Valve clearance adjustment*, by Nicholas Carrer, March 2019
*the ongoing debate on how to successfully describe feeler gauge use.
Over the years of working as a motorcycle mechanic (and subsequently as a teacher/instructor at a technical college), I have come across the difficulty to explain how to accurately measure clearances using a feeler gauge. As the name implies, the whole point is to “feel” said clearance… And this is where the whole matter stumbles into difficulties:
It is extremely hard to pass-on any live experience, especially one that involves the totally subjective matter of “finger grasp and feel”! In a desperate and mostly optimistic effort to compare this feeling to an already known or easily duplicated one, some refer to the sensation as similar to a feeler blade stuck between the bottom pages of a large phone book and slowly being dragged out… Close, but no cigar! How big a phonebook? how many pages from the bottom to insert the blade? Also, what sort of a blade? A stiff, 0,40 mm blade is easier to manipulate than a flimsy 0,05 one, so what gives? The issue still remains subjective and foggy. Others claim it should be like cutting warm butter with a knife (or cold butter with a warm knife), again giving rise to questions like, what sort of butter? Exactly how warm is “warm”? etc.
A feeler blade measures properly only if it is inserted parallel to the surfaces that form the gap and not at an angle. Pulling or pushing the blade at an angle creates extra friction (=resistance to movement) that masks the pure clearance feel. If tip #1 (below) doesn’t help, you must rely on (high) experience or a pair of tweezers to grab and move the blade as parallel as possible to the surfaces forming the gap. The goal here is not to simply (and brutally) pass a blade through the gap (given enough force, almost any blade will pass through, overwhelming the spring). Instead, it should be slowly and gently (but firmly) pushed-in, with a slight “biting” feel as it is moved (again, slowly!) back and forth inside the gap. I’m afraid that sums up any reference to the matter I can offer. Best practical way is to have the apprentice touch your hands as you measure (to better understand “slowly”/“gently”/“firmly”) and have him check the clearance afterwards, using one-step thicker and thinner blades as well, to better understand the “wrong” feeling (too tight and too loose, respectively).
The following refer mainly to threaded adjuster (screw) type systems:
- In case of limited access to the valve, as per OHC Honda’s C/SS/CD family and early in-line fours, where it has to form a tortuous “S” bent to reach the valve tip, consider gently bending the (working) tip of the feeler blade (the last 10 mm from edge end), to a 30°- 90° angle depending on the case. It can be done (in two or more steps) as long as you are careful not to break it (Photo 1). But if you do break it, it can still be serviceable though, after you lightly grind the broken edge, then measure it using a micrometre. By the way, measuring feeler blades can be extremely beneficial, as I have discovered myself, desperately trying to record clearances on a Ducati 851 (another notorious example of almost inaccessible valves) using a Chinese set that was all over the place.
Photo 1. A homemade bent-blade feeler gauge essential to set difficult to reach gaps. Note that, on top of the custom angle, it is actually formed by two blades glued together along the “handle” portion (not the measuring tip) to strengthen it. Unless desperate, never use two blades to measure a gap (i.e., do not sandwich a 0,08 mm and a 0,10 mm blade to measure a 0,18 mm gap). Inaccuracy is guaranteed.
- If present, take up any play between the holder (tool) and adjuster against the direction of rotation of the locknut, to ensure that it absolutely stays put where intended. Even if the adjuster doesn’t move at all, tightening its locknut, slightly “pulls” up the adjusting screw (clearance increases); it is best to set the gap on the tight side before you nip it (feeler blade still inserted) and check it again afterwards.
- Do not lean-on said locknut; it is easily deformed or stripped, damaging the adjuster’s threads as well, making further attempts to properly set clearance, a nerve-wracking experience! I used to cut to about 70 mm in total length, Z-type ring spanners for the job (before I discovered that good quality short ones are readily available), to avoid accidental over-tightening.
- It is easier to increase the gap (=unscrew the adjuster) than to decrease it, take my word for it! Act accordingly!
- A case of 0,05 mm specified clearance: “if you can feel side, but not a vertical play of the rocker arm, then you are done!” Tip for OHC – not OHV engines, given by a Japanese mechanic some 40 years ago and still exploitable! Do be careful on this though: it is very important that there is a gap after all, so I would suggest experimenting having the appropriate feeler blade handy and check it still. As you repeat this procedure time and again, you will master the exact feel, saving time and keeping yourself serene!
As far as Honda’s OHC 50-110 cm3 series is concerned (C, SS, ST, Z, etc.), take into account that in some official Honda service manuals (see Honda ST50 Dax, recently uploaded, among few others), specified clearance is given at 0,05 ± 0,02 mm not strictly 0,05 mm as stated in most. These two-hundredths of allowance give a certain amount of freedom, although a 0,05 mm intake - 0,07 mm exhaust clearance combination is widely (and successfully) used.
- Never use a deformed or rusted blade. Feeler gauges are precision instruments and should be treated as such! Have them cleaned and oiled before storing. Consider that acids present in used oil and body fluids from your fingers will attack steel, leaving visible and tangible marks on their surface, thus rendering them useless. By the way, a deformed feeler blade (one that looks as it has received the scriber treatment) is absolute proof that the user has no idea of the correct use of the instrument! An exception to this (rule), are many vintage, mainly automotive, engines that had to have their valves adjusted with the engine hot and running (most Opel GMs from the early 60s almost up to the 90s and some early Toyotas, among others). Mechanics had to be extremely familiar to the “feel” of when the valve is closed (a very short time indeed, but some have developed the ability to “tune” their hand movement to the engine’s running cycle at a remarkable level, using up as many feeler gauges as shop rags in the process).
- In case you get a “scratching” sensation as you drag the blade between adjuster and valve tip, remove the adjuster very carefully (not feasible with swivel-type “elephant foot” - see photo 2, below - or ball-ended adjusters, whose end diameter is far greater than the threaded portion), as to not drop it into the engine (have a small, strong magnet handy), and examine both its working end and the valve stem tip for wear, pitting or other irregularities. And this, dear chaps, explains why it is imperative for your feeler blade’s surface to be immaculate, to not mistake the feel of a damaged or misused one, for valve component wear.
Photo 2. “Elephant foot” type adjusters. Large valve tip contact area in conjunction with a swivelling end (arrowed), dramatically lessen local wear as well as valve stem to valve guide side loads (courtesy KTM). However, this set-up is a bit more difficult to set than conventional, rigid adjusters. Note width of exhaust rocker roller (left), almost twice as that of intake side, to compensate for extra loading since the cam lobe is acting “outside” the valve tips and not between them, as on the inlet (although heavily biased towards the top in the photo, loads are better accommodated nonetheless.)
- If you discover a “dugout” valve stem tip, one that exhibits crater-like wear, a feeler gauge is useless to measure actual clearance. The only dependable solution is to use a dial gauge fitted to measure rocker arm (or adjuster) play if space allows. Or, you can rely on the “feel” of the gap (experienced hands only).
- Last and certainly not least, it is better to hear some (little) valve clearance noise than none at all, a rather long topic worth debating on its own.
Suzuki 4-valve TSCC (Twin Swirl Combustion Chamber), early to mid-‘80s: As a result of using comparatively soft valve springs (a benefit of a light valvetrain), one can easily be misled into thinking he has done them “nice and tight” whereas, in reality, has them so tight (the feeler is actually pushing open the valve, against spring force) that some (or all) valves do not actually close. A reminder of why a feeler blade must never be forced into the gap to be measured!
Forked (paired) rockers (most with threaded adjusters): To name a few (!), many ‘80s 4-valve Hondas (VF400 –1100, VT250, XL/XR250/400/500 - non RFVC, CL/CB250RS, CB/CM250/400N [3-valve heads, intake side only], CBX400/550, pushrod-operated CX/GL500/650 as well as XL250/350 Motosport ’74 –’78), all Suzuki TSCCs as above, DR/SP/GN250, DR350 – 800, SG250/350 Goose, 1st generation GSX-Rs (air/oil cooled engines) plus water-cooled 400, VS/VL/VZ400 –800, VX800, LS650 Savage, XF650 Freewind, Kawasaki KLR250, EN450, GPz/KLE/EN500, ER-5, GPz600/900 and all variants including 1000RX, Yamaha T135, many BMW 4-valve boxers, 4-valve Moto Guzzis, KTM original LC-4s (400 – 660, up to ‘07) and early Racing series (400-525), Husabergs (up to ’08) and Husqvarnas 570/610 ’99 – ’07 (these are a real pain to do, as access is very limited and specified clearance is 0,05 mm, a combination that can lead to disaster as per broken feeler gauge blade inside the head - bad, very bad news!)
As each valve pair is operated by a single camshaft lobe, the rocker arm should be acting on both simultaneously, otherwise side loads will exert immense pressure on rocker arm sliding surface (or roller, if present), rocker arm shaft and cam lode. In case heat treatment of parts involved and/or lubrication is insufficient, wear will occur shortly, audible as a loud ticking sound from the top-end, similar to that of a way too loose valve gap (which is exactly the case, as related components wear badly and quickly). It is essential thus, to set both valves to the same clearance by using two identical feeler blades at the same time (see below, photo 3).
Note that the method described in Tips, #5 above, “Case of 0,05 mm specified clearance” should never be applied to forked rocker arm configurations.
Photo 3. A service bulletin from Honda, regarding first generation 750 V-fours, a monumental U.S. market failure!
Valve clearance misconception: In most OHC (or DOHC) units, valve clearance increases (or, stays close to cold specs, and certainly does not decrease) as the engine gets hotter. Valve clearance is not given to compensate for heat expansion but rather to ease valve train life, as hinted in tip #9, above. Again, a quite complex, as controversial issue that can be looked into later on.