Inlet valve problem

Hope somebody may have a suggestion as to what inlet valve I need to get .....

Months ago with a view to reconditioning my ST70 cylinder head, I bought new valves, valve guides, rockers, springs, etc. I had an engineer machine the valve guides into the head so as to suit the valves.

Today I put the valve assemble into the head and found that the inlet valve is 4mm longer than the original. Consequently the rocker arm is in direct contact with the top of the valve which is no good at all. The old valve measures 64mm and the new one 68mm.

The new inlet valve that I have is part number 14711087000. Having researched the CSM site it appears that this part number is the same throughout the range.

The engine number that the old valve came out of was 119389, which according to the CSM site is a ST70 General Export model. The bike I have is a 1972 K reg.

I need to get a valve that's 64mm to suit my cylinder head. Does anybody what's going wrong ? What valve do I need ?

Thanks......... Steve

That's very strange indeed...
All the OT70 heads had the same valves.

Is the diameter of the valve the same from your old one? (should be 25mm)
What's the engine number prefix? And... are you 100% sure that it is the original OT70 cylinder head?
Maybe it's swapped with a Chinese version, or a OT50 or,... head troughout the years.

Brand: Honda
Type: OEM C70, ST70
Intake Valve: 25mm
Outtake Valve: 22mm
Valve Stem: 5mm
Cam Bearing Left: 29mm
Cam Bearing Right: 29mm
Combustion Camber: 47mm round
Sparkplug Type: 10mm
Intake Port: 24mm
Outtake Port: 20mm

Thanks for the reply 'Waynus' and all the technical specs...........

I believe that I've unravelled the problem. I telephoned 'David Silver Spares' today and told them what had happened and asked them to measure their valves. All their valves on that part number were 64mm long. It seems that they sent me the wrong valve (68mm). They're going to send me the correct one.

This has answered the question as to what was going wrong, and although they sent me the wrong part, it's my fault for not checking it.
I wish I had because not only was the one they sent me 4mm longer in length, but it had a 27mm head. Now as you say in the specs the valve head should be 25mm. When the engineer put the new valve guides in, he seated in the larger valve head. Now when I put the 25mm valve in the seat, it sits lower. The saving grace is that it appears to me that all the seat face on the valve is in contact with cylinder head valve seat.

Thanks once again for your reply. I'll post how things develop with the rebuild. It's just taking such a long time because of work commitments. Hope to get the bike on the road for spring. At least I'm getting full use of my new Honda valve spring compressor tool !!!! ........ Steve.

waltergallons wrote:At least I'm getting full use of my new Honda valve spring compressor tool !!!!

Nice one!
I'm not sure if refitting a smaller diameter valve would be a good idea.
(should see and check the head for myself)
What you could do is take the bigger valve (and adjust it, or look for a 27mm but shorter in length as your current one) and increase the inlet port to 26-27mm as well.
Fit a PE24, VM24 or VM26 and a 52mm cilinder / piston for 6v OT70 head.

Why do I tell you all this? Because this little 'accident' gives you actually a good opportunity to make your engine LOT more powerfull if you can afford to spend the extra cash.
The bigger inlet valve will give you a 88cc engine which will -when set up right- gets you very easily to 60mph.
Just an idea.

Thanks for your help and ideas 'Waynus'........................ Steve