Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Nurse Julie on December 17, 2018, 11:22:37 AM
-
Graham is stripping a very, very early engine today. This engine has been stood for not far off 50 years after sustaining chain damage to the crank cases. The engine has done probably less than 1000 miles as other parts are original and in excellent condition, although most parts will need replacing due to age related deterioration. The pistons were seized to the bores and 1 piston in particular has sustained a very similar type of damage to what I found in my CB400/4. Graham always insisted my damage was caused by water ingress a long time ago, with rust forming, eating in to the metal, with the resulting catastrophic damage when someone 'unseized' the engine probably with a big breaker bar and a lot of force and then started the bike and rode off into the wild blue yonder.
I think this just shows what damage can NOT be seen, when buying an old engine that has sat around for many years.
Photo 1 shows totally destroyed rings, lands and piston. Photo 2 shows the top ring being eaten away.
[attachimg=1]
[attachimg=2]
-
Wow. Makes one wonder what else might have gone the same way.
-
Wow that's servere,
Sent from my SM-G930F using Tapatalk
-
Ouch!
No doubt it ran fine when treated to a little easystart squirted into the carbs. ;)
-
Wow, that's really far gone.
Be interesting to see the top of that piston on the combustion face. If it's just been damaged by corrosion and removing the barrels then it should have no other signature of damage if it's not been run after the event.
-
It will come out with a bit of T-cut and some elbow grease. ;D
-
Nasty, a real horror show.
Cheers
Dennis
-
Ouch!
No doubt it ran fine when treated to a little easystart squirted into the carbs. ;)
Yeah right...... ::)
-
Wow, that's really far gone.
Be interesting to see the top of that piston on the combustion face. If it's just been damaged by corrosion and removing the barrels then it should have no other signature of damage if it's not been run after the event.
Some more photos Nigel. You can see all the rust on the top of the piston and then i cleaned it up on the wire wheel. You can actually see where the rust has 'eaten' in to the piston and where the few deeper holes are, i picked out rust particles corresponding to those shapes.[attachimg=1][attachimg=2]
-
[attachimg=1]
[attachimg=2]
-
What do you think then Julie?
I bet you can guess my view :D
-
What do you think then Julie?
I bet you can guess my view :D
Nope, can't guess your view Nigel.
-
The taper shape of wear on the top ring lands shows its been running with the ring getting progressively more movement.
If it had been corroded to the bore and snapped when it was tapped down the cylinder to get it free, then the piston material would just snap off without any wear pattern. The wear is there without the top surface of the piston being displaced above the taper region, so material missing next to the ring with piston crown still in place.
Detonation craters visible with molten edges on the piston crown.
-
That's interesting Nigel as i know this engine has been out of the frame sitting on the floor somewhere for 49 years. Graham has suggested the pattern of damage is consistent with water getting in and the resulting chain of events that causes rust to eat away at metal, which in turn lets more water down and so on and so forth. The worse piston was the one seized but actually freed off quite easily compared to some we have in. Also, on the other piston where the top ring has been eaten away by rust, there is no evidence on the piston crown of detonation.
Whatever has caused it, my point with these photos is that so many times, i read or hear of people that say 'the engine was a bit seized but it freed off easily' and they do not know what damage has been caused until it is too late and major damage has been done.
-
Sweet baby Jesus .... I don't see the problem.
Stick it on E-Bay as a good runner in need of slight attention. Has compression and will fire on Easy-start .... might need carbs cleaning. Make good Cafe racer. Barn Find...and stick a price tag of around £5K on it.
One thing I am not sure about, though, is which model of Submarine was fitted with these engines....I mean..it did come out of a subamrine? yes?
-
One thing I am not sure about, though, is which model of Submarine was fitted with these engines....I mean..it did come out of a subamrine? yes?
No, no, you really must brush up on your product knowledge. This is the model fitted to the submarines (a totally different CB750/4 engine than the one above :'()
We get some cracking examples through here ;D
[attachimg=1]
[attachimg=2]
[attachimg=3]
-
You all expect miracles, after all, it is complete apart from the odd piston ring.
-
Julie got this a little wrong it was number 2. Here is a picture before the barrels were removed and it was the only piston with a pond ;D[attachimg=1]
-
Oxidation to any alloy causes pitting and not only on pistons.
Here are some pictures of a piston from a engine that had been rebored and then left for about 40 years, without been started. The oxidation had started and the rings had seized to the piston. If this engine had been started, the corrosion would of burnt off and the detonation would of caused a chain reaction ;)
As you can see from the underneath of the piston it is untouched.
[attachimg=3]
[attachimg=1]
[attachimg=2]
-
Probably blend it out with some wet & dry and a bit of autosol eh Graham?
-
A lot of damage on that piston crown.... probably why it was separated from the frame in the first place.
-
Wow, that is some severe damage. I think endoscope inspection before any startup attempts on unknown motors from now on in my shed. Admittedly it won't show the rings but piston crown and bore inspection will give a good insight as to if sutch horrors lurk beneath the crown. My own far more limited experience is that once engines contain that much damage and corrosion they often won't turn over well enough to run, still it's a worthy warning of what can be amiss in an engine that seems reasonable. I had a cb500 engine that kept eating tensioners but otherwise seemed extremely clean and low mileage. One day I'll tear that down and see what similar horror lives down in the crank end. Top end was mint, hardly even carbon on the head. The flip side of the coin is that for us less experienced engine dabbles can do more damage looking inside a motor that was fine before we set to with spanners.
-
Matt most common reason for 500 damaging tensioners is if the holder gets a bit bent and does not locate in the correct place in crankcase.
-
A lot of damage on that piston crown.... probably why it was separated from the frame in the first place.
If you are talking about the piston pics i put on, nope. The drive chain made a hole in the crank cases the size of a football which is very common on these early engines. The engine was removed from the bike having covered less than 1000 miles (approx).
If you are referring to the ones Trig put on, i have no idea.