easily have enough tension against the cylinder wall to scratch off the oil film which will begin the scoring/seizure process. A piston seized in this way will have heavy scoring around the entire diameter of the piston, with the ring usually locked into the groove all the way around as well.
 
Got the new base gasket and put everything back together, and this time i switched the cylinders to see if that would switch which side had low compression. Except this time they both did, both at 130. Low, but at least they were the same. I tried to start it just to see if it would, and again I got nothing, not even a pop or anything. Confirmed it has spark. The plugs were wet when I took them out after, so I'm thinking it has fuel (plus I rebuilt the carb so I know its not plugged and it should be working correctly).

So I'm not really sure what to do next. I guess warranty the cylinders and pistons maybe? Could it possibly be something else that's giving me the lower compression? Battery is strong and cranks strong, and I know my compression gauge is good because it reads all of my other machines correctly.
 
Yeah I would contact sbt and start there warranty process unless the other guys have any ideas. It’s weird that they are both low now and your piston to cylinder wall clearance is good and your ring end gap is sounds kinda flooded it should run with 130 psi still even though it’s not ideal compression numbers I had a ski that had a fresh top end and it run great but it said 120 psi so I have no idea on that one. But it ran good had full power and started good in the water. So I’m not exactly sure what that was
 
Hi, I am new here and I am not a mechanic. I did 720 rebuild recently. Did you try to start putting gas mixture directly through plug holes?
 
Well.... I found the REAL issue, and the news is not good. The previous owner must have done a top end themselves, and were not careful to catch all the cageless needle bearings and a few fell into the crankcase. Looks like a piece made its way into the rotary valve, took a chunk out of the engine-side mating surface, carved a gouge out of it and then seized it in place. Turning the engine over by hand does not move the rotary valve at all.

Now... that mating surface I believe is part of the engine block, correct? So is this damage even repairable? Or am I looking at a new reman engine?

PXL_20240909_162627105.jpgPXL_20240909_162722098.jpg
 
Yeha that would do it. Some of the more experienced guys would know better than me but it looks pretty bad. It is part of the engine cases.
 
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