750 Sport, New Motor, Burning Oil
Posted: Mon Jun 24, 2013 9:39 am
I have owned this '74 Sport since Apr 1975, parked it in '88, started a ground-up refurbishment around 2000, finished last year, and have only had the chance to ride the machine 50 miles since then. Last week I pulled the spark plugs to see how all looked in there & found oil on the plugs. As I was tightening up the forward cylinder's plug the threads pulled out, so I pulled the head to get it repaired. I decided to use a threaded insert repair that was developed by a company known as Lock-N-Stitch, who are more commonly known for their cracked cast iron block repair process used extensively on Cummins Diesels. They developed the plug thread repair process to fix a common problem with the Ford Triton motors and the 4 cyl engines used in Dodge PT Cruisers. The spark plug insert they developed is made from hard anodized aluminum to match the expansion characteristics of the aluminum head, reducing the possibility of the insert coming loose. Also the aluminum insert more closely matches the spark plug heat transfer characteristics of the original parent material. They sell kits that include the tooling but I could not find anyone locally who had the tooling and buying a kit for one repair was cost prohibitive, so I opted to strip down the head & ship it to their in-house service department in Turlock, CA. I'll post pictures & results when completed.
When I pulled the head I noticed a small amount of oil in the cylinder and it looked like the end of the intake valve guide was wet with oil. While pulling the head apart I found the oil seal on the intake valve guide was split, but I found it hard to believe that the amount of oil I found in the cylinder was getting through the guide. I decided to pull the cylinder & piston for an inspection - wasn't too pleased with what I found.
There are wear marks at the top & bottom of the cylinder, perpendicular to the wrist pin axis. The wear is not scoring, but a patch about 3/4" wide on both the top & bottom, and running the length of the stroke in the area where the bottom of the skirt rides. I took the piston & cylinder to a local automotive machine shop to ask their opinion. They ran a bore gauge down the cylinder and it showed about .002" taper in the areas of the wear, but no wear was indicated along the axis parallel to the wrist pin. A cursory visual inspection of the piston showed no evidence of contact of the skirt with the cylinder. The machinist did notice uneven wear around the circumference of the 2nd ring. The piston is an 81mm JE assembly that I believed was sourced from Syds back in '03. At that time I supplied them to MMS for fitment into the cylinders, requiring an overbore out from 80.4mm. Since it was past closing time on Saturday afternoon, the machinist didn't have the time to closely measure the piston & check for piston-to-cyl clearance.
Yesterday I spent a little more time looking at the piston, cylinder, and rings. I pulled the rings off the piston & they appear to have been installed correctly. I then checked for end gap; the top ring had .014", the second had .020", and the oil control rails had .017". I was surprised to see .020 on the 2nd, but that was the ring which had the funny wear pattern. Looking at the contact surface of the 2nd ring, in some areas it showed wear completely across the face, and in other areas the wear was not completely across the face. This seems to indicate that the ring hadn't completely bedded in yet - don't know why.
As I was measuring end gap I used the piston to square up the rings in the bore, and while doing so I noticed the wear in the bore aligned with the most bottom portion of the piston skirt on the thrust sides (perpendicular to the wrist pin axis). Careful visual examination of the piston skirt revealed wear on the very bottom edge; a very narrow strip less than 1mm wide. I had missed it earlier because I was looking for a wider contact patch on the skirt.
To me this indicates there may have been a manufacturing defect with the piston. I will need to take measurements to verify, but it appears that the bottom edge of the piston is too tight in the bore. JE recommends taking piston measurements ~ 1/2" above the bottom of the skirt; if the diameter flares out significantly below that point the piston was probably too tight in the bore & may have been close to seizure. I will get some more measurements and talk to customer service at JE.
One last thing that bears mentioning - I did not baby this motor in the short time it was on the road. In other words, the wear may have been induced from a heavy throttle. There were multiple occasions of running rpm past 4500rpm in the lower gears and perhaps 2 or three short blasts with rpm up to 6000 or so, but in either case, never on a cold motor. I used 15-40 wt diesel oil for break-in and changed to straight 50 wt at 40 miles.
Open to comments. Thanks for your time.
When I pulled the head I noticed a small amount of oil in the cylinder and it looked like the end of the intake valve guide was wet with oil. While pulling the head apart I found the oil seal on the intake valve guide was split, but I found it hard to believe that the amount of oil I found in the cylinder was getting through the guide. I decided to pull the cylinder & piston for an inspection - wasn't too pleased with what I found.
There are wear marks at the top & bottom of the cylinder, perpendicular to the wrist pin axis. The wear is not scoring, but a patch about 3/4" wide on both the top & bottom, and running the length of the stroke in the area where the bottom of the skirt rides. I took the piston & cylinder to a local automotive machine shop to ask their opinion. They ran a bore gauge down the cylinder and it showed about .002" taper in the areas of the wear, but no wear was indicated along the axis parallel to the wrist pin. A cursory visual inspection of the piston showed no evidence of contact of the skirt with the cylinder. The machinist did notice uneven wear around the circumference of the 2nd ring. The piston is an 81mm JE assembly that I believed was sourced from Syds back in '03. At that time I supplied them to MMS for fitment into the cylinders, requiring an overbore out from 80.4mm. Since it was past closing time on Saturday afternoon, the machinist didn't have the time to closely measure the piston & check for piston-to-cyl clearance.
Yesterday I spent a little more time looking at the piston, cylinder, and rings. I pulled the rings off the piston & they appear to have been installed correctly. I then checked for end gap; the top ring had .014", the second had .020", and the oil control rails had .017". I was surprised to see .020 on the 2nd, but that was the ring which had the funny wear pattern. Looking at the contact surface of the 2nd ring, in some areas it showed wear completely across the face, and in other areas the wear was not completely across the face. This seems to indicate that the ring hadn't completely bedded in yet - don't know why.
As I was measuring end gap I used the piston to square up the rings in the bore, and while doing so I noticed the wear in the bore aligned with the most bottom portion of the piston skirt on the thrust sides (perpendicular to the wrist pin axis). Careful visual examination of the piston skirt revealed wear on the very bottom edge; a very narrow strip less than 1mm wide. I had missed it earlier because I was looking for a wider contact patch on the skirt.
To me this indicates there may have been a manufacturing defect with the piston. I will need to take measurements to verify, but it appears that the bottom edge of the piston is too tight in the bore. JE recommends taking piston measurements ~ 1/2" above the bottom of the skirt; if the diameter flares out significantly below that point the piston was probably too tight in the bore & may have been close to seizure. I will get some more measurements and talk to customer service at JE.
One last thing that bears mentioning - I did not baby this motor in the short time it was on the road. In other words, the wear may have been induced from a heavy throttle. There were multiple occasions of running rpm past 4500rpm in the lower gears and perhaps 2 or three short blasts with rpm up to 6000 or so, but in either case, never on a cold motor. I used 15-40 wt diesel oil for break-in and changed to straight 50 wt at 40 miles.
Open to comments. Thanks for your time.