Author Topic: 1973 CB750 head on 1975 CB750 cylinders (Read 8238 times)

Leanier · « **on:** December 30, 2010, 11:03:14 AM »

I was just wondering if my 1973 head will work on a set of 1975 cylinders that I purchased. I figured they would be the same, but once I got my new set of used cylinders, I noticed that the holes for the cylinder studs in the middle of the cylinders, (4 per side) have dowel pins in them, and rubber gasket things, that my 1973 cylinders do not have. I tested to see if my head would fit, but it seems the dowel pins hold the head like 3mm up or so (thats an estimate) from seating perfectly together. Is this an issue? or is that space taken up by the head gasket anyway? Or do I also need to get a new head or something as well.......

Leanier · « **Reply #1 on:** December 30, 2010, 11:15:05 AM »

Ok, looked on bike bandit and both cylinders and head from 73-75 are the same part number. Also, noticed that the dowel pins in the central area are the same length as the two dowel pins on the outer bolt holes, (that actually go into the head) are the inner ones supposed to be shorter?

MCRider · « **Reply #2 on:** December 30, 2010, 11:27:55 AM »

Quote from: Leanier on December 30, 2010, 11:15:05 AM

Ok, looked on bike bandit and both cylinders and head from 73-75 are the same part number. Also, noticed that the dowel pins in the central area are the same length as the two dowel pins on the outer bolt holes, (that actually go into the head) are the inner ones supposed to be shorter?

I don't know the exact answer to your question. If you're in the parts books, they list the dimensions of the dowels/knock pins. I would just say, make sure the holes the pins go into are clean from gunk and goo and that you can easily drop the knock pins in both the cyl and head with just a friction fit. Shouldnot be a force fit. The head should mate easily to the cylinder, the pins seating in their holes. Sounds like you've got pins that are too long and are holding the head up. Don't force them whatever you do.

Leanier · « **Reply #3 on:** December 30, 2010, 08:47:41 PM »

Well, I got the cylinders online, supposedly just taken off of a bike, and fully stock. so the pins should all be the stock ones.. anyone have a stock 75 head they could take a look at and see if the gasket face has any holes with slight counterbores to them? mine only has the two outer ones counterbored and the rest smooth. if 75 heads are the same as my 73 head, then i'd suppose the head gasket just takes up the slight mm or two of space.

HondaMan · « **Reply #4 on:** December 30, 2010, 09:12:22 PM »

The head gasket will not take up that spacing: it is possible that the 1973 head you have is the last of the K2 series heads, which did not have the recesses around the stud holes that fit the later cylinders. The K3 model year started with many K2 engines (not by serial number, but by the components inside), became K3 bottom ends with K2 heads (different clutch and shifter drum and final drive bearing), then became K3 all by about February of 1973.

During the K3 era, the 750 went through several changes as the early heads disappeared and the new cylinders with the longer rubber seals and dowels appeared. The earlier heads can usually be machined slightly in depth around the stud holes to receive the dowels and rubber seals: I can find a head over the weekend and make some measurements of these recesses, if someone here doesn't beat me to it. That little bit of machining is all it takes to make them fit together. The early heads and cylinders had O-rings in those seal zones, where the later ones had several dowels and rubber seals added to reduce the effects of high-detergent oils, which washed away the sealant on the head gaskets around those holes over time. If you make new recesses, the head can still fit on the earlier cylinders if you select a thicker O-ring to "go back" to the original pair. These recesses varied between 1973 and 1976, but were mostly a little over 1mm deep, with the 1974 and later variety being stepped to receive the dowels and the rubber seals on 2 separate surfaces. These later heads are identified by the squared-off edges of the fins, as opposed to the thin, sharper-edged fins of the earlier heads, and they were usually painted with silver paint.

You might find that the K5 cylinders are also about 1mm taller than your K3 cylinders, which was done to lower compression a bit when the U.S. was suffering a severe gas shortage in those years (and premium was all but impossible to find). If so, and you wish to restore the compression, have a machine shop mill off about 0.030" from the cylinder deck to restore the 9.0:1 ratio, if you care...but, don't do this if your cylinders are already the same height, as the pistons and valves will get too friendly.

Leanier · « **Reply #5 on:** December 31, 2010, 07:44:09 AM »

Thanks a bunch for the info!! those measurements would be great, as I really do not want to purchase a new head, as I already have more into this bike engine than I wanted. (this is my 3rd rebuild on this engine in a year) thats what I get for buying a bike with a seized engine I guess. I figured that the head was counter-bored, just wasn't sure. its great to get a definitive answer. Thank you again, and if you could get those measurements I would be very grateful.

HondaMan · « **Reply #6 on:** January 01, 2011, 12:19:05 PM »

Here's some pix: they say each one is worth a thousand words?
I have a K4 and an F2 head open at the moment. The F2 head is representative of the K5 and later types, while this particular K4 head looks like the K0-K4 dowel holes.

BTW: the long dowels go into the outboard holes, the shorter ones in the center-head holes.

Leanier · « **Reply #7 on:** January 02, 2011, 03:31:27 PM »

Thanks!! Awesome diagrams and pictures! makes it very clear. I should be fine counter boring my head to the K3 specs correct? wont have any adverse affects?

Leanier · « **Reply #8 on:** January 02, 2011, 05:51:16 PM »

Actually, are the center of the head holes also counter-bored to these same measurements? these are the ones that I have been wondering about. my cyls have the original dowels and rubber seals in them, but they still stick about 1-2mm above the surface of the cyl deck. making the head not sit perfectly flush. I'll try to get some pictures of this later today.

Leanier · « **Reply #9 on:** January 04, 2011, 03:45:57 PM »

Ok, here are two pictures, one with just those center 8 dowels in, and the head ontop, showing the size of the gap. the other shows the pins, and kinda how much above the cyls they are.

Gap size: (doesnt seem to be more than 1mm or so.)

Leanier · « **Reply #10 on:** January 06, 2011, 08:41:27 AM »

Anybody know if this gap is normal or ok? Otherwise I'll probably just throw the engine together and hope for the best once I get a few parts in... any input is appreciated.

MCRider · « **Reply #11 on:** January 06, 2011, 09:07:42 AM »

Quote from: Leanier on January 06, 2011, 08:41:27 AM

Anybody know if this gap is normal or ok? Otherwise I'll probably just throw the engine together and hope for the best once I get a few parts in... any input is appreciated.

I can't imagine that its normal or acceptable. If I were to guess I'd say the head should lay flat on the cylinder block without the head gasket. Maybe just a little resistance from the rubber Orings, but you should overcome that with weight.

If the head is being held aloft from the cylinder block by something hard, I would suspect the dowels are too long or the holes they set into are imparied with gook.

But I'm just killing time with a guess, if it were mine I'd want a confirm from someone who knows for sure.
Curious.

Leanier · « **Reply #12 on:** January 06, 2011, 09:17:07 AM »

Thanks for at least giving your opinion, I appreciate it, every thing is perfectly cleaned, dowels are all the stock ones, so I dunno. and at the time of the picture I did not have the rubber o ring thingys in, just the dowels.

Leanier · « **Reply #13 on:** January 06, 2011, 11:11:27 AM »

could I perhaps grind the 8 pins down to where the head would sit flush? dont know if that would cause problems..

750 · « **Reply #14 on:** January 06, 2011, 12:09:59 PM »

You have to put your seals in these gaps between the dowel and head. The seals will fit right over the dowels. Check out the schematic on www.cmsnl.com.
Hope this helps

MCRider · « **Reply #15 on:** January 06, 2011, 12:33:58 PM »

Quote from: Leanier on January 06, 2011, 11:11:27 AM

could I perhaps grind the 8 pins down to where the head would sit flush? dont know if that would cause problems..

Rereading HondaMans post, seems the proper fix is to machine the head to accept the dowels. He seems to imply as I have speculated, that without the head gasket and Orings, the head should meet the cylinder with nothing holding it up.

I'm guessing that you could trim down the dowels. The earlier bikes only had 2 outboard dowels and 2 inboard ORings IIRC and all the extra ones are upgrades but not necessary. But, its not my bike. If it were I'd have the head machined to accept the dowels.

HM offered to get the measurements of the needed machining, but hasn't got to it yet it seems.

Leanier · « **Reply #16 on:** January 06, 2011, 12:57:10 PM »

I did see hondamans post. just seemed that those measurements were closer to the measurements needed on the outboard pins, as at most it seems that these would need holes only machined a few mm at most, I was hoping he would reassure me that those are for the middle pins, as looking at his pictures, the holes on the middle of the head don't seem to be machined, but the outboard ones look exactly to what he was saying. Just wanted clarification before I go machining my head haha..

MCRider · « **Reply #17 on:** January 06, 2011, 01:40:10 PM »

I'm probably past my point of knowledge, which stops mostly at K2. But I'd be very uncomfortable with the idea of cranking down the head nuts to close that gap, which the headgasket is not designed to fill. Good luck with an answer.

What's it like if you set up the 75 cyls as tho they were the 73 cyls, as far as dowels and Orings. Will the head sit down on the cylinders then? Without the gasket.

Leanier · « **Reply #18 on:** January 06, 2011, 03:16:50 PM »

Yup, if I take out the dowels, and set it up just like my last head, sits perfectly flush. really seems like they need little 1-2mm machined counter-bores, or something.

MCRider · « **Reply #19 on:** January 06, 2011, 03:51:58 PM »

Quote from: Leanier on January 06, 2011, 03:16:50 PM

Yup, if I take out the dowels, and set it up just like my last head, sits perfectly flush. really seems like they need little 1-2mm machined counter-bores, or something.

Well that answers the question. Now what to do? Make the 73 head like a 75, by machining. Or treat the 75 cylinders like they were 73, without the extra dowels and Orings, if that's doable. (I think it is). As HondaMan said the later engines had the extra dowels and ORings to make the engine more oil tight. But (hundreds of) thousands or earlier engines were sold without the extra dowels and ORings.

Bottom line is, you need some remedy that allows the head to sit on the cylinders, without the gasket, unimpeded. The gasket needs to be free to be squished without anything to stop it from being squished.

MCRider · « **Reply #20 on:** January 06, 2011, 04:01:10 PM »

DANG, I could have had a V8!

I've got a head with the ORings sitting on the shop floor. I bot it from eBay a year or so ago, and was puzzled with all the extra hardware, dowels and ORings it came with. Stripped it and set it aside for later.

I'll compare it to my early head and let you know the differences. Probably tomorrow. Maybe even pictures.

But really, I see now HondaMan has already done that. And the proper answer is to make the head accept the extra dowels.

HondaMan · « **Reply #21 on:** January 06, 2011, 08:28:26 PM »

I just measured a couple of things I had here, to make an educated guess at it: the Honda head gasket(s) I have here (used) are thicker than the Athena, Vesrah, and the unlabelled ones I've recently received with some 836 piston kits. The plot thickens...

Here's what I surmise, as I never had any interference issues when I used only Honda's parts: the new aftermarket gaskets don't "understand" that there were various different engine designs. I have seen this with aftermarket parts for my Fords before, too. The Honda gasket I just measured, fully collapsed after 25k+ miles on its engine, is 1.20mm thick. This means it cleared those dowels, compressing only their rubber collar seals, when the head was torqued down. I don't have that head handy: it is running around town on a cafe' bike I just finished last Fall. If I did have it, I suspect there would be no dowel marks on the head.

The (new) Athena gasket I have is 1.15mm thick at the same place, and the (new) Vesrah is 1.8mm thick. I think the Vesrah would work OK in this scenario.

Here's what I would do, if I were assembling that combination (and I might be, soon, on a build I'm gathering together now):
1. Measure the thickness of the new head gasket.
2. Set the head and cylinders together (as you already have), measure the open distance.
3. The head gasket will not compress more than 25% of its thickness, typically. So, I would shorten the dowels by 10% more than the projected thickness of the assembled head gasket. This should leave the dowels captured in place to support the rubber seals without hitting the head and stopping the closure.

One of the things we must deal with these days are all of these (young) vendors who are jumping into the burgeoning vintage markets without benefit of original knowledge. In particular, I have noticed that in the area of gaskets and seals, there are some uneducated products appearing to make our work a little more challenging (but still completely do-able). The gaskets in the new "red" packaged kits we are seeing from Japan have thicknesses that correspond to modern bikes' parts, which are far more accurately machined than 1960s parts, and which use thinner gaskets. Some seals we are seeing are actually built backward (tach drive seals on the CB750 come to mind), so they need close inspection before installation.

The old gaskets (and Honda's own, today) come with a paraffin-like sealant embedded in the material, and are nearly twice as thick (about 1.7 times, actually) as the afore-mentioned "unnamed" Japanese gaskets I have here. The new ones do not, so I think they will need Hondabond or the like on them at assembly. This is not usually done for head gaskets, though: they should have a sealant on areas around the oil ports already. If they don't, use Felcobond (from Fel-Pro) thinly on those areas to prevent oil leaks around the passages: this stuff is similar to the stuff normally painted on the Honda gaskets.

In this particular area, there were 4 dowel length used on the heads and cylinders over the years. The longest ones were 22mm (I think), while the shortest ones were 12mm, and the "others" are 18mm long. The parts fiche I've been looking at don't have it right, either. The K0-K1 engines had one "set" of dowels, the K2 (after about 11/71 production) through early K4 had another "set", and interspersed with the early K4, through the K6, a third "set" appeared with 3 different lengths. The "F" engines all shared a common 2 types, too, but it seemed they differed because the F0 used 4 (short) and 8 (long) of the different types, while the later ones used just 4 and 4 of each, but added O-rings in other places and little recesses in the head (the F2/F3 engines) to help reduce hot oil leaks.

In the end, almost all of the heads and cylinders (after K1) will interchange with a bit of puzzling through the lengths of the dowels (or lack thereof) and the thicknesses of the O-rings needed (although some combinations may not be the best idea, like mixing "F" and early "K"). Let your calipers guide you: I recently discovered, for instance, that the O-rings Honda now sells for the head/cylinder interface for the K1 engines are not the same thickness as their present head gaskets require: they are too thin at 1.8mm. A standard 2.0mm thickness fits the Honda head gasket thickness, letting the O-ring compress about 20% during the torquing (the former is about 6%). If you had used the thinner Athena gasket, the same 2.0mm O-ring would be too thick: those gaskets will require about 1.5mm of cross-section thickness to prevent over-squishing, splitting after a couple of years and causing a leak.

So, measure the gaps, measure the head gasket you will use, and use the rule of 8% to 18% of "squish" for non-pressurized O-ring seals, and 12% to 22% for pressurized O-rings (under the cam rocker towers, too) and you'll be rewarded with a dry engine for a lot longer.

Leanier · « **Reply #22 on:** January 06, 2011, 09:40:25 PM »

Wow, your knowledge amazes me. Thank you a ton for the information, all seems very concise and clear. I am very glad to have had you helping, as this seems to fully answer my question. I will set out to doing the work on this tomorrow, Thank you again for all your work on this one question of mine, I am very impressed at the great lengths you went through to help this problem out. Seems like I should be able to manage to fix mine, with a bit of figuring.
Sounds great!
Thank you!!!

Leanier · « **Reply #23 on:** January 07, 2011, 08:05:05 AM »

Ok, aftermeasuring my parts. the gap between my head and the cyls is about .023 at the intake side, cant get my calipers anywhere else, going to double check with some feeler gauges if I can find them... and my head gasket it .047" thick. So, if it compresses 25% as you say is approx. the head gasket is still .0353" or so. which leaves plenty of room for the pins. even if the gasket compresses 50%, it will still theoretically have enough room, with the gasket being about .0235" thick. a little close for comfort, but I cant see the gasket compressing 50%. So, if all measurements and everything are correct, I think I should be fine without even grinding the pins down at all. I will double check the measurement of the gap with my feeler gauges, as I could only measure at one point, not near the pins.
*update* - after finding the feeler gauges.. checked the gap around the entire head, after reseating everything and making sure everything is in proper places, the gap turns out to be never more than .020" with the head just resting on the dowels. Seems that everything is in order, and there was never anything to worry about after all. I will start the assembly of the engine shortly, and in a day or two let you all know how it works out.

Leanier · « **Reply #24 on:** January 07, 2011, 04:32:23 PM »

Engine is fully back together, everything seems to be in order. Approximate thickness of the gasket now, fully ready to run, is about .037 or so. so plenty of dowel space. Hondaman, you seem to have been almost exact in your figuring, hopefully get the engine back in the bike and run it later tonight, or tomorrow, depending on how many people I can get to help me haha.

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Author Topic: 1973 CB750 head on 1975 CB750 cylinders (Read 8238 times)