Topic: Stripped rocker arm shaft bolt.

[niels.d]

So I was mighty surprised when I took my valve cover off to find an incredibly clean, newly rebuilt engine with tight new chains, tensioners and sprockets inside, especially after how rough the exterior of my 72 CB750's engine was. So, looks like a rebuild isn't going to be in order. However, I encountered one big problem: one of the four bolts that keep the rocker arm shafts in place but allow them to still rotate is stripped. I noticed when it wouldn't tighten. I pulled out the bolt and saw a broken helicoil on the bottom. I am pretty new to the whole mechanics things and I'm not really exactly sure how helicoils work. Since the coil itself broke and came out with the screw, could I just put another helicoil on? Will it have to be tapped/drilled? Any other options out there for a stripped rocker arm shaft bolt?

[bollingball]

Could be the PO used the wrong size drill. I would try to put another coil in get a good bolt and try to torque You may have to go next size over if you have room. Or maybe the coil was not long enough How do the threads look where the coil was? if the new coil gets tight you may be good to go.

[gane]

Niels, Just to be sure we're on the same page here, are you SURE, that what came out is a broken heli-coil ? SOHC's pull the threads on cam tower bolts regularly... Usually blamed on ham handed PO's but not always the case. the amount of contact area/depth of hole/age/corrosion/aluminum head, are all factors. I've seen heli-coil inserts "unwind", "follow bolt out", & rarely "pull" recieving threads. but I've never seen one "break", as they're  a "square section" mild steel wire,..... the bits which come out of the hole w/bolt are usually the broken aluminum "threads" which were once head material.  cheak w/magnet, or crush w/ fingers to verify. If aluminum, fine, simply heli-coil or time-sert. (my preference). &  If you have to pull the engine to repair one, do "em all". ( I like time-serts, because  they seldom "back out", but because they have 2 outside diameters are a true  PIA to install in the bottom of holes which are also the homes of locating sleaves.  my 2 cents , luck G

[niels.d]

Thanks for the tips guys. Gane, I'll have to flip my engine back over but I'll double check with a magnet and then go from there.

[HondaMan]

One thing not covered in any manual except the 1969 CB750K book and mine: those bolts are to be tightened to just 7 ft-lbs. max. All they do is keep the rocker shafts from sliding sideways: they don't hold the towers down. That job is accomplished by the same nuts and bolts that hold the cam bearing caps in place.

That's why so many on the K0-K3 bikes are stripped. 

Starting on the K4, Honda made 7 threads for those bolts: before that, it was just 3 threads.

[mcswny]

One thing not covered in any manual except the 1969 CB750K book and mine: those bolts are to be tightened to just 7 ft-lbs. max. All they do is keep the rocker shafts from sliding sideways: they don't hold the towers down. That job is accomplished by the same nuts and bolts that hold the cam bearing caps in place.

That's why so many on the K0-K3 bikes are stripped. 

Starting on the K4, Honda made 7 threads for those bolts: before that, it was just 3 threads.

Sorry to re-open an old thread, but what would happen if these bolts came loose over time? I know they don't hold the cam towers down, just the rocker shafts in place.

I just finished my top-end rebuild (for the first time). I just hand tightened these ever so slightly for fear of stripping them (I have no experience with heli-coils, although I believe I read that MCRider has used SAE 1/4-28 bolts as a solution to these). For some reason I'm having nightmares about these bolts that they'll shake loose and cause damage. What COULD happen?

[MCRider]

If one were to loosen, it can't rise high enough under the cam cover to do anything. If you could see in there with a minicam, the top of that bolt is real close to the cover. Might be noisy though, clicking around.

[mcswny]

If one were to loosen, it can't rise high enough under the cam cover to do anything. If you could see in there with a minicam, the top of that bolt is real close to the cover. Might be noisy though, clicking around.

MCRider,
Thanks! This will at least ease my mind for the moment.

As of now (about 100 miles) my rebuild on my k1 is riding better than ever (New gaskets, seals, cam chain, tensioner, rings, hone and lapped valves) and I put in a CycleX MLS gasket so wasn't planning on pulling and re-torquing my bolts.
It's been a brutal winter up here in NYC and the last thing I want to do is re-pull my engine when we're finally getting a chance to get on the road.
But I'll keep an eye on them and if they do come loose I'll at least wait until a rainy weekend and try and power through in a weekend.

Thanks so much!!!

[AJK]

If one were to loosen, it can't rise high enough under the cam cover to do anything. If you could see in there with a minicam, the top of that bolt is real close to the cover. Might be noisy though, clicking around.

+1
MCRider is correct. You will notice that these 4 special bolts have a certain length and head height. If you try and use bolts with the same length but different (taller) heads, you will not be able to get the cam cover on. So Honda has really made this a tight fit.

Also agree with Hondaman, these 4 bolts don't do much in the way of holding down valve spring forces. Its the 8 x main cap bolts that do that.

I've made notes to not go more than 6ft.lb on these bolts (and use a little locktite).

PS: I mentioned the head height, because if you have stripped threads, it may be possible to use a different bolt that actually grabs in the threads properly (whereas the honda bolt may just keep spinning). I found this to be true on one of my heads using another bolt in place of the honda one, however you need to make sure the bolt height is no taller than the honda's bolt height. This method might work for you, but the best bet is something like a timesert.

[Hon3ybadger]

If one were to loosen, it can't rise high enough under the cam cover to do anything. If you could see in there with a minicam, the top of that bolt is real close to the cover. Might be noisy though, clicking around.

+1
MCRider is correct. You will notice that these 4 special bolts have a certain length and head height. If you try and use bolts with the same length but different (taller) heads, you will not be able to get the cam cover on. So Honda has really made this a tight fit.

Also agree with Hondaman, these 4 bolts don't do much in the way of holding down valve spring forces. Its the 8 x main cap bolts that do that.

I've made notes to not go more than 6ft.lb on these bolts (and use a little locktite).

PS: I mentioned the head height, because if you have stripped threads, it may be possible to use a different bolt that actually grabs in the threads properly (whereas the honda bolt may just keep spinning). I found this to be true on one of my heads using another bolt in place of the honda one, however you need to make sure the bolt height is no taller than the honda's bolt height. This method might work for you, but the best bet is something like a timesert.

AJK couldn't be more right on this one. I just rebuilt my top end and the bolt in the front right corner under the tach gear was spinning. I think the stock honda bolt was a 6x1.0x55mm. The hole bottoms out at 76mm. Simply clean the hole and threads, get a 6x1.0x75mm flange bolt (hex head or allen) I used allen and done. It will use all the thread for more strength and fit like stock under the cover.

P.S. I have 10mm wear marks on the bottom of my valve cover from a cam tower grenading and hitting it. but not from changing bolts.

[AJK]

Yep, grind the heads down and your problem is solved.

[mcswny]

I've decided To go for piece of mind and to time sert them. Figure take this chance to learn something new. Even though the 1/4" bolt trick could work ill always know in the back of my head it's not correct and could fail and that will drive me absolutely insane.


Sent from my iPhone using Tapatalk

[MCRider]

I've decided To go for piece of mind and to time sert them. Figure take this chance to learn something new. Even though the 1/4" bolt trick could work ill always know in the back of my head it's not correct and could fail and that will drive me absolutely insane.


Sent from my iPhone using Tapatalk

timesert (keensert?) or helicoil are good fixes. But tapping it to a 1/4-28 will give you a fresh hole with clean threads too. and no fixture.  My memory was it was a clean fix. But...

It was a long time ago that I did that, so any one contemplating doing it proceed at your own risk. First you'd have to confirm that a 1/4" bolt will fit through the cam tower. 

[Don R]

I did the 1/4" bolt trick on my 836. The bolt does fit tight in the notch on the rocker shaft. If you want the shafts to rotate like early heads they need clearancing. I sanded the bolt slightly until the shafts rotate freely. works fine.

[MCRider]

I did the 1/4" bolt trick on my 836. The bolt does fit tight in the notch on the rocker shaft. If you want the shafts to rotate like early heads they need clearancing. I sanded the bolt slightly until the shafts rotate freely. works fine.

Thanks Don, I thought there was something like that, could't remember for sure.  Did you tap the hole to SAE, or just run the bolt in?

[ekpent]

I did the 1/4" bolt trick on my 836. The bolt does fit tight in the notch on the rocker shaft. If you want the shafts to rotate like early heads they need clearancing. I sanded the bolt slightly until the shafts rotate freely. works fine.

I did it once also

I did the 1/4" bolt trick on my 836. The bolt does fit tight in the notch on the rocker shaft. If you want the shafts to rotate like early heads they need clearancing. I sanded the bolt slightly until the shafts rotate freely. works fine.

Thanks Don, I thought there was something like that, could't remember for sure.  Did you tap the hole to SAE, or just run the bolt in?

I did it once also on an 836. I ran a tap in the old hole first.

[mcswny]

I ended up just putting a set of (I believe) 9.8mm time sert inserts in for these on all 4 bolts when I re torqued my head bolts today. So easy and super tight.


Sent from my iPhone using Tapatalk