Camshaft end float and lobe position

[RandalColman]

Browsing the Jowett technical bulletins recently (http://jowett.net/Parts/TechNotes-Part24-ServBul.htm), as I'm sure we all like to do from time to time, I was arrested by the following:

Bulletin Issue Date: September 1950
Item No. 23. Camshaft
To avoid the possibility of the operating cams of the camshaft fouling adjacent tappet assemblies, the width of the operating cams has been reduced from ½" (12.7 mm) to 7/16" (11.11 mm) with effect from Javelin car Engine Number E0 PB 9332.

Now it is true that there is a relatively short offset fore-and-aft between the opposed inlet tappets of cylinders 1 and 2 [actually between no. 1 inlet and no. 2 exhaust], and again between cylinders 3 and 4, so a wide enough cam lobe could conceivably activate both tappets, but measurements on my engine show a clearance of 2–3mm here. Camshaft end float would never exceed 1mm – this engine had quite a distinct knock from the camshaft, yet I measured the end float at less than half a millimetre. I believe that what looked like chipping from accidental contact was actually fatigue damage to the tappet rim as a result of an excessively wide swept area under the cam lobe. The causes and remedies for this would be similar to those for accidental fouling of neighbouring tappets, but the remedy above did perhaps not go far enough.

The cam lobe must always remain within the area of the face of the tappet – if it even gets within perhaps half a millimetre of the edge it is liable to produce fatigue cracking, and there is the risk of lubrication failure at the extreme edge. If the tolerances on the axial position of the cam lobes are too loose, then the swept area of the tappet face may extend beyond its circumference. In another thread I mentioned that the lobes were offset 1.7mm along the camshaft from the tappet axis: this was obtained by comparing the lobe spacing with the tappet spacing. But on measuring more carefully, I find that on my engine the offset varies from 0.8 to 2.4mm. The lobe width also varies, by about +/-0.4mm. Taking the original ½ in. lobe width and applying these tolerances, the edge of the lobe could be up to 9.1mm from the tappet axis in the direction of the camshaft axis. In the lateral direction (i.e. in a horizontal engine, the distance above or below the camshaft centreline) the offset in mm is equal to the lift rate in mm/radian, which I reckon to be about 7.7mm/rad for the hydraulic tappets half-way through their stroke of 4.9mm. Combining these two offsets by Pythagoras' theorem, the furthest part of the contact line is at a radius of 11.9mm – or it would be if it was still on the tappet. The tappet radius is 10.3mm.

My measurements may not be very accurate, but it does seem likely that the cam lobes were running off the edge of the tappets. The reduction in lobe width would certainly help matters, but this was quickly followed by the introduction of solid tappets, and if I interpret the service bulletin correctly (no. 34), the lowering of the cam base circle meant an increase in cam lift from 4.9mm to 5.6mm. So the end of the cam-to-tappet contact patch was moved in by 0.8mm but then moved sideways by up to 1.1mm, so the cams were again running off the edge of the tappet. Add to this the fact that the cam lobes in my car (late 1951) were not 11.1mm wide but over 12mm, and it is clear why the edges of the tappets were crumbling away. If anyone is playing around with high-lift camshafts then it is worth paying particular attention to the axial position of the cam lobes and possibly grinding the side faces of the lobes.

In my car some fragments of the fatigued tappets could be found in the sump, and here I am thankful for the fairly fine mesh of the oil pickup strainer, which protects the oil pump, even if we rely on the oil filter to protect the rest of the engine. On the other hand, the strainer was three-quarters blocked when I finally got round to pulling the sump off.



The car had probably only travelled a thousand miles or so since the previous partial rebuild. However, that rebuild was 40 years ago, so I should really have had a look at the oil pickup before I started using the car. It is also worth alluding to Mike Allfrey's excellent notes on engine rebuilding http://jowett.net/Parts/TechNotes-Part0 ... gOhaul.htm, where he recommends checking the clearance between the pickup and the sump and ensuring that 10mm exists. In my case it was probably only a couple of millimetres, since all cars of this age seem to have had a few whacks in the sump, no matter how generous the ground clearance.

[Forumadmin]

Great post. I think I need a few diagrams to visualise this!

Also I changed your link in the img BBCODE to

Code: Select all

[url=https://jowett.net/forum/download/file.php?id=8692&mode=view]https://jowettnet/forum/download/file.php?id=8692&mode=view[/url]

[Forumadmin]

Diagram from Randal.
The 7mm dimension I think was meant to represent the maximum excursion of the cam on the follower.
But I think the diagram is incorrect as this will occur at the top of the cam when it first contacts the follower and will also be the farthest point when it eventually just leaves the follower.
As the point of the cam wears, less of the follower is swept. When round there is no sweeping except due to the offset between the centre lines and the turning of the follower.

The turning of the follower is induced by the sweeping of the cam being more across one half of the diameter of contact than the other. Thus causing a turning moment of force on the follower due to the friction between the surfaces. This would happen if both the surfaces were flat and parallel.

As can be seen from the diagram the swept area would contact the edges of the follower unless the follower was ground slightly convex . The convex grind leaves a gap at the circumference thus saving the edge from destruction. I think this convex grind will also create a curved motion at the contact patch as the swept arc rides up the convex shape and down the other side. But this would be in conjunction with the turning of the follower and slippage at the contact point, but the motion would be constrained by the camshaft end-float. The convex shape would also accelerate the tappet as the lobe rode up the convex hill.

I have been assuming here that the centre line of the follower does pass through the centre line of the cam. If not more geometry is called for!

[RandalColman]

Thanks for posting the diagram, Keith. It may have been my use of a degree symbol in the text that prevented me from posting it myself. As you suggested, the diagram would have helped a lot with the first post and it might have helped me avoid the error (subsequently corrected) where I claimed the camshaft lobes would need to be displaced axially by 7mm to hit the wrong tappet, rather than just 2mm. (I was looking at just one half of the crankcase.) But the essential point remains, i.e. that an overly wide lobe is likely to produce damage by running off the edge of its intended tappet rather than hitting the wrong one.

Yes, the 7mm in the diagram represents the maximum excursion of the contact line away from the plane of the tappet axes. (And yes, it is safe to assume that the camshaft axis also lies in that plane.) But I firmly believe that the contact line will only move progressively up from the middle of the tappet as it is accelerated, returning to the middle at full lift and then onto the lower half of the tappet as the valve is lowered. We are talking here about the tappets of the left bank, cylinders 1 and 3. The only mistake that I will admit to in the diagram is that I should have shown the cam rotated slightly further clockwise for the maximum rate of lift. (For a cam duration of 180 + 65 = 245 crankshaft degrees, the lifting of the valve takes place over 61 camshaft degrees and the maximum rate of lift would be when the cam lobe is only 30 degrees from horizontal in the diagram above.)

It is true that relieving the rim of the face of the tappet would protect it against damage, for a while at least; I will probably adopt this measure when I come to reassemble my engine. Doming the whole tappet is another matter: this would put the centre of pressure in the middle of the tappet (in the fore-and-aft direction, that is) and might prevent it from rotating. There would still be some viscous drag from the oil that might be sufficient to rotate the tappet, but I wouldn't want to rely on it.

If anyone is interested in reading more on valvetrain mechanisms there are some good articles at https://www.highpowermedia.com/RET-Moni ... valvetrain

[Forumadmin]

And from that link this VERY important warning about having ZDDP in your oil. Also that a different ZDDP additive with a lower operating temperature is needed for running in.

It is easy to forget that not all modern race engines are double overhead camshaft, lightweight alloy wonders. There are still many series, notably in the US, that rely on the use of pushrod-actuated valvetrains and solid mechanical valve lifters, the most high profile of these being the NASCAR Cup championship.

This presents some unique challenges from a lubrication perspective, not least in the area of oil selection. Modern emissions legislation has seen the make-up of engine oils change considerably over recent years, with an ensuing unforeseen impact on the reliability of engines running solid lifter. This month I want to investigate these issues and the solutions devised to counter them.

Thanks to regulation changes by various legislating bodies, particularly the US Environmental Protection Agency, the quantities of ZDDP (zinc dialkyl dithiophosphate) used in most engine oils has been restricted. The intention behind such legislation is to protect emissions-reducing components such as catalytic converters, which can be damaged by the inclusion of such additives. However, the reduction in ZDDP has had an unforeseen impact on racers running so-called ‘flat tappet’ race engines, ranging from NASCAR Cup racers to Californian drag racers running air-cooled VW engines.

ZDDPs are a family of coordination compounds, originally developed by Castrol, that feature zinc bound to the anion of dithiophosphoric acid. These uncharged compounds are not salts; they are soluble in non-polar solvents, and the longer-chain derivatives dissolve easily in the mineral and synthetic oils used as lubricants. It is important to remember that in boundary lubrication, surface asperities make contact with each other even though the lubricant supports much of the load. The level of friction is determined by the shearing forces necessary to cleave these adhering asperities, and wear and friction can be reduced through the use of additives that reduce this contact.

In most modern engines, whether for road or race use, the loss of ZDDP is not a major problem. These engines feature overhead camshaft designs that do not experience the same boundary lubrication issues at the cam lobe-lifter interface as old-fashioned pushrod V8s, which often feature solid flat tappets [Fig. 1]. Compared to engines that use roller or hydraulic lifters, the loadings at the point of contact between a flat tappet and the camshaft lobe are exceptionally high, so there is a greater need for additives in the oil to help protect them. Over the years, ZDDP has proved to be the perfect solution here, and thus its removal from most motor oils caught some racers by surprise. Suddenly they were starting to experience previously unseen problems, particularly excessive wear of cam lobes.

To counter these problems, several oil manufacturers started producing specialist oils for motorsport use, with substantial ZDDP content. However, it is worth noting that not all zinc (ZDDP) additives are the same. ZDDP does not begin to be effective until it is subject to heat and loading and, depending on the exact formulation used, different additive packages react at varying levels of heat and load.

ZDDP also has different ‘burn’ rates. Some zinc additives have slower burn rates that require more heat and load to activate than others. With this in mind, several manufacturers of engine break-in oils make a point of using ZDDP formulations that activate at relatively low heat and load levels, providing the maximum possible protection for components when they are at their most vulnerable.

Another article explains how the critical time is where the cam lobe reverses relative direction as it goes over its high point thus loosing the wedge of oil lubricant. This is where ZDDP is so effective. But it also warns the oil temp needs to be above 60deg C to be effective.

Of all the components in an engine, the cam - flat tappet interface, whether in the form of the direct acting mechanical bucket in overhead cam engines or to a slightly lesser extent mushroom tappets in pushrod engines, is that most at risk. In these circumstances the oil entrainment velocity, the relative velocity of the cam-tappet contact point on the tappet with respect to the velocity of that point on the cam, drops to zero and then reverses at slow speed. If this relative velocity is insufficient to generate a ‘wedge’ of oil between the two, then these surfaces will rely on the presence of the highly polar ZDDP molecules to keep the surfaces apart and protect them.

To carry the story on further this article explains some of the complexities of legislation and oil, summarized in this diagram.

Check out to see if the oil you are using has enough ZDDP. I think 0.12% (1200ppm) is about right, too little- bad, too much- bad. If not then Frost do a bottle for £10

Castrol HD 30 is good for running in.
Castrol GTX 20W-50 is good after running in on old engines.
Redline 10W-40 or 10W-30 synthetic is good after running in on new engines.

[Keith Clements]

Here is another article with an impressive test on assembly grease. OK it is by the manufacturer!

[Keith Clements]

http://wiki.seloc.org/a/Oil_Labelling_Explained

A concise article with some good explanations. Jowetts recommended 30 SAE in UK and 40 SAE in hot countries. Multigrades mean you do not need to change oil between winter and summer which I doubt anybody ever did anyway in Jowett. We are bombarded by much marketing techno blurb from classic car oil suppliers which I am pretty sure is not factual and is simply a way of selling oil made to old (poor) specifications at a higher price.
The fact is that most classics rarely get their engines warm so would probably benefit from a technology such as Castrol Magnatec. Jowetts will certainly benefit from a multigrade so the oil at least gets to where it is meant to be earlier and thus reduce the damage done in the first 20 minutes of a journey. Also there is no reason for any extended life technology which is designed for 30000 km service intervals as all Jowetts will need a few replenishments in that period.
It is probably better to spend your money on frequent oil and filter changes than on expensive synthetic or specialist oil, but if you do put the expensive stuff in then I cannot see any harm in it. Just make sure you change it like it says in the manual every 2500 miles and the filter every 5000 miles!!! Maybe with modern pistons and liners installed in your Jowett you could extend that period, but if your oil is black then change it.
Many classics have so few long (100 plus mile) journeys and run rich that a lot of petrol gets into the oil. Also, since they do not get hot, water condensate weakens the oil. Thus changing the oil every year is probably advisable which seems a bit extravagant when you only did 100 miles! The answer is to use your classic more often on long journeys.

[Forumadmin]

https://www.ryeoil.co.uk/shop/classic-2 ... ine-oil-2/

High Zinc 20-50W

I have just bought 25L from ebay as it is cheaper and 'free' postage.
We will see how it performs.

[Nick Webster]

By a coincidence "Rye" brand oil came up for discussion during an evening meeting of my local group of the TR Register, to which I also belong. The following observations were made: The oil in question appears to have its origins in the agricultural market. The zinc content is 1500ppm which is actually a tad on the high side. Too much zinc can actually be detremental and affect oil performance in parts of the engine other than the camshaft. For this reason randomly chucking additive in is not a good idea. In any case it is not simply zinc and the chemical is different depending on whether the oil is synthetic or mineral. I note that big name companies (who probably have better lab testing facilities) like Castrol peg their classic oil ZDDP at 1300ppm, Mobil 1000ppm while Duckhams describe theirs as "optimised. Rye oil also claims to be low detergent. In the case of the Javelin / Jupiter engine the use of a disposable cartridge oil filter means that without a doubt higher detergent oils are beneficial. It could be that Rye oil offers advantages in running in new camshafts but in my personal opinion there are probably better spec oils for longer duration every day use.

Nick

[Forumadmin]

Too much zinc can actually be detrimental and affect oil performance in parts of the engine other than the camshaft.

Can we be more specific? Like which parts and what level of zinc?

I know that ZDDP affects catalytic converters but there is not one in my Jowett.

I found this ..

Most engine and engine component manufacturers recommend zinc and phosphorus content of more than 1,200 PPM for break-in; in fact, many will void warranties on camshafts or crate engines if this minimum is not found in the oil sample you supply when returning broken parts for warranty.

[Nick Webster]

The higher the ZDDP the greater the carbon build up in the engine (and I assume cat if you had one). There is also the the fact that excess zinc does not give greater protection, just longer lasting, which who knows probably exceeds change intervals. As you state, 1200ppm is adequate for cam run in which leads me to think that just north of this mark is indeed "optimum". It is not my intention to "rubbish" the Rye oil but ZDDP isn't everything. There could be a whole new debate on whether quality goes up the more you pay, but the outstanding performance of Mobil 1 would seem to suggest a link. (Synthetic, I know).

Nick

[Srenner]

Oil is a huge can of worms. What counts are the ACEA or SAE certifications that the oils meet or exceed.

ZDDP content has been falling for several reasons, and one cause is the incompatibility of ZDDP and the catalytic converter. The zinc is bound to and carried by phosphoric acid and is considered a sacrificial layer on parts that slide over each other. Modern cars have roller actions in the places our cars have sliding.

Synthetic on a label is an advertising ploy and bears ZERO relationship to the origins of the base oil. Decades ago, polyalphaolefins (PAO) was the new truly synthetic (as opposed to mineral) base that most oil companies were using when they first started selling oils labeled Synthetic. A lawsuit in 1999 in which Mobil challenged Castrol changed all that and the industry standards dropped ANY definition of synthetic. Today, labeling anything "synthetic oil" is about the same as food labeled "natural".

Certainly there are oils that are PAO based and these are generally considered "better" as the molecules are all essentially identical or uniform. There are five primary groups of base oils and some Synthetics use a percentage of PAO, but there are others that are 100% mineral based. Even Mobil One is claimed to have 50% to 100% mineral based oil stock.

In short, higher priced synthetics may not be what the buyer thinks he is getting and may not offer any better protection that lower priced mineral oils.

For sure, oils are way better than the sludge used back in 1950.