750 Sport, New Motor, Burning Oil

If you need technical information or help with your roundcase Ducati 750 engine - post your FAQs, comments & questions here.
Post Reply
jumpjg
Cucciolo - the Lil Pup
Posts: 14
Joined: Wed Dec 05, 2007 1:08 pm
Location: St Louis

750 Sport, New Motor, Burning Oil

Post by jumpjg » Mon Jun 24, 2013 8:39 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.
Joe in St Louis

wdietz186
888
Posts: 658
Joined: Sun Sep 10, 2006 4:40 pm

Re: 750 Sport, New Motor, Burning Oil

Post by wdietz186 » Wed Jun 26, 2013 7:18 pm

I would have the machine shop check to see if the bore is actually round. In addition you could have a piston that was dropped or otherwise mis handled and the skirt distorted. It doesn't take much to tweak the skirt [ask me how I know] but the ring wear would make me suspect the bore is ovaled. The end gap being greater on the 2nd ring isn't all that critical unless the gap is waaay bigger and .004" isn't really all that much.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Fri Jun 28, 2013 7:43 am

Also: Look carefully in the port at the guide/head (not guide/valve) interface. Any engine with leakage there can burn a lot of oil,because of the suction involved,and this includes the exhaust side. Ducati bevels,with their miniscule valve guide bore length,are very prone to this,especially after somebody works with the guides,proper technique or not.If you see a trickle of oil on the outer length of the guide or along the port wall,it's a dead giveaway.Fortunately,it is a piece of cake to fix,with little or no engine dismantling involved.Up in the rockerbox,paint on Ye Olde Brown Permatex (do they still make it?) or Loctite's guide sealant or something similar around the guide/head interface,and the problem is solved. Never put up a fresh guide work job in any engine without sealing this area.Many guides nowadays come with a proper o-ring for this purpose.

jumpjg
Cucciolo - the Lil Pup
Posts: 14
Joined: Wed Dec 05, 2007 1:08 pm
Location: St Louis

Re: 750 Sport, New Motor, Burning Oil

Post by jumpjg » Fri Jun 28, 2013 9:55 am

Ray,

I think you are on to something. Last night I pulled the engine out of the frame and pulled the rear head, cyl, and piston. This morning I disassembled the head & noticed a substantial amount of oil in the intake manifold - it seems that on the rear cylinder the low point is in the manifold and evidently the oil puddled up in there. I found the intake valve seal intact, but I noticed oil on the valve face and it looks like it's coming from where you said; between the head and the guide. It looks as though the guy who did the valve job was aware of this problem, as he installed an o-ring between the head & guide but it doesn't look to have been done right. Looks like the o-ring was just slipped over the guide then the guide was pressed into the head, squeezing the o-ring between the head and the guide flange that would normally seat against the head. Seems like a half-baked attempt to seal the guide to the head & evidently it was not successful. I'd rather not punch out the guides & get into re-doing the valve seats again. There is plenty of room in there to lay a bead of sealant to supplement the leaky o-ring. Brown permatex? Really? Anything else that might work better, like Yamabond or the like?

I also noticed that the spring seat is sitting on the top side of the guide flange, and that is keeping it elevated off of the head by ~ 1.0mm. I was expecting to see the seat sitting on the head, but perhaps the gap is to insulate the valve spring & allow oil to cool the valve guide. I'd appreciate any enlightenment on the subject.

Thanks for the recommendations. I'm still trying to figure out what's up with the piston & cylinder. I'll get some dimensions on the rear cylinder parts today & that may shed some more light on the subject.
Joe in St Louis

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Fri Jun 28, 2013 10:14 am

The valve spring pack MUST rest against the head(altho you'll notice you don't have a complete circle of material there,probably) . So,you've got a valve job to do and maybe spring pack parts to acquire.Not the end of the world.You probably had more seat pressure there than needed.(80-88 lbs for stock machinery)Try to avoid carving channels in the valve guide bores if/when ditching the old guides. Caveman car mechanic knucklehead methods are not recommended. Get bronze alloy guides and set the valve to guide clearance at nil-plus.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Fri Jun 28, 2013 11:06 am

You might consider having somebody weld and machine a "full circle" spring pack base while you're doing all this. I never did it to my racer,but probably should have.On the other hand,with 80-88 lbs of seat pressure and stock cams,etc.,there's thousands of springers out there with no issues caused by the seat pack base not being all there.

jumpjg
Cucciolo - the Lil Pup
Posts: 14
Joined: Wed Dec 05, 2007 1:08 pm
Location: St Louis

Re: 750 Sport, New Motor, Burning Oil

Post by jumpjg » Sat Jun 29, 2013 7:57 am

Ray,

Couple questions...

Why is it important that the spring pack be resting on the head, given there is such a small support pad for it on the head?

Could you be more specific about valve to guide clearance? The 860 shop manual states .0005" - .0022".

Where did you get the 80-88 lb for seat pressure. All I have found in the Ducati books is a free length - an installed height spec from the factory would have made more sense. Naturally with cutting the seat the height would change & the spring pack would require shimming to get the height, and that leads to a seat pressure.

Appreciate the help,
Joe in St Louis

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Mon Jul 01, 2013 4:41 am

Well,these are all just my opinions.The forces involved in valve trains are astronomical.Even Ducati when they were at their worst didn't sell you an engine with the spring pack not resting on the head.As for that area being incomplete, I think Desmo was all they cared about and as we now know,it was all they could do in that era to keep the doors open and make a few batches of bikes. 88 psi is what I measured for seat pressure on an 860 and a 750,all with the same part numbers.With stock cams a bit more or less won't be any problem.My experience with bronze guides is that a "polished in" nil+ fit is best,in BMW,Ducati,Norton,etc .Also,that's how the sainted Jerry Branch did it.You want as little wiggle and wobble as possible,for you will have more of that that soon enough.

jumpjg
Cucciolo - the Lil Pup
Posts: 14
Joined: Wed Dec 05, 2007 1:08 pm
Location: St Louis

Re: 750 Sport, New Motor, Burning Oil

Post by jumpjg » Mon Jul 01, 2013 5:21 am

Ray,

Just wanted to say thanks for your help. I don't have access to other bevel drives to verify configurations, and obviously I'm having problems with machinists who are/were known as Ducati specialists and who could be trusted to do top quality work. I'm afraid that I'm starting down a slippery slope and I'll be looking at guides, valves, and perhaps valve seats. But most of all I need to find a trusted machinist and, perhaps even more importantly, good information on technical details beyond the workshop manuals so I don't end up with similar results.

Appreciate your time,

Joe
Joe in St Louis

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Mon Jul 01, 2013 6:00 am

Your existing seats and valves probably should not need replacement,just a fresh cut or grind. There are lots of guys out there who know how to remove and install a valve guide properly (there are a couple of good methods).Mike Dusek in Pa.can certainly fix you up.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Mon Jul 01, 2013 10:50 am

I can't get through the "private e-mail" procedure,Joe. All the guys you mention are probably fine,as would be any guy who does a lot of 911 Porsche head work..If you want to e-mail me at my regular e-mail rno2m@virginia.edu,I will add a further suggestion.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Re: 750 Sport, New Motor, Burning Oil

Post by Ray O'Donohue » Mon Jul 01, 2013 11:25 am

It's kind of a shame that there are "Ducati Specialists" or "machinists" or whatever out there screwing up cylinder head work-just as I also did when I was a beginner on Hondas,with no good training.There is nothing AT ALL exotic about a springer 750 or 860 Ducati or 911 Porsche or Harley cylinder head.All the fundamental things apply,just as they do with any other head. Doing valve guide work or cylinder bore work without causing problems is as much a matter of common sense and caution-what NOT to do-as much as anything else.Lucky is the young mechanic who comes along in the hands of the right guru.

Post Reply

Return to “Roundcase Engines > 750 GT, 750 Sport & 750 Super Sport”