Honda CB750 Sandcast
The Restoration Shop => Engine Mechanical => Topic started by: markb on October 31, 2017, 11:39:58 am
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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.
I’ve 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 K1’s and #12027-300-330 for K2’s. The drawing also shows the grooved exhaust guide (but that probably doesn’t 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 K1’s did not have seals. Then K2’s and after had seals again.
The only reason this matters to me is I’m 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) I’m 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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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 I’m 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?
(http://cb750sandcastonly.com/coppermine/albums/userpics/10002/CIMG1390.JPG)
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If you put a seal on top of the left one, wouldn´t it be about as long as the right one?
/Ken
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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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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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Sorry to revive an old thread, but what was the consensus on this? Did the sandcasts run with an exhaust valve guide seal?
Thanks Steven.
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Hi Steven ;)
Yes, The exhaust valve seals were removed during the Diecast period after engine number E-1014495 >>
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Many thanks, this confirms Marks observations. I read elsewhere the early bikes didn’t use them and I wanted to be sure before buying some.
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The first reference of the exhaust guides is 12027-300-300, and superseded by 12027-300-330 and superseded today by 12241-413-305 (see CMS). We can still find the first references on old stocks with patience...
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Gerard is correct. i ran into this issue a couple years ago rebuilding a K3 engine, the exhaust guides used no seals. Putting the micrometer to the guides's i.d.'s and stems o.d.'s, measured to the near outer limits of serviceable wear. Running the engine on the test stand, 2/4 cylinders smoked. Turned out the head on this K3 engine had been replaced at some point with an early K2 head. I do not know Honda's rationale for departing from using seals on all 8 valves guides to only on the intake, but as it turned out, the longer seal-free exhaust guides were prone to wear. The use of these seal-free guides ended around Evin 20004800.