Uh-oh!

Larry S

Larry S

VIP MEMBER
Joined
Apr 15, 2020
Messages
685
Country flag
1975 850 mk3. Last ride out I was passing a semi at about 70 mph when suddenly I had a major loss of power. I immediately pulled over and got trailered home. I pulled the plugs and checked compression. 145 psi left and 65 psi right. Ok, figured a blown head gasket and ordered one up of the composite type. Current one being copper. I received the new gasket today and pulled the head and this is what I see after rotating to TDC. The left piston is at TDC and the right is significantly lower. The right piston also bears a slight witness mark from the intake valve. Disgusted, I took pics and ceremoniously draped clean shop towels over the barrels so I didn't have to view it any longer. What am I likely to find when I do remove the barrels? Best case, worst case scenario. Here's the pics.
 

Attachments

  • Uh-oh!
    20230719_193636.jpg
    259.8 KB · Views: 375
  • Uh-oh!
    20230719_193644.jpg
    317.8 KB · Views: 380
  • Uh-oh!
    20230719_193655.jpg
    290.7 KB · Views: 349
  • Uh-oh!
    20230719_193925.jpg
    388.2 KB · Views: 388
Very disheartening after all the time you spent building that thing on your hands and knees. Take a day off, don't take it out on the dog or the Mrs.. Remove the jugs and you'll find the answer. Kinda like Cracker Jacks, there's a prize in there somewhere. Good Luck!
 
Hmm. The RH piston has been leaping up and down in an unconstrained way, which points to a problem with the gudgeon pin or the big end bearing. There must have been a mechanical clattering noise as well, though you may have been a little preoccupied at the time.

Best advice would be to remove the barrels and try moving the conrod, it will probably have several mm of vertical play due to a collapsed big end bearing.

I bought a Trident in pieces many years ago which had a similar problem Needed a crank regrind, replaced the rod, rebore + new pistons. At least there's only 2 of them here :cool:
 
Really sorry to see this
My guess is it's more than just a big end shell
The piston is too far down for just a shell IMO
I'd check that valve very carefully that the piston kissed
They can easily be very slightly bent
 
Rod bearings. What was the oil pressure gage reading? (Not while passing the truck, but earlier in the day)

The pistons look light grey...
Bare aluminum?
Grey deposits?
A bit blurry.
 
Larry S said:
1975 850 mk3. Last ride out I was passing a semi at about 70 mph when suddenly I had a major loss of power. I immediately pulled over and got trailered home. I pulled the plugs and checked compression. 145 psi left and 65 psi right. Ok, figured a blown head gasket and ordered one up of the composite type. Current one being copper. I received the new gasket today and pulled the head and this is what I see after rotating to TDC. The left piston is at TDC and the right is significantly lower. The right piston also bears a slight witness mark from the intake valve. Disgusted, I took pics and ceremoniously draped clean shop towels over the barrels so I didn't have to view it any longer. What am I likely to find when I do remove the barrels? Best case, worst case scenario. Here's the pics.
Click to expand...
Bummer Larry - Keep us posted with pics please. Does seem too big a difference for rod bearings alone …
 
baz said:
Really sorry to see this
My guess is it's more than just a big end shell
The piston is too far down for just a shell IMO
I'd check that valve very carefully that the piston kissed
They can easily be very slightly bent
Click to expand...

I Wonder what the oil looks like?
 
You need good news right now......other than mentioning that AN now has cranks I can't help.....sorry......also expect bent inlet valve!

The sooner you look, the sooner you start on the route to resolving it!
 
On second thoughts, if the rod bearing shell spun up onto the rod cap shell it could do what the photo shows. Larry says he did compression check, if it was crank or wrist pin issue I would think you could hear strange noises just kicking it through. Might also explain why just one mark in piston crown from the valve. It only hit once as the shell rotated in the rod. I had a Mack engine let go on me like that once. made a terrible knocking noise for about 15 seconds then sounded OK. The teardown was not pretty though.
 
Remove engine and tear it down. Sooner started sooner you know where you stand.
One way or another most of us have been here before...
 
Ouch!!

Those pistons are much cleaner on top than mine have ever been. No valve reliefs in flat tops?

It must have sounded like hell for a second or two when it lost power. Wise to shut it down. I did not shut my motor down when I heard some new sounds. Not the best use of my brain.

Warm up your credit card and clear some space in the garage for the engine disassembly. I think you have a late summer fall project.
 
You must log in or register to reply here.
Back
Top