Author Topic: CB750 K1 Cylinder Head Gasket and Oil Seals (Read 11356 times)

CB750F2 · « **on:** December 08, 2007, 10:04:04 PM »

I have recently removed the cylinder from my CB750 K1 motor because oil was seeping around the head gasket and some of the valve stems were badly worn on the ends and had to be removed for refacing in preparation for the fitting of valve lash caps. I think that the oil leak was due to a badly distorted "O" ring. This "O" ring is one of two fitted over the head studs between the cylinders and head to provide a seal from supply oil pressure. Later heads were fitted with 12mm knock pins and packing around the cylinder stud holes that form the return oil passages from the head to the crankcase. The new head gasket that I have - Vesrah brand - has the larger holes to accomodate these knock pins and packing.
Can I use this new gasket or do I have to purchase another similar to the old gasket?
If a new gasket is required can someone supply me with the part number?
Are the extra seals effective in reducing oil seepage?
I am reluctant to modify the head and cylinders to allow fitting of the extra knock pins and packings because the cylinders would need to be removed thus desturbing the piston rings. I have 185psi compression pressure on all four cylinders so I don't want to go there unless absolutley necessary. Thanks, Pat

754 · « **Reply #1 on:** December 08, 2007, 10:37:38 PM »

By knock pin, do you mean hollow dowel pin??

CB750F2 · « **Reply #2 on:** December 09, 2007, 03:03:07 AM »

754, Yes the Knock Pin is a 12mm dowel and there are eight of them. The packings go around the dowels and I think that it was a Honda modification designed to reduce oil seepage around the head gasket. Pat

CB750F2 · « **Reply #3 on:** December 09, 2007, 01:37:17 PM »

G'Day. I have just realised that the Vesrah gasket that I have is not suitable for the 64.5 big bore kit fitted to this motor so the questions about the head gasket do not apply. I will have to purchase a suitable head gasket from Wiseco or maybe Dynoman. Thanks, Pat

HondaMan · « **Reply #4 on:** December 09, 2007, 03:24:29 PM »

Quote from: CB750F2 on December 09, 2007, 01:37:17 PM

G'Day. I have just realised that the Vesrah gasket that I have is not suitable for the 64.5 big bore kit fitted to this motor so the questions about the head gasket do not apply. I will have to purchase a suitable head gasket from Wiseco or maybe Dynoman. Thanks, Pat

Pat: you'll likely find that you want to replace the cylinder base gasket, because removing the head also disturbs this seal just enough to make it start leaking later, anyway. $:-\$

I did it many times the wrong way in racing, just to get through the next race, but it nearly always leaked unless replaced.

CB750F2 · « **Reply #5 on:** December 09, 2007, 11:49:11 PM »

Hondman, thanks for the suggestion. What about the extra seals and dowels that are fitted to the later heads? Do they reduce the chance of oil leaks? I guess I will have to wait and see the configuration of the Wiseco head gasket before deciding on fitting the extra seals. At this stage it is looking like, a hone, new rings, base gasket, head gasket, and new seals between the head and cylinders. This together with refacing of the valve stems, lash caps, valve stem seals, 8 valve adjusters, and lapping of all valves should give me a strong motor. It was strong before I dismantled it to fix some serious oil leaks. Then I found damaged valve stems and the job just "snowballs" from there. Thanks, Pat

bryanj · « **Reply #6 on:** December 10, 2007, 12:11:12 AM »

dont forget the "O" rings in the base gasket and the "pucks" under the cam towers, plus a tacho drive seal whilst its easy!!

When cleaning the base gasket surface make sure no bits get into the oil ways or they will block the oil restrictors. FOR THE SAME REASON DO NOT USE "GOO"

CB750F2 · « **Reply #7 on:** December 10, 2007, 12:54:48 AM »

Thanks for that Bryan. I have those bits already. What do you mean by "GOO"? Are you referring to sealents in general or the sealent called "GOO"? Thanks, Pat

bryanj · « **Reply #8 on:** December 10, 2007, 07:08:22 AM »

Didn't know there was one specifically called GOO but i meant EVERYTHING----Seen a few wrecked cams after home builds back in the day

HondaMan · « **Reply #9 on:** December 15, 2007, 02:49:22 PM »

About the earlier question: yep, Honda did add the dowels and O-rings to stop oil leaks and weeps. They were becoming an issue for touring riders, in particular. IIRC, the K4 was the first fully oil-free 750.

The little 6mm bolts on the outside edges of the head also helped: don't put any sealant around these, in particular, as they help pull the otherwise-flaring head edges down that last tiny bit to help seal those oil passages. Adding sealant around these will make the area sit higher, which you don't want. What you will find: after about 1000 miles, these little bolts will be quite loose. You can reach in with a thin, flat 10mm wrench to snug them back down, then, and it will help prevent future edge leaks. I have occasionally fixed leaks altogether by tightening these during tuneups! (Long ago, of course...)

While it's apart: check studs and make sure they are well-set into the crankcase, tighten if necessary by adding 2 nuts to the other end, locking them together, and torque the stud, all 8. Then, clean the upper threads well, add a drop of oil to each stud thread and nut when reassembling (I normally use anti-seize instead of oil). This will ensure a full, even torque that won't need retorquing later on. Dirty threads will create just enough offset to cause the need for retorque in about 1000-2000 miles to prevent oil leaks. Proper installation will solve it "up front", like when Honda first built the engine!

scottgarland · « **Reply #10 on:** December 16, 2007, 04:43:46 PM »

Hondaman,

Ever used Blue Loctite? I have pre-unit Triumphs and use it on studs and other places subject to vibration loosining trouble with good results. Old English bike are as a general rule all alloy though. Dose this make products like loctite enadvisable ?

Scott

kaysystems · « **Reply #11 on:** December 16, 2007, 05:45:47 PM »

Honda, in one of their Service Bulletins (posted here on the forum) introduced O-rings for the early engines. These O-rings fit in the larger holes in the head gasket around the oil drain studs. Western Hills Honda had some left in the summer. They are actually not round in cross section, but the typical Honda square ones. This should stop the leaks, as well as a bit more torque on the head nuts. K0 was 14 ft-lbs, but later was more.

HondaMan · « **Reply #12 on:** December 16, 2007, 07:45:51 PM »

Quote from: scottgarland on December 16, 2007, 04:43:46 PM

Hondaman,

Ever used Blue Loctite? I have pre-unit Triumphs and use it on studs and other places subject to vibration loosining trouble with good results. Old English bike are as a general rule all alloy though. Dose this make products like loctite enadvisable ?

Scott

Yep, the "Blue stuff" was a requisite on 350s and 450s. The Fours never needed it, though.

CB750F2 · « **Reply #13 on:** December 17, 2007, 03:33:55 AM »

Hondaman and Kaysystems. Thanks for your info and advice. I pulled this motor to fix some oil leaks and in doing this I found wear and damage to the ends o fthe valve stems. So I remove the head to fix the valves and find that one cylinder is burning oil so now I have removed the cylinders to replace the rings. So, what started out as a relatively minor job has turned out to an expensive overhaul. I am going to start a new post about MTC Pistons so it would be appreciated if you could provide input. Thanks, Pat

HondaMan · « **Reply #14 on:** January 03, 2008, 08:12:54 PM »

Quote from: CB750F2 on December 17, 2007, 03:33:55 AM

Hondaman and Kaysystems. Thanks for your info and advice. I pulled this motor to fix some oil leaks and in doing this I found wear and damage to the ends o fthe valve stems. So I remove the head to fix the valves and find that one cylinder is burning oil so now I have removed the cylinders to replace the rings. So, what started out as a relatively minor job has turned out to an expensive overhaul. I am going to start a new post about MTC Pistons so it would be appreciated if you could provide input. Thanks, Pat

There's several others here who know a lot more than I do about the 750"F" series engines, Pat. I've only worked on a few of those, mostly in shop maintenance situations (not racing and mods). I'm better at the K0-K4/5 series.

CB750F2 · « **Reply #15 on:** January 04, 2008, 12:58:34 AM »

Hondaman. The bike is an F2 with a K1 motor. I have all the info I need. MTC Engineering can supply rings and they are on their way. Pat

kaysystems · « **Reply #16 on:** January 04, 2008, 09:37:04 AM »

The O-rings mentioned in the service bulletin may be avaiable from Western Hills Honda. That's where mine came from.

David

HondaMan · « **Reply #17 on:** January 04, 2008, 08:43:03 PM »

Quote from: CB750F2 on January 04, 2008, 12:58:34 AM

Hondaman. The bike is an F2 with a K1 motor. I have all the info I need. MTC Engineering can supply rings and they are on their way. Pat

Ah, then maybe I can help!
Do you also have the "F" carbs on that "K" motor?
The "K" engines are, I think, more forgiving in hop-ups than the "F" engines, because the early ones were pretty overbuilt. And, they were designed to be worked on without special tools, or just the ones in the toolkit (did you know the front axle's thread taper is there so it can be used to pull off the alternator? There's lots of that sort of thing on the "K"s.).

kaysystems · « **Reply #18 on:** January 05, 2008, 07:02:56 AM »

Quote from: HondaMan on January 04, 2008, 08:43:03 PM

(did you know the front axle's thread taper is there so it can be used to pull off the alternator? There's lots of that sort of thing on the "K"s.).

Gee, that's cool. Any thoughts of writing all this stuff down. I'm sure we won't find it anywhere else.

David

HondaMan · « **Reply #19 on:** January 05, 2008, 08:27:31 PM »

I'll try to make a list of it. It always seemed to me that Honda was thinking about such things at the time, maybe to enhance the 750's chances? It wasn't true with their earlier popular bikes, like the CB/CL72-77, CB/CL160, the 350 or 450 bikes. Heck, the bikes before the 350 all required a 12mm or 14mm wrench to remove the seat to get to the battery, and the bike's toolkit was housed in a little triangular case with either a screw-on or latched lid, under said battery, open to the public. Many bikes were stolen with their own tools, so we used to remove the tools to the house or garage. Which, of course, meant that no one ever checked batteries or adjusted chains....

CB750F2 · « **Reply #20 on:** January 05, 2008, 08:58:12 PM »

David, I will be travelling to Townsville tomorrow to pick up some parts from Rising Sun Honda so I will order the "O" rings then. Thanks, Pat from Australia

birdmannn101 · « **Reply #21 on:** October 09, 2009, 01:32:31 AM »

Morning Hondaman,

The little 6mm bolts on the outside edges of the head also helped: don't put any sealant around these, in particular, as they help pull the otherwise-flaring head edges down that last tiny bit to help seal those oil passages. Adding sealant around these will make the area sit higher, which you don't want. What you will find: after about 1000 miles, these little bolts will be quite loose. You can reach in with a thin, flat 10mm wrench to snug them back down, then, and it will help prevent future edge leaks. I have occasionally fixed leaks altogether by tightening these during tuneups! (Long ago, of course...)

Are these the pan screws (phillips head) or the cylinder head screws you talking about? What would be the torque on these bolts (if any)?

Dan

HondaMan · « **Reply #22 on:** October 25, 2009, 08:24:18 PM »

Quote from: birdmannn101 on October 09, 2009, 01:32:31 AM

Morning Hondaman,

The little 6mm bolts on the outside edges of the head also helped: don't put any sealant around these, in particular, as they help pull the otherwise-flaring head edges down that last tiny bit to help seal those oil passages. Adding sealant around these will make the area sit higher, which you don't want. What you will find: after about 1000 miles, these little bolts will be quite loose. You can reach in with a thin, flat 10mm wrench to snug them back down, then, and it will help prevent future edge leaks. I have occasionally fixed leaks altogether by tightening these during tuneups! (Long ago, of course...)

Are these the pan screws (phillips head) or the cylinder head screws you talking about? What would be the torque on these bolts (if any)?

Dan

Yes, the bolts often have crosspoint screw slots in them (all of the inner ones did, some of the outer ones did, too). The torque is 80-90 in-lbs. max, if the threads are still good (check them during disassembly). The screwdriver bits can only get these up to about 30 in-lbs, so many leak if rebuilt. I have a Honda KOWA 10mm socket box Tee handle that just barely fits these: it was brought out in 1969, no doubt to service these heads, when I bought the whole set. I remember it being painfully more expensive than the other Tee handles in the set! It must be in the "special tools" category, or something.

wick · « **Reply #23 on:** December 22, 2010, 01:41:10 PM »

hi-- so what was the outcome- about- the " knock pins " I have a 75 cb750.. with 836 kit- seepin- oil on side- and- i know- that- i only used- " 0 -rings but no- knockers?
thanks - 4 ur - time/```````

CB750F2 · « **Reply #24 on:** December 23, 2010, 04:10:16 AM »

Wick, this is an old thread that I started. Read through the replies and you should find your answer. In my case I just replaced the head and barrel gaskets together with the O rings - I used viton o rings. I did not use knock pins which came in the later models. I suggest that you check the service manual for your model to see what is required. Pat

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Author Topic: CB750 K1 Cylinder Head Gasket and Oil Seals (Read 11356 times)

scottgarland

kaysystems

kaysystems

kaysystems