Author Topic: Engine rebuild question for the Pros (Read 3736 times)

hapakev · « **on:** February 24, 2010, 11:38:33 PM »

I am rebuilding a 1973 CB750K3 engine. I purchased a gasket kit from cycle x for the rebuild. I thought I was missing the steel dowel/knock pins, between the head and cylinder, from my motor. I checked the parts fiche over and over. The 12mm dowels dont fit in the cylinder, also the hole does not taper down to keep the dowel in place. I checked another fiche from the manuals FAQ section, that covers the K3-K5 motors, and found that there are no dowels for this motor? According to the "Reqd. No." under K3. So my question is, "What goes there?"

Please help. I am at a loss. do I just install the head with head gasket or do I need to install something there?

wrenchmuch · « **Reply #1 on:** February 25, 2010, 08:00:09 AM »

I am familiar with both K1 and K4 motors and they both use dowels between the cylinder and head ( 12mm x 22 mm ) ,and between the cylinder and the case ( 12 mm x 15 mm ) . This is a link to bike bandit for a K3 head you can see the part description at the bottom of the part list . You will see numbers in the view which don't have a corresponding number in the list . That is because they use the same view for a years and just omit the part from the list if it does not pertain to the year you are viewing . The dowels are absolutely required for correct alignment of the head , cylinder ,and case . Don't omit them .
http://www.bikebandit.com/houseofmotorcycles/1973-honda-motorcycle-cb750k3/o/m9234
I looked at 78' 750's and they still use dowels .
Hope this helps .
Mike

wrenchmuch · « **Reply #2 on:** February 25, 2010, 08:01:39 AM »

I should add that I ain't no pro

hapakev · « **Reply #3 on:** February 25, 2010, 08:15:33 AM »

While i did see the same thing on a few different sites, I measured the holes and found that the "dowel" holes are 11mm, so the 12mm dowels listed on the parts fiche dont fit. Also the holes are not tapered to keep the dowels from falling into the crankcase. I found a parts list for the K3, K4 and K5 that stated they are not required for this year motor. It made sense because the dowels dont fit. The locating dowels on the front two studs are there and fit.

mystic_1 · « **Reply #4 on:** February 25, 2010, 09:19:50 AM »

What parts fiches claim that you don't need those knock pins? www.cmsnl.com says you need 94301-12150, PIN,KNOCK 12X15 for your K3, just like every other 750 motor I've taken apart.

Item #12:

mystic_1

Bodi · « **Reply #5 on:** February 25, 2010, 09:54:20 AM »

I think he's talking about the barrels-crankcase pins, if there's a worry about dowels dropping into the crank.
You must have the wrong holes in mind. The pins are really tubes, and go around a stud on both barrels-crank and barrels-head. There's no possible way for these to drop into the crankcase. The upper ones are #12 (12mm x 22mm) on the drawing, bottom are #11 (12mm x 15mm).

(mostly amateur)

markb · « **Reply #6 on:** February 25, 2010, 11:09:23 AM »

Quote from: Bodi on February 25, 2010, 09:54:20 AM

The upper ones are #12 (12mm x 22mm) on the drawing, bottom are #11 (12mm x 15mm).

For what its worth, those 22mm long pins are not available. And if they were they wouldn't fit. If you measure the depths of the holes and the thickness of the head gasket its right at 20mm (in otherwords the 20mm is too tight). The ones that came out of my last two motors were 18mm long and thats what I replaced them with.

mystic_1 · « **Reply #7 on:** February 25, 2010, 11:11:55 AM »

Quote from: Bodi on February 25, 2010, 09:54:20 AM

I think he's talking about the barrels-crankcase pins, if there's a worry about dowels dropping into the crank.

But, he said "between the head and cylinder" in the original post:

Quote from: hapakev on February 24, 2010, 11:38:33 PM

I thought I was missing the steel dowel/knock pins, between the head and cylinder

mystic_1

Bodi · « **Reply #8 on:** February 25, 2010, 11:21:08 AM »

Bloody confusing then.
I wonder what dowels/holes he's looking at? Not much is going to fall into the crankcase from up top through a stud hole, anyhow.

wrenchmuch · « **Reply #9 on:** February 25, 2010, 02:26:25 PM »

Are you talking about the pins labeled 3 in Mystic's post above ?

Quote from: mystic_1 on February 25, 2010, 09:19:50 AM

What parts fiches claim that you don't need those knock pins? www.cmsnl.com says you need 94301-12150, PIN,KNOCK 12X15 for your K3, just like every other 750 motor I've taken apart.

Item #12:

mystic_1

If so that is a head change that came in sometime in the k4 I think .The dowels are used with rubber cylinders to seal around the studs .Most gasket kits include the rubber cylinders . A k3 won't use these . I have 2 k4 motors . One used the seals and one doesn't . I don't know if the dowels where the rubbers go come out easily . I haven't tried . The dowels labeled 11 and 12 are needed in all these engines . They are hollow cylinders with chamfered ends . They are not tapered . They fit around the two outer front studs that hold the head and cylinder to the cases . The holes they fit into are not tapered .
Mike
Rank amateur

markb · « **Reply #10 on:** February 25, 2010, 02:44:12 PM »

Quote from: wrenchmuch on February 25, 2010, 02:26:25 PM

Rank amateur

Yes, but a good professional answer!

hapakev · « **Reply #11 on:** February 25, 2010, 03:35:09 PM »

All the other parts fiche I have looked at call for 12mm dowels, but they don't fit in either the head or the cylinder. The parts list is copied from a published parts list. Its in manuals FAQ, under one of steve's posts. Let me find the link.

hapakev · « **Reply #12 on:** February 25, 2010, 03:50:45 PM »

Ok, I can't get on a computer right now (on my blackberry) to find the link, but wrenchmuch has confirmed my situation. The dowels at the two front outer head studs are there. The "knock pins" between the head and cylinder are not there. The spare ones I have don't fit. And the K3 parts list does not "list" them. Do I need anything there besides the head gasket?

mystic_1 · « **Reply #13 on:** February 25, 2010, 04:27:50 PM »

Again, the parts labeled 11 and 12 in the posted diagram ARE "knock pins". Sounds like you have those.

Are you talking about the parts labeled 3 in the posted diagram? If so, see the following service bulletin:

http://data.sohc4.net/SB750/750_42.pdf

The additional knock pins were introduced starting at engine number 2352923. Earlier motors did not use them.

Compare with the following K2 diagram:

With both of the posted diagrams, you can click them to go to the parts fiches at www.cmsnl.com.

mystic_1

hapakev · « **Reply #14 on:** February 25, 2010, 05:55:28 PM »

OK Mystic-1 that is what I'm talking about. A diagram that shows what I have. I couldnt see the diagrams before because I was on my blackberry (work phone). That makes me feel al whole lot better. Thanks man

mystic_1 · « **Reply #15 on:** February 25, 2010, 07:55:48 PM »

Cool, sounds like you have an earlier K3 motor, or a K3 bike with a K2 motor.

What's your motor's serial number?

mystic_1

hapakev · « **Reply #16 on:** February 25, 2010, 08:01:15 PM »

2236689- definitely before the cutoff. I picked up the motor by itself. Thanks for the help.

bikerbart · « **Reply #17 on:** February 26, 2010, 12:22:55 PM »

In taking your motor apart did you lose the original ones?

hapakev · « **Reply #18 on:** February 26, 2010, 05:35:18 PM »

The o-rings? They were missing.

wick · « **Reply #19 on:** January 12, 2011, 02:31:01 PM »

I need those- K5 dowels also- wheres good place- to get-- are they` all OEM?
thnx-

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Author Topic: Engine rebuild question for the Pros (Read 3736 times)