Topic: Valve kissed piston after 100 miles....

[samm_j2]

Hey everyone,

Hoping yall can do what you do and help me figure out what happened now... just can't seem to catch a break with this damn bike...

Bike is a 78 cb750f, after a second total rebuild 100 miles in the exhaust valve kissed the piston.

Here is possibly the pertinent information. Head was put together by a reputable SOHC engin builder. Head is ported, high rev springs, upgraded retainers, and a high lift cx11 cam.
Cam was not degreed in just lined up with timing marks.

I am pretty cautious when breaking in a motor. I put the first 50 miles on with moderate rpm not going much past 5 or 6k and varying the rpm. Changed oil and started to put more miles on ramping up the rpm. At about 100 miles exactly the bike lost significant power and I could tell it was no longer running on all 4 cylinders. Took it home to figure things out. At this point all cylinders had 100% leak down and equal compression. Turns out the lash on cylinder 1 exhaust was incredibly loose... thought maybe just the jam nut back way out. Reset the lash and engine was super happy. Took the bike back out to keep putting miles on and about 5 miles out the same thing happened, loss of power and sounding like only 3 cylinders running. I limped the bike back home and again the lash on cylinder 1 was huge. I reset the lash, leak down was still solid but this time 0 compression.

 Called the gentleman who put the head together and he said i probably bent a valve. Took the motor apart and sure enough the piston and exhaust valve hit. I sent the head back to him for a look aver and he assure me that all his work was correct after he double checked it.

I am wondering if anyone has any insight into what happened here, I really don't want this to happen again... why did the piston and valve hit only after 100 miles and not before?

The only thing I can think of would be valve float? But the bike maybe only saw 8000 rpm and this bike should be able to rev much higher than that?

I plan to degree in the cam this time but I am not sure that it was a timing issue, if it were wouldn't it have hit more than just one piston and happened almost immediately?

Any insight would be much appreciated! Thanks!

[samm_j2]

Here are some pics...

[bryanj]

Has to be too much lift or cam timing

[david 750f]

Did you use an adjustable sprocket for the cam or the stock one? That cam must be degreed in as it is a high lift cam.
The Cycle X website also stipulates high lift valve guides for that cam. Perhaps one of the experts can explain the necessity of the high lift guides.
 
http://www.cyclexchange.net/camshaftpage.htm

[Tracksnblades1]

Advance camshaft timing puts intake valves
with inadequate clearance in harms way..

Retarded camshaft timing (worn cam chain)
puts exhaust valves with inadequate clearance
in harms way.

block decking (top or bottom in this case), cylinder
head resurfacing, piston design, aftermarket rods,
 head gasket-selection, and high lift long duration camshafts
are a few more concerns that can put pistons and valves
in contact. 

My Grandad always said, “Trust, but Verify”….
Works well with other matters too…😁

[samm_j2]

Thanks for the replies everyone,

The head was set up for that cam shaft, so all the valves and guides and spring pressure was done with this cam in mind and then re checked after this happened.

I understand all these issues about timing and head gasket but if it was timing or something wouldn't the valve and piston hit a lot sooner? Like immediately? And happened on more than one piston?That is just my assumption and I am probably wrong and would really like to understand what happened so it does not happen again.

Do I need to like torque down the head and put clay on the piston and then measure how much the clay was smoothed?

Thanks again

[Don R]

 My cylinder seemed to have an unusually low deck height, the piston was not in the hole far enough, one missed shift later I tagged one exhaust valve. I added gasket thickness under the cylinder, got a set of APE springs and retainers, degreed in an even bigger cam and checked the clearance with clay.
 So far, so good. 

[PeWe]

Check the timing of previous setup so you know how it was adjusted, how much wrong it was. That can be the explanation.

Stock sprocket only and as written here earlier, decked cylinder and ev head shaved too will retard the timing.

Cam is probably not grinded to fit a stock sprocket. Must use an adjustable sprocket and time it correctly even if cylinder and head has stock height.

I have a cam where the notch in right end is way wrong placed.

[samm_j2]

Thanks you guys!

Definitely did this all with the stock sprocket! Hopefully that is the issue here. I have ordered an adjustable sprocket and will degree in the cam.

I will maybe use the old head gasket and torque everything down and measure with clas to be extra safe!

[PeWe]

My 970cc setup has pistons with the outer edge rather low.
Had to deck cylinder 1mm, from 85 to 84mm.
Piston crowns shaved to fit chambers with OK compression.  Around 3mm of the flat crowned JE 12.5:1 pistons milled off.

Pockets still deep enough to handle 33.5/28.5 mm valves and 125-75 cam.  But it depends on the piston design.

I tightened the adjusters another 2-3 mm in the  +/- 20* around TDC to ensure clearance.

[Jerry Rxman Griffin aka MuthaF'er]

Am I seeing the hit OUTSIDE of the pocket?

[samm_j2]

Jerry, that is what ot looks like to me also!

Since it lost power twice i am wondering if it hit the pocket the first time and bent and then the second time it hit out side? I have no idea though...

[Tracksnblades1]

Samm,

How did it run before it lost power?

Did you determine why lash was changing?  Piston tapping valve?

Are all the pistons the same brand and marked correctly?

Does the top of piston and cylinder chamber
depict the actual carbon deposits of only 100miles.?

The light valve spring/dial indicator method may
facilitate determining clearances while degreeing
camshaft.

[samm_j2]

Tracks,

The bike ran incredibly well before power loss. Better than ot had ever run!

No idea why valve lash was changing, probably due to valve and piston contact.

All pistons pr weisco 836, and yes they are oriented correctly. Unless I totally installed them wrong haha but I am sure they are in there correctly.

Yes, this is the carbon from only 100 miles. It seems like a lot to me? I thought this was strange as evey time I check the plugs they were lean. I was also trying to get the carbs with a good AFR so I was stepping up the jets and needle positions.the carbs are mikuni rs34.

Does this information give any ideas??

Thanks!

[Tracksnblades1]

I would compare that exhaust lobe lift to another exhaust lobe that didn’t
contact its piston.

Not beating up on any manufacturer, but I’ve seen new camshafts
with errors. One luckily preventing installation in a SBC. The distributor
end bearing journal was .010 larger than the cam bearings..

I would think all of the exhaust may be near contact. Unless the other
three were sunk during the head work..

Do all the pistons deck height look similar?

[samm_j2]

I will check all this and report back. Thanks!

[PeWe]

The only damage is one valve?
If yes, not that bad. It could have ended up much worse.

About the sooty chanbers. Did you tighten head enough?
MLS need good torque to crush and seal.
I have learned min 20 ft lbs  the hard way.  It took lots of oil, no visible leak outside with less.
Last time 21.5 ft lbs.

Another headgasket to the bill as well. If not that dirty or damaged reuse the MLS again tighten it a tiny bit more like 22 ft lbs.  If bike has frame kit easier to work and redo if needed.

Important to verify that everything is OK and find out why it happened.
-Springs are race springs, shimmed for correct install height/seat pressure?

Good idea to measure the IN/EX timing on the cylinder with bent valves too when timing the cam for same numbers to verify cam grind as mentioned.

Important to tighten the advancer screws equally when time at 0 lash.
A little less give lower lift numbers and later opening, earlier closing.
I tighten until I see the needle moves just a little like 0.02mm.  This to get numbers as the camcard specify and get max lift.
 
If measure the final timing with running lash can be done afterwards.

- One more detail. Have you verified that the MLS gaskets rivets are not located where cylinder and head sit tight together?

I had to hammer them flat and drill small dents with a 10mm drill in both head and cylinder for the rivets.
If not, head will leak when rivets sits in the mating surface and obstruct head to be tightened and gasket to seal.
It depends on how the head and cylinder mold look like.

[MRieck]

The only damage is one valve?
If yes, not that bad. It could have ended up much worse.

About the sooty chanbers. Did you tighten head enough?
MLS need good torque to crush and seal.
I have learned min 20 ft lbs  the hard way.  It took lots of oil, no visible leak outside with less.
Last time 21.5 ft lbs.

Another headgasket to the bill as well. If not that dirty or damaged reuse the MLS again tighten it a tiny bit more like 22 ft lbs.  If bike has frame kit easier to work and redo if needed.

Important to verify that everything is OK and find out why it happened.
-Springs are race springs, shimmed for correct install height/seat pressure?

Good idea to measure the IN/EX timing on the cylinder with bent valves too when timing the cam for same numbers to verify cam grind as mentioned.

Important to tighten the advancer screws equally when time at 0 lash.
A little less give lower lift numbers and later opening, earlier closing.
I tighten until I see the needle moves just a little like 0.02mm.  This to get numbers as the camcard specify and get max lift.
 
If measure the final timing with running lash can be done afterwards.

- One more detail. Have you verified that the MLS gaskets rivets are not located where cylinder and head sit tight together?

I had to hammer them flat and drill small dents with a 10mm drill in both head and cylinder for the rivets.
If not, head will leak when rivets sits in the mating surface and obstruct head to be tightened and gasket to seal.
It depends on how the head and cylinder mold look like.

I have seen this interference with the rivets.....Cometic warns people about it. Also.....torque to 22lbs from the get go. Studs lubed with motor oil, 3 stages...all pulls smooth until click. If you cannot get it to click on one smooth pull loosen it up and start again. Using a torque wrench requires technique.....I learned this after reading Smokey Yunick.

[Don R]

 hmmmm, rivets, this might be problem Kemosabe. 

[samm_j2]

Interesting that it could be the rivets...?

I am pretty sure a small pocket has been machined out to allow for those little rivets. At least on the cylinders.

The springs pressure was set by the gentleman who put the head together, I trust he did it right and I sent the head to him for inspection after all of this and he assured it was good on his end.

I no longer have the bent valve so trying to take measurement with that can't happen...

I am by no means an expert but I feel pretty good about the head torque. I will step it up to 22ft-lb though, from the get go.

And yes only the one valve which I just think is really interesting, I figured it would have also happened on exhaust number 4 in addition to number 1.

This cam called to sink the vales with the f head which was done. I initially thought maybe the valve was not sunk as much as it needed but it was apparently in spec.

Thanks for the continued ideas everyone! I really appreciate it

[PeWe]

22 ft lb....
I did only 21.4 ft lb (29Nm) last time.
Retorqued the day after. Released 1/4 turn and tightened direct in one smooth movement until click.
If it was disupted by something before the tool clicked  I did it again.

I tightened the head in steps by 5Nm from inside and out. First my small 1/2" wrench from 5- 20Nm, next the bigger one 1/2", 25 and 29Nm.
I felt the gasket to crush just after 25Nm maybe (20 ft lb), it suddenly moved  lighter and longer until  next click.

So I guess that gasket crushing feeling is the important thing here. So I'll not take everything apart again to tighten a little more   (30Nm is 22.12 ft lb)

I did not notice that crush when I tightened too little with a huge oil consumption  that started after around 1000km  and got worse with black plugs.

The modern less smoking oils  can trick you. No smoking, but horrible oil consumption.
It took hours to clean pistons and head with all EX valves  off to brush the ports  and valves clean.

It was de-ported by oil carbon.

I was shown how to use a torque wrench at my job back in early 90's by a tool dealer. Very important to not tighten with a too quick twist. Final torque could land at +20% than the click wrench was set at.
At quality check we used non clicking wrenches, gauge. +/-2% tool.
Too expensive when I bought mine German made Hazet that work fine.
2.5-25Nm 1/4" wrench and a 10-60Nm 1/2" for not that much.  I have not checked them after purchase.
I'm very careful to release to 0 direct after use.

[samm_j2]

I just want to make sure I am following things here as all the mentions of head torque.

Are you guys saying that head torque is the cause of the valve hitting the piston? Or is it part of why there was so much carbon for only 100 miles?

And is this an excessive amount of carbon for only 100 miles? I mean I think it is but I don't know if my opinion on this matter carries much weight...

I will say when I sent the head back to Ken I left the head gasket on it as it was stuck there and wanted to touch things as little as possible. He thought I had used some sort gasket sealer as it was really stuck on. So I am guessing I torque it enough?

And I definitely have a new head gasket for when things go back together...

[PeWe]

No, maybe sooty pistons and chambers if oil can pass due to gasket did not seal properly.

[scottly]

Please show a picture of the top of this piston including it's neighbor's tops. The valve reliefs are different for #1 vs #2, and there is also a front and back orientation.

[Tracksnblades1]

Reply #13