Author Topic: oil leaks on a cb750 F2 head  (Read 3701 times)

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Offline mazingerzeca

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oil leaks on a cb750 F2 head
« on: May 16, 2009, 12:26:11 PM »
Hello all.
I've just finished rebuilding my cb750 F2 engine, and the oil comes (a lot) through the head fins.
All the 0-rings are new, and the rubber pucks have been replaced. The only way I can imagine the oil to pass through is the holes for these studs, which communicate with the outside of the engine.
Do you think this is the place where the oil comes from? If so, is there any way of sealing it? Thank  you.



Offline MRieck

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Re: oil leaks on a cb750 F2 head
« Reply #1 on: May 16, 2009, 12:51:46 PM »
Oil can leak from there. I use teflon paste on the threads. You also need to put some sealer on the rubber pucks (1194, Hondabond etc)
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Offline Johnny5

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Re: oil leaks on a cb750 F2 head
« Reply #2 on: May 16, 2009, 12:56:56 PM »
I had a recent problem with the rubber pucks. They were new but I didnt put sealant on them and they leaked a lot. Took it apart, used sealant, and have been leak free since.
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Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #3 on: May 16, 2009, 01:12:01 PM »
Thanks for the replies.
I had put sealant on the rubber pucks. It doesn't seem to leak from there. I will use teflon on the threads. I can't imagine any other way for the oil to escape.



Offline Ace

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Re: oil leaks on a cb750 F2 head
« Reply #4 on: May 18, 2009, 05:55:06 AM »
Make sure your tubing for your crank case breather isn't kinked or block.  This can add pressure to the engine and help it leak.  Doesn't seem to be the problem though from what you're saying.

I too have a leaking F2 head after a rebuild, well a few hundred miles later at least.
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Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #5 on: May 18, 2009, 12:10:01 PM »
It doesn´t leak from the gasket. It looks like the oil comes from the fins, and a lot of.



Offline MCRider

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Re: oil leaks on a cb750 F2 head
« Reply #6 on: May 18, 2009, 12:34:27 PM »
It doesn´t leak from the gasket. It looks like the oil comes from the fins, and a lot of.
You're back to the pucks and the threaded holes that open into the fins from the cam tower hold down studs and bolts. I had one that leaked heavily from "the fins", between the cam cover gasket and the head gasket. I had helicoiled some of the cam tower hold downs. I could never get it to stop and replaced the head.

Once those threads are compromised, fatigued, etc, I think they may be a chronic leak problem.   :(

If you haven't used sealer on those threads, and they are not otherwise compromised, that may do it.
« Last Edit: May 18, 2009, 01:11:47 PM by MCRider »
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Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #7 on: May 18, 2009, 01:01:34 PM »
Yes, some of the threads have been helicoiled. I'm afraid I will have to search for a replacement head, which is very hard to find here, in Spain :(



Offline MCRider

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Re: oil leaks on a cb750 F2 head
« Reply #8 on: May 18, 2009, 01:10:38 PM »
Yes, some of the threads have been helicoiled. I'm afraid I will have to search for a replacement head, which is very hard to find here, in Spain :(

Bingo.  Sorry. 

My head was ported by Yoshimura back when they had a shop in New Jersy. It performed wonderfully. I had used it on my roadracer then my streetbike. I had swapped out several cams, fatiguing those threaded holes beyond salvation.

I'm hoping someday for old times sake, I can find a machine shop that can weld it up and redrill those holes.  I've actually found several on the internet that advertise aluminum cylinder head thread repair. Its just a matter of getting it into their hands for evaluation.

Maybe a good machine shop in Spain...?
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Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #9 on: May 18, 2009, 01:27:28 PM »
I took the head to two machine shops locally, and they are not used to this kind of job. I've talked today to some "famous" machine shops, one of them in Barcelona and the other one in Bilbao. They told me that the work can be done, that it won't be cheap and that they want to see the head to evaluate it. I will take the head to Bilbao, where my parents live, this summer. Anyway, if a second hand head appears meantime, I will buy it.
It has been an expensive engine rebuild, and I don't want to end with a defective head leaking everywhere, so I will have to wait.



Offline MCRider

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Re: oil leaks on a cb750 F2 head
« Reply #10 on: May 18, 2009, 02:02:20 PM »
Yeah, I figured it might be pricey. I don't understand welding but I've seen welders do magical things. My guess is they would weld up the holes and redrill and retap. And somehow assure that it is level with the rest.

In my case, I would pay $200 just because I want to and $200 for salvaging a ported head.

A good used head could cost $100 to $250, and you're not for sure what you've got.

Good luck.
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Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #11 on: May 18, 2009, 05:08:25 PM »
I will post the results. Thanks for your help.



jtcb750K6

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Re: oil leaks on a cb750 F2 head
« Reply #12 on: May 20, 2009, 12:17:25 AM »
Ok, I have a cb750 K6 that had the same issue. The root of the problem comes down to one word: "tolerances".
If the gasket is below or above the correct tolerances, the rubber standoffs (rubber cylinders0 for the bolts will not do their intended job, I will explain why below.

Let me take a step back and explain the oil distribution system:
1. oil travels up through the two center back bolt holes [intake side] at about 60psi from the oil pump (bolts are easy to recognize by the o-rings with a countersunk hole, not rubber cylinders), this is why your bolts are significantly smaller in diameter than the "clearance holes" (keep this in mind because all are like this...hint...).
2. the nut seals the top end, therefore oil is forced to travel through the walls through a small hole that meets the bolt hole [think of a Y), which ends in a smalll injector. Check to see if this is clogged (more than likely, otherwise there would be no leaks, but this is not the only issue at hand), how? by sealing the end where the nut sits with you finger and blowing air through the bottom (where the o-ring sits), air should come out through the injector (small silver injector with a small o-ring, this sits below the cam tower - one injector per tower).
3. then, the injector (aided by the o-ring) forces oil through the holes in the cam tower, which have two small holes pointing towards the cam to keep the parts lubricated.
4. once it has done its job of lubricating and stealing heat, gravity allows it to travel back to the crankcase through the other bolt holes (again, think of a Y), you will see holes at the bottom of every "pool" below/around the springs, this meets with the bolt holes and travels back to the crankcase to be filtered and re-circulated.

so, why did I mention this? because, if you do not check step #2 (oil injectors), the 60 psi goes out through the easiest way it can find...your gasket!

Now, I said the issue does not end there, and I also mentioned "tolerances", and here is why:
Honda has head gasket thickness specifications, a maximum and a minimum, why? well we go back to the bolts again. See, the rubber cylinders around the bolts have to deform to do their job of sealing and transfering oil. If the gasket is too thick (rare), then the rubber cylinders nor the o-rings deform enough, not sealing, therefore creating leaks. If the gasket is too thin (common), then the metal bushings that keep the rubber cylinders in place stand taller than the gasket. how to check? simple, remove the rubber cylinders and leave the metal bushings and the head gasket, get down to eye level with the gasket and press on it, if the bushing stands taller than the head gasket, this is the "source of evil" because the bushing sits flat with the head, leaving your head gasket floating (not noticeable to the naked eye because we are talking about less than a millimeter. Here you have two options, either buy a new gasket, or file down the bushings. If you opt to file down the bushings, take it one bit a a time, only file enough to bring it right below the gasket, too low will allow the rubber cylinders to deform radially, creating leaks (mine were tapered, so I removed one tapered end and made it my point of reference at the grinder).

Also, with the head gasket in place, place the o-rings in their corresponding countersunk spot, again, at eye level, press on the gasket, and make sure the o-ring shows past the gasket by a hair below or at least 1/2 of its diameter, if not, find a new pair of o-rings, this is a rule for a good o-ring seal.

Now that tolerances are taken care of, spray the gasket with "copper spray-a-gasket" which you find at a local auto supply store, spray it like paint, following the can instructions (make it sticky), this will take care of small imperfections on the surface that could cause leaks, not noticeable to the naked eye, usually created when using sandpaper to remove debris. Install the head and let it cure for 3-4 days. This material will not dissolve with gas, like an RTV gasket would (this is why RTV is not used to seal heads).

If you helicoiled the cam towers and the holes made it to the other end, a dab of RTV will take care of it, this area does not undergo pressure, so RTV should be enough.

Good luck, I hope this helps. Let me know if you need anymore help, I have had all sorts of issues, from stripped bolts to cracked crankcase, I have done all work on my garage, so shoot away...



jtcb750K6

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Re: oil leaks on a cb750 F2 head
« Reply #13 on: May 20, 2009, 12:38:55 AM »
I meant to say "helicoiled the holes for the camtower bolts" not "helicoiled the camtower", its getting late, I hope I did not make other errors...



Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #14 on: May 20, 2009, 07:41:48 AM »
What you mention makes sense, I mean, about the pressure. I installed HD studs on this engine, and given they are larger than the original ones, I thought that it would be more difficult for the oil to pass through the holes, resulting in bigger pressure, but I've read about a lot of rebuillds here and never read a problem about HD studs related to oil restrictions, so I forgot about it.
But I think that the problem on my engine is not related to pressure, because when I dismantled the head (after all the leaks), the head gasket was absolutely clean, no oil came through it. Even the cylinder fins were clean, near to the gasket, they were dirty on the outer sides, from the oil that was dropping from the head, so I am sure that the leak is not from the head gasket.
All the leaks come from the head itself, from the front side, and I suspect that from the holes I signaled on the picture, so I will use RTV or teflon paste, as MRieck suggested, there.
Thank you very much for your help.



Offline MCRider

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Re: oil leaks on a cb750 F2 head
« Reply #15 on: May 20, 2009, 08:00:33 AM »
What you mention makes sense, I mean, about the pressure. I installed HD studs on this engine, and given they are larger than the original ones, I thought that it would be more difficult for the oil to pass through the holes, resulting in bigger pressure, but I've read about a lot of rebuillds here and never read a problem about HD studs related to oil restrictions, so I forgot about it.
But I think that the problem on my engine is not related to pressure, because when I dismantled the head (after all the leaks), the head gasket was absolutely clean, no oil came through it. Even the cylinder fins were clean, near to the gasket, they were dirty on the outer sides, from the oil that was dropping from the head, so I am sure that the leak is not from the head gasket.
All the leaks come from the head itself, from the front side, and I suspect that from the holes I signaled on the picture, so I will use RTV or teflon paste, as MRieck suggested, there.
Thank you very much for your help.

Yes I was going to mention to JTCB that I appreciate his well laid out explanation. But the key point came near the end where he references the holes for the camtower bolts. These are the culprits. And in my case, having run them thru several use cycles and then helicoiling them, further softening and distorting them, no amount of RTV would save them. 

THese holes are tapped all the way thru the head to the atmosphere from the factory. While it seems that since there is no pressure there it wouldn't be a problem, it is. Otherwise they wouldn't have the rubber pucks. Must be a form of capillary action.

I could look in between the fins on mine with a flashlight while it was running and actually see the oil drip drip out of those holes, which had the camtower hold down bolts in them. Actually, 4 of the 6 come with studs in them from the factory, but they gave up early and were withdrawn to be replaced with bolts.

Once they are that bad, I see no way to fix them without major surgery. Weld up and redrill.

HD cylinder studs are not a problem, IMO I had them in my street bike for 50k+ miles, never a problem with head gasket leak. Thousands of sets are in use. Also HD nuts are available, though the stock nuts are hardened. I don't tighten them past the high end of the stock range. THey provide stability as well, even at lower than maximum torque.
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Offline bryanj

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Re: oil leaks on a cb750 F2 head
« Reply #16 on: May 20, 2009, 08:53:07 AM »
Rather than helicoils i have used "Keenserts" which used a standard (But odd pitch) tap and then used a Lock N Seal on the outer thread with a TINY dab of silicone on the bolt thread.

I have had porous castings before now but its rare
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Offline MCRider

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Re: oil leaks on a cb750 F2 head
« Reply #17 on: May 20, 2009, 08:57:36 AM »
Rather than helicoils i have used "Keenserts" which used a standard (But odd pitch) tap and then used a Lock N Seal on the outer thread with a TINY dab of silicone on the bolt thread.

I have had porous castings before now but its rare

I've used keenserts before as well, great product. Won't quite fit in this application. Would require engine removal (I had an APE removable frame tube kit at the time I was messing with mine) to get a straight shot with the drill. The larger hole required by Keensert I think IIRC was also a problem, not enough meat in that area.
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jtcb750K6

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Re: oil leaks on a cb750 F2 head
« Reply #18 on: May 21, 2009, 06:07:19 PM »
ok, my stubborness did not let me understand that you had already determined it was the bolt, so here I go at an attempt to redeem myself.
Helicoils and keenserts, unless a sealant is applied, will leak oil (in small amounts, but that is besides the point).
Here is an easy and cheap solution that requires bolts, a grinder and RTV.
Given that the concavity for the spark plugs provide you with access to the bottom of the tapped hole, remove the spark plug (if you have not already done so) and do the following:
1.Purchase fully threaded short bolts with a wide head (make sure the head is bigger than the hole), and grind the length until you have only two threads left in it (this is all you will need for thread engagement).
2. Grind down the studs by 3 threads.
3. Apply RTV to your new short bolt, and with a ton of patience, bolt it to the bottom of the hole.
4. After curing, install stud.
This will prevent RTV to remove itself through vibration and leaking in a few miles.
I have included a poor drawing to see if it helps.
Let me know if this helps.
 



MotoMartin

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Re: oil leaks on a cb750 F2 head
« Reply #19 on: May 22, 2009, 02:38:25 AM »
This is from a previous post (Cheers KaseyF2), and seems relevant to your problem. Try the simple things first! Are you sure the O rings are on the right studs? Are the domed nuts on the correct studs?

"hya there serge, to help you understand, look at your f3 head, there are only four oil return holes...
Each "pair" of  valves share one hole, whereas on the k series heads,there is a return oil hole next to EVERY valve.....
Be VERY VERY carefull that you do TWO THINGS RIGHT. Down at the crankcases BEFORE you put the barrels on, make sure you have installed the four tiny   "o" rings on the studs that do not have the knock pins, I believe from memory these are labelled 5,6,7and 8 in the tightening sequence, but NOT IN EVERY MANUAL!! ( I have two manuals that show different tightening sequences!)
The purpose of these "O" rings on the studs is two-fold, the stop oil rising up the way AND believe it or not (BUT ITS ABSOLUTELY TRUE!!) they stop water and weather from entering down the way so to speak, this can be proved by looking at your cylinder head in a moment!

IF, you take a screwdriver or other probing device (but not your personal tool so to speak) and poke it down "those" (5,6,7and8) stud HOLes ON YOUR CYLINDER HEAD, AND LOOK through the fins,
"OH MY GOD" YOU MAY WELL EXCLAIM, "why can i see my screwdriver?"
You will then be in the enviable position of being in the f2/3 "know" This is because "those" holes are not fully enclosed like in the k series, and should you forget OR neglect to replace those copper washers and domed nuts with new copper washers OR put the domed nuts on the wrong studs, you
WILL get oil pissing out between the fins when the motor is started.........
MORE THAN ONE person has experienced this! ( including me!) and I now have a mission to stop other people from the pain this causes!!
If ever you get the chance to see UNDERNEATH the barrells of a k and an f2/3 side by side, DO IT!
If ever you get the chance to see a k series AND f2/3 series head side by side DO IT!
Only close inspection reveals the differences and its well worth the looking and then the knowing!
kindest of regards,
kaceyf2"
« Last Edit: May 22, 2009, 02:47:53 AM by MotoMartin »



Offline mazingerzeca

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Re: oil leaks on a cb750 F2 head
« Reply #20 on: May 22, 2009, 07:44:04 AM »
Yes, I had read this post yet. Before assembling the engine, I think I read all the posts related to F2 engines, I'm too anal.
But the problem doesn't come from there. If you forget the 0-rings, for sure you will have oil leaks, but they will come between the head or the cylinders, or through the barrels fins.
But the problem on my engine is that the oil comes from the head itself. For sure is passing through the holes made on the head for the cam towers bolts.
jtcb750K6: I thought about tapping these holes. I wonder why Honda left them open. Given that the head has been dismantled, maybe I will take it to a machine shop to have them tapped with welding.



Offline MCRider

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Re: oil leaks on a cb750 F2 head
« Reply #21 on: May 22, 2009, 08:05:51 AM »
Yes, I had read this post yet. Before assembling the engine, I think I read all the posts related to F2 engines, I'm too anal.
But the problem doesn't come from there. If you forget the 0-rings, for sure you will have oil leaks, but they will come between the head or the cylinders, or through the barrels fins.
But the problem on my engine is that the oil comes from the head itself. For sure is passing through the holes made on the head for the cam towers bolts.
jtcb750K6: I thought about tapping these holes. I wonder why Honda left them open. Given that the head has been dismantled, maybe I will take it to a machine shop to have them tapped with welding.
Its hard to explain isn't it?  Those cam tower hold down bolt holes emerge from the head between the fins about the 3rd one, at an angle, on the exhaust side. There is no way to plug them from underneath. They are the ones that do double duty, not only hold down the cam towers, but also hold down the exhaust side of the cam bearing cap.

There are no Orings, washers, etc involved. On the K at least the original setup had 4 studs into the head from the factory. It may be that the shoulder on these studs provided a sealing effect. Like many, the top threaded end of the stud where a nut would go would strip from multiple use, and I removed my studs to be replaced with a hardened bolt from the top down to the head. Perhaps we should always replace them with studs to preserve that shoulder sealing effect? Too late for that if the threads are compromised.

(FYI: On the intake sides, these holes have a bottom and hence don't leak. But if one accidentally puts too long of a bolt oin that hole it can emerge in the intake port.)

I'll try to post a picture showing these later today.

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