Bore Score Inspection Through the Sump for a PPI – Is It Practical?

OK, so it’s a bad weather day, like everywhere in the US right now, so let’s have a little discussion about bore score inspections “thru the sump”. Nothing definitive here, just my thoughts. I’ve just finished rebuilding my 2007 Cayman S M97.1 engine so this discussion is biased towards that engine design. But from what I can tell, it applies to the earlier M96 engines from the Boxster and 911 as well. I am not here to argue whether bore scoring is a major concern or not but I agree that when buying one of these cars, getting a Pre-Purchase Inspection (PPI) done is a good idea and a PPI to check for bore scoring adds piece of mind. And many folks recommend getting this inspection done through the sump versus going thru the spark plug holes. Now in both cases, the inspection is done with a fiberoptics borescope/endoscope. I have one that casts its image to my cellphone. It is a Depstech Dual Lens wireless endoscope. About $60 from Amazon. The diameter of the camera head is about 5/16” and it is about 1 ½” long. It has built in LED lighting. The lead wire can be bent into almost any position, once you get past the camera head, and will hold that position. Dual Lens means you can look straight ahead or to the side. The side view is nice, as you don’t have to attach a right angle mirror, which might come loose or get caught. It can record still pictures and video. Anyway, I’m not here to laud this camera, I’m just saying that is the kind of tool many shops will be working with. As we will see, a valuable feature would be the ability to “command” it to articulate the lead wire. I don’t even know if such a thing exists in a remotely reasonable price range or package size.

With the scope through the spark plug hole, you must remove the coil pack and obviously the spark plug. Once you get the camera into the spark plug hole, it is a straight shot into the cylinder bore. With the dual lens, you should be able to see the walls of the cylinder pretty readily. You may need to rotate the lead wire 360 degree to see the entire bore. This will be a little bit awkward on the forward cylinders just due to access but it can be done. Each cylinder is done one at a time, with the engine rotated to place each piston at its lowest travel point, i.e. bottom dead center (BDC).

The standard complaint about “through the spark plug hole” is that even with the piston at bottom dead center, there is an area of the lower cylinder wall where the cylinder bore cannot be seen. Fair enough, this is absolutely the case. We have all seen pictures of bad bore score. Trust me, it is not hiding under the piston. It goes all the way up the wall of the cylinder. Indeed, the maximum “side thrust” on the piston occurs halfway between BDC and TDC, just based on the geometry of the crankshaft. But the beginnings of bore score probably do start lower in the cylinder wall and can be best captured from below, with the piston at top dead center (TDC).

Which leads of course to the recommendation to go “thru the sump”. Well, herein lies my concern. No arguments, a bore score inspection done through the sump will allow the lower wall of the cylinder to be inspected. I keep saying lower, really we are saying the portion of the cylinder wall closest to the crankshaft. And of course, in this case the top area will be obscured by the piston. So when I had my engine apart and was putting it back together, on the bench, I tried to scope the bores with my endoscope. The middle cylinders (2 and 5) not so bad. Not exactly a straight shot but you only have one major bend. You go vertical a bit and then have to make to 90 degree turn to get into the axis of the cylinder bore. Of course, there is an oil baffle and a crankshaft and its carrier in the way. It seemed doable, although already more difficult that through the spark plug hole. Then I started to think about how to get to the outer cylinders and my brain just kind of went “no way!” You have to make multiple turns to get there. You can’t really pre-bend the scope wire. It just did not seem possible.

A few days ago, I Googled this subject. By far the most authoritative hit on the subject was Jake Raby. In one video he said “I just spent 10 minutes threading my endoscope into cylinder #6”. Well, if it takes Jake Raby 10 minutes, it surely might take a more inexperienced person much longer or even forever. Then I found a follow-up video by Jake where he said folks had questioned how he did it. In this case, he demonstrated, with the crankshaft carrier removed! Sorry Jake, I love you man, but that is cheating. And once you get your camera head that far, what are the odds that it is going to be pointed in the right direction? Also once you get there, twirling the lead wire to get a full 360 degree view may not be possible. My conclusion is that it is hard and time consuming to inspect cylinders this way. And the one most at risk, #6, is the hardest one to get to.

So what is your point HarveyF? My point is, if you are looking to buy a Porsche with one of the engines that is potentially subject to bore scoring, you should get a PPI. But I have a bad feeling that if you insist on one through the sump, you may not get what you pay for and you may be paying a lot. If it really requires all the above described effort, I would suspect the cost is going to be hundreds of dollars/pounds/euros. And maybe they show you pictures of #2 and #5 and say they did all six. For sure, there are no landmarks in the individual cylinders that say clearly “You have arrived in bore #6!” I struggled with the word “practical” in my title. I could have used “cost effective”. Or “do you get what you pay for?” All things to be considered.

BTW, to go in through the sump requires dropping the sump cover. Not a bad thing, in that it gives you a chance to check the oil pump intake screen for chunks of metal and plastic. It does mean you are probably going to be paying for 9 quarts of fresh oil and the associated sump plate labor. Granted, thru the spark plug holes there is the labor of removing/installing the coil packs. Maybe a wash?

Also BTW, on my 5 owner 100K mile car, I did not find any bore scoring. I did find that the special coating on the skirts of the pistons was starting to fail, which is the first stage of bore scoring. I’m glad I found that, as it justified the cost of the engine rebuild. But that type of wear can’t be seen with either type of endoscope inspection.

In conclusion, if you are looking to purchase one of the cars that is subject to bore scoring, for sure, do your homework and exercise due dillegence. As is often the case, if the car you are considering is local, it is at least possible to get the PPI by a local shop you trust. But if it is far away, we are often at the mercy of the recommendation of the Seller for PPI services. In that case, just to reduce the chances of ineptness or downright hanky-panky, I would default to an inspection thru the spark plug bore. A compromise but as discussed, maybe the more prudent approach. And that, 1000 words later, is my suggestion, up for discussion or whatever.

Update 1- Brandon at Slakker Racing identified a reasonably priced endoscope that allows the head to be articulated. This would certainly help as one manuvers from the sump to the cylinder bores. Go to Amazon and search on “Two way articulating borescope”.

Brandon also posted this photo showing the depth that the top of the piston goes to in the cylinder at bottom dead center. See the red line. As you can see, you are not missing very much of the total cylinder volume when doing your visual inspection through the spark plug hole.

Update 2 – One thing that puzzled me when writing this article is how a mechanic going in through the sump would be able to accurately position the cylinders at Top Dead Center (TDC) for the inspection of each bore. The answer may lie in the annotated photo below. There are markings on the crankshaft pulley that are used during the timing of the engine. There are also holes, where you can insert a pin and accurately lock the engine in a TDC position for each cylinder. I got this from a guy on Rennlist that goes by Porschetech3. Thanks to him! He also points out that you can put a bore scope into the access hole for the Bank 2 piston wrist pins (see arrow to the right in the photo) and, due to the fact that the 3 connecting rods are staggered on the crankshaft at 120 deg increments, there will be a path through the same system of holes used to insert the pins to get a camera into bores 4, 5, and 6. This is undoubtably the least invasive method that I have heard, although it cannot be done on bores 1, 2, and 3. He says that it is his preferred approach from a cost/benefit standpoint. Anecdotally, folks think that bore 6 is the most likely to bore score.