Honda CB750 Sandcast Forum
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Author Topic: Early CB750 Exhaust Valve Guide Confusion Seals or No Seals?  (Read 248 times)
markb
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« on: October 31, 2017, 11:39:58 AM »

I had a discussion with an old mechanic recently and have since read several posts that all said the early CB750 engines did not have seals on the exhaust valve guides.

Ive done some research and found that the 69.4 (sandcast) parts manual calls out 8 valve guide seals and the drawings show the groove on the exhaust valve guide for the seal (part #12027-300-300).

Also the CB750 ~ K2 parts manual calls out 8 valve guide seals up to number 14955, then 4 seals from 14956-2000000 (up to K1) and then 8 again for K2. It shows exhaust valve guide #12027-300-310 up to K1s and #12027-300-330 for K2s. The drawing also shows the grooved exhaust guide (but that probably doesnt mean much).

Of the six engines I have worked on in the last few years, numbers 100, 748, 1490, 1789, 5491 and 12029, all of them had the grooved (seal) type exhaust valve guides. It is possible but I think highly unlikely that all five of the sandcast engines had the exhaust guides changed out from seal less to seal type guides. The early diecast I can say with 100% certainty that it was born with the seal-type guides. I bought it from the original owner who I knew in1975 (and still have) and the engine had never been touched.

My conclusion from all this is that the engines had exhaust valve guide seals in sandcast and early diecast engines up to 14955. Then engines up to K1s did not have seals. Then K2s and after had seals again.

The only reason this matters to me is Im working on the heads for a couple of these engines and there is a good chance I might have to replace some valve guides and as usual (for me) Im going for correct. To add to the confusion I have found pictures on eBay (and others) for all three part numbers listed above and the -310 number is the only one that had no seal (which coincides with above) but the other two numbers had pictures with and without seal grooves. I know that sometimes the picture shown is not correct and in fact I did buy some -310 guides from two different sources and got two different parts.

So my questions are:

Is my conclusion correct?

Has anyone seen a sandcast engine with seal-less exhaust valve guides?

What is the correct part number for the seal-type exhaust valve guide?

What is the difference between the inlet and exhaust valve guides that have the seal grooves (length, diameter)?
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Mark B
1969 CB750 sandcast #97 current project Restoration thread link
1969 CB750 sandcaxt #576 next project
1969 CB750 sandcast #1553 next project after that
1969 CB750 sandcast #1990 next project after that
1969 CB750 sandcast #5383 Sold Restoration thread link
1970 CB750 K0 restored
2010 H-D Tri Glide Ultra Classic (Huh?)
markb
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« Reply #1 on: October 31, 2017, 01:42:23 PM »

Here are pics of the two types of exhaust valve guides. The one on the left is the one that uses the seal. Now that I have looked at them side by side Im curious as to why the one that uses the seal is shorter. I checked the length and it (the shorter seal one) is the same length as what is on my heads. Why would the no seal one be longer or is it an incorrect part?

« Last Edit: October 31, 2017, 01:44:27 PM by markb » Logged

Mark B
1969 CB750 sandcast #97 current project Restoration thread link
1969 CB750 sandcaxt #576 next project
1969 CB750 sandcast #1553 next project after that
1969 CB750 sandcast #1990 next project after that
1969 CB750 sandcast #5383 Sold Restoration thread link
1970 CB750 K0 restored
2010 H-D Tri Glide Ultra Classic (Huh?)
kenhan
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« Reply #2 on: November 01, 2017, 03:21:34 PM »

If you put a seal on top of the left one, wouldnt it be about as long as the right one?

/Ken
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1969 CB750 Sandcast #5128 - Running
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Honda CB750 Sandcast Forum
« Reply #2 on: November 01, 2017, 03:21:34 PM »

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markb
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« Reply #3 on: November 01, 2017, 04:47:32 PM »

Hmmm. I see what you mean. But starting from the retaining ring groove the right one is about 4mm longer on each end, in other words about 8mm longer total. I guess that would give you more bearing support on the valve stem.
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Mark B
1969 CB750 sandcast #97 current project Restoration thread link
1969 CB750 sandcaxt #576 next project
1969 CB750 sandcast #1553 next project after that
1969 CB750 sandcast #1990 next project after that
1969 CB750 sandcast #5383 Sold Restoration thread link
1970 CB750 K0 restored
2010 H-D Tri Glide Ultra Classic (Huh?)
kenhan
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« Reply #4 on: November 02, 2017, 01:05:37 PM »

Yes you are right about a longer guide will better support the valve stem! I have not checked but how deep is the valve (and valve spring retainer) going down to the guide in full motion? It must be a minimum play between the valve end and the valve guide. To be able to put a seal on top you maybe had to have a shorter guide to match that. Just a thought!

/Ken
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1969 CB750 Sandcast #5084 - running
1969 CB750 Sandcast #5128 - Running
1970 CB750 next project
1975 K2 (K3-5 dont exist in Sweden) - Running
1981 Goldwing 1100 Interstate
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