Inlet/Exhaust Valves!?

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SarahMac
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Inlet/Exhaust Valves!?

Post by SarahMac » Sat Nov 17, 2018 3:07 pm

I'm trying to check the valve clearances on my '93 NC31 CB400.
However, getting a bit bogged down in identifying which are the inlet/exhaust valves.

Photo below is of the left hand side of the engine with the cylinder head cover removed, so carbs go in the RHS of the photo and exhausts come out LHS.
So my logic says LHS valves (as you look at pic) should all be exhausts and RHS ones should be inlets, but as you can see this doesn't match the machined labeling!
My very patient but stubborn 'repair supervisor' (read 'boyfriend'!) insists that I should believe the labeling on the engine, but when cranking the engine round by hand it appears that both the valves marked 'EXL' in this image are open at the same time. So... they can't possibly both be exhausts... right!?

Anyone come across this before?
Any advice much appreciated!

Image

Superjack44
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Re: Inlet/Exhaust Valves!?

Post by Superjack44 » Sat Nov 17, 2018 8:16 pm

As you describe the lefthand side of the engine the exhaust valves are on the left hand side there are two exhaust valves and there are two inlet valves on the right.
The engine has 16 valves 8 exhaust and 8 inlet, it has 2 camshafts 1 for the exhausts and 1 for the inlets.
Before you dismantle anything measure the gaps between the cam lobe and the cam follower with a feeler gauge write down the clearance for each valve ie Cylinder 1 (Which is the cylinder in your photo) Ex No 1 valve 020mm No2 valve 018mm the clearances should be ex 20mm/022mm so valve No2 should have the shim under the cam replaced by a shim that is 002mm smaller(Thinner).

Exhaust valves clearance should be 019mm/022mm
Inlet valve clearance should be 012mm/015mm

Check all exhaust clearances and write down the clearance for each one for all 8 valves .
The same procedure for all 8 inlet valves.
If any clearance is less than the above then they should be adjusted with new shims.

A word of warning from your post I assume you have no working knowledge of this engine or how its put together.
If you are intent on doing this work yourself be aware that the camshafts are held in place by 10mm bolts these must be released gradually ie half a turn at a time across the length of the camshaft (from memory 8 bolts) per shaft.
Tap the holder gently in order to gradually free the cam shaft watch for it lifting away from the head take your time.
If you try and just release the bolts all at once there is a danger that the pressure from the valve springs will try and lift the camshaft from one end and crack the camshaft holder (eek major headache)
Before releasing the camshafts trap the camchain to the sprockets with a small zip tie so that you do not lose the valve timing.
If you have never done this sort of work before it may be a good idea to get a mechanic to do it for you.
Keep us informed of your progress , might be a good idea if you mention where you are in the country just in case there is someone nearby to give you a hand.

SarahMac
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Re: Inlet/Exhaust Valves!?

Post by SarahMac » Sun Nov 18, 2018 6:32 pm

Thanks very much for the comprehensive response, really helpful and definitely appreciate the tips you give!

I've measured all the valve clearances and today managed to remove the cam shafts and get to the shims, so will be doing some part ordering tonight.

Wondering where you got those values for the correct valve clearances from?
The manual I have says IN 0.12-0.18 and EX 0.17-0.23, based on these I have two exhaust valves out (0.23 & 0.25). Plus one borderline inlet (0.17) and 3 borderline exhausts (0.22).
But if I go with the values you give below 10 of the 16 clearances are at the top end or above! (eek! :shock: )

You're right, I haven't done anything this involved on this bike before ( only on my previous 125, which was a little more straight forward ;) )
But hey, there's only one way to learn! :grin:
I'm in the south west, Bristol area.

Superjack44
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Re: Inlet/Exhaust Valves!?

Post by Superjack44 » Sun Nov 18, 2018 9:55 pm

There is a 003mm -/+ tolerance on the gaps.
If you do a search on here there is a post covering the gaps but I am blowed if I can find it.
Remember when you come to replace the camshaft holders you must pull it down equally and smoothly.
Did you notice the timing marks on the camshaft cogs.
Remember the cogs must be replaced on the camshaft in the place they were removed from otherwise the valve timing will be out.

SevenThreeSeven
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Re: Inlet/Exhaust Valves!?

Post by SevenThreeSeven » Thu Nov 22, 2018 12:21 am

In the picture you posted (in the first post of this thread) isn't it obvious to you that on each camshaft holder there is a designation, either the "INL" or the "EXL", that has been "struck-through" with two lines of little indentations? In Honda-speak this means you should completely ignore the "struck-through" designation.

It sounds like you're misinterpreting the markings on the camshaft holders. But Yes, you've correctly identified the valves in your picture: exhaust valves on the left in the pic and intake valves on the right in the pic. You will see that this corresponds perfectly to the designation on the camshaft holders when you realize the following:
- every valve located underneath a camshaft holder marked "EXL" with a struck-through "INL" is an Exhaust Valve
- every valve located underneath a camshaft holder marked "INL" with a struck-through "EXL" is an Intake Valve

The camshaft holders are cast in a way that allows them to be used on either the Intake Camshaft or the Exhaust Camshaft and on either the engine's left side or right side of either camshaft. So the engine prep technician who is tasked with setting up the engine for boring of the camshaft bearings simply grabs a set of four camshaft holders from the stockpile and attaches them to the engine. He then he marks the camshaft holders by striking out the appropriate "INL" or "EXL" designations and he adds the engine's unit identifier (U46 in the case of your engine) and he adds the "left" or "right" character to each of the camshaft holders (just after the U46 designation).

Then the engine is passed along to the technician who does the actual boring of the camshaft bearings. The markings prevent confusion later when the camshaft holders are removed by another technician who is tasked with installing the camshafts and associated items.

If you accidentally mix up the camshaft holders in a different order/location than they were originally bored during pre-assembly you might have problems.

But if you read the camshaft holders correctly there's no chance of re-installing them in the wrong location. The holders with the "struck-through" "INL" are Exhaust side holders, and the holders with "struck-through" "EXL" are Intake side holders. This takes care of the Exhaust vs. Intake question. Then there's the Left-side-of-engine vs. Right-side-of-engine question which is answered by the letter that follows the "U46" engine unit designation.

BacheMAB
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Inlet/Exhaust Valves

Post by BacheMAB » Tue May 28, 2019 2:40 pm

Its the motor Im building for my swap. Basically doing a stock rebuild with an upgraded cam. Im trying to do as much of the work myself as I can. The intake valves cleaned up nice, I could probably just lap them in, but I dont know enough to know if that would work with the pitted exhaust valves.


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