Topic: K8 Engine Damage - cam shaft sprocket in pieces

[anotherCB]

As the title says. This is what I found after pulling the engine and taking the valve cover off. One ear (or how would you call them?) of the cam where the sprocket attaches broke off. The chain must be torn and down there somewhere. All this happened at 6000 rpm. Haven't taken the heads off yet, who knows what I find in there, possibly piston/valve contact. Halleluja!

I bought the bike "running when parked" but I have the feeling that the cam sprocket wasn't tightened down properly. Or what do I know what happened....What am I looking at? Do I need to get into the bottom end to get the new cam chain around the crank? That's why I need more than 1 bike 

[754]

If you pul, the rockers , you can get tge cam out.
 Then roll the chain around and inspect.
 Yeah be good to get the parts out if the bottom of motor.

[anotherCB]

The chain is not around the cam anymore. It must be torn.

[seanbarney41]

ok, that cam sprocket was in bad shape for a really long time now as easily seen by the heavy rust pitting.  I doubt that is what caused it to fail but a builder who would use that, god knows what else is in there...I would have to tear that down or find another engine.

[anotherCB]

I agree. I am questioning everything at this point, will probably take the time and do a good rebuild.

[Alan F.]

That big piece would make a great key fob, the end looks like you could open a beer with it.

Sorry to hear about all this, were you riding or just reving up?

[anotherCB]

Not a bad idea!

Happened at around 6,000 rpm in second gear....luckily I only had to push it home 0.5 mls but it was 100 degrees 

[scottly]

That type of damage can happen when one of the bolts backs out. Make sure you find the missing bolt!!
http://forums.sohc4.net/index.php/topic,173115.0.html

[anotherCB]

Yeah the guy in that thread was luckier than I am!

[scottly]

Did you read the last page?

[scottly]

Actually, it's reply #141 on page 6..

[ekpent]

 I went and checked it out Scottly after seeing your link. What a shame for her with all the work she put in making that bike her own.

[anotherCB]

Actually, it's reply #141 on page 6..

Now I see, I didn't read that far. That's probably what I will be looking at. Thanks for the link, will read the whole thread tomorrow.

[bryanj]

Engine out, complete strip with several valves or used head maybe pistons/bore all as minimum

[PeWe]

Another method to get same result.
http://forums.sohc4.net/index.php/topic,125377.msg1424921.html#msg1424921

This case helped me to cut my frame and install Franks frame kit to replace my overtightened HD cylinder studs.
A borrowed bad torque wrench was my root cause.

I bought new wrenches that I have used a lot since then. Nothing bad that does not bring good in the end.
With my own wrench, 20-20.6 ft lbs (27-28Nm) for APE and Kibblewhite cyl studs. Same torque I tortured my K6 stock studs back in the 80's.

[anotherCB]

Here is where things stand. I pulled the jugs off, fished the cam chain out of the bottom end and got my sprocket puzzle almost back together. Looks like the reason for the failure was that one of the two screws that hold the sprocket on the cam came lose.

When loosening the head bolts i realized that 4 bolts were only hand tight! Also the upper halfs of the cam bearing holders (or whatever you call them) were mismatched. Someone did a lousy job putting this together.

Two valves have made contact with the pistons as well, however it doesn't look that bad. Pistons and jugs both look good so I don't think I need to do anything there. So what are my options:

1) Take bottom end apart, make sure I find all the sprocket pieces to complete the puzzle, get a new cam chain and cam. Valves and pistons look ok, do I always have to replace valves if there was contact?

2) Would it make more sense to just buy another engine? I think for around $500 I could probably get a running engine.

Let me know your thoughts. I usually work on Airheads, parts seem to be more plentiful and cheaper for this bike, so i would love y'alls input. Thanks!

[PeWe]

Take cases apart, look for damage. No damage, replace primary chains and tensioner. Look if crank bearings and crank journals are worn.
Feel if rods have lash due to bad bearings. You should have heard that before, but a rod might been bent at the last event.

Check bearings in engine and look if transmission has worn dogs, shifter forks OK, neutral switch OK, no leaks.

Maybe you have ridden the bike enough to know that 2nd and 4th gear never jumps as they can.

A known restored engine better than an old you do not know the history of.

I should replace the primary hub cush rubbers too when at it.

If clutch steel basket has loose fit on the primary hub splines, another tighter basket can be found. Possible to change when assembled too, but it usually cost a gasket that will sit too hard to go off in one piece.
 (I changed clutch steel baskets on both my CB750 this year.)

The bottom end need probably a good clean from sludge, old fiber material.
Hopefully no pricey costs, in the end an improvement.

Oil pump repair kit might be something too. Take off and on pump when in bike is possible, easier when engine is on the bench.

Head with seats, all valves and guides might need a restore?
Exhaust worn more than in.

If your pistons are stock 61.0mm, there is a low cost set for $120US + shipping from Japan. Maybe 40 in shipping.
+0.5mm. +1.0 available too

https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

I should add a clutch at the same time.  My K2 has one work fine. I had one in my K6 when 836cc and +80whp.

https://www.cruzinimage.net/2017/12/20/76-78-honda-cb750k6-cb750k7-cb750f2-clutch-friction-plate-2/

[anotherCB]

PeWe, thanks for the shopping list, good to have it all in one place! I wonder what that would cost if I added it all up...I will also need a new camshaft and chain.

One more thing to consider, it was a high mileage engine. I have an out-of-state title which says 75,000 and the PO thinks that's correct.

I guess right now I'll look at both options, rebuilding or buying a used running engine, which I would then go through (top only not bottom).

Thanks!

 

[Alan F.]

75K is just broken in so I hear.

I also hear a 750A cam provides more low end torque.... maybe it's an option that shouldn't be discounted too early?

[seanbarney41]

Valves that contacted pistons are most certainly bent.

[anotherCB]

I also hear a 750A cam provides more low end torque.... maybe it's an option that shouldn't be discounted too early?

That's good to know, if I decide to go that route. Thanks!

[mrbreeze]

Wow! If you weren't an engine rebuilder you're gonna be now! I agree with the others as to it being a crap shoot putting a used engine in. I would say if you go that route scour through these forums before you buy one. Arm yourself with as much info as possible. You'd be extra pi$$ed off if this happened twice.

[PeWe]

I have found 2 K7 cams in very good shape for max $50US.
Seems they are cheaper than the earlier cams.
Work fine in my K2 with stock sprocket, no advanced timing to move power earlier.

[bryanj]

I built a 120000 mile engine with in spec bores, pistons and rings.
At your mileage the above list is good add oil seal kit for engine.
First job is strip head and decide how many valves are bent as it may be cheaper to get a used head, cam and towers ets complete but even then i would remoce valves for a clean up and new stem seals, getting a damaged head and transfering bits may be even cheaper.

Do check out your valve guides as its not unheard of for those to crack when valves get bent

[PeWe]

I found a very good looking K6 head. In valves and guides looked really fine as not much used while ex valves were bad, guides too with huge play.