Hadn’t they already had about three goes at getting the head gaskets right?
railstaff:
Carryfast:
While I was only referring to the example of the opposite extremes of fixed head OHC v pushrods and seperate heads regarding maintenance.While single head or not major engine work is going to take more time and aggro in the case of single head and OHC/CIH than even single head and pushrods.While going for a fixed head OHC design effectively makes the engine uneconomic to overhaul which seems to have been confirmed in the case of the 500.Which notwithstanding the issues of OHC/CIH v pushrod design is probably why no other manufacturer has gone for the fixed head option.Its a known fact a one piece head stiffens the block face,this reduces flexing and liner fretting,thats why nearly all are now single piece.As for maintenance every OHC engine ive been involved with(D20,D13,Cursor,ISX)the camshaft has been left in to remove the head.Valve clearance set up is easier because the inner base is viewable on the camshaft.In certain cases less head bolts,and to top it all off ease of removing the camshaft unlike an OHV set up.In essence the 500 had a good spec but the technology to produce the block wasn’t available.
Blimey I’ve never known an OHC motor that didn’t mean removing the camshaft/s to access the head bolts and remove the head nor removing the cam drive from the cam shaft let alone disturbing the cam drive train and timing in one way or another which seems to hold true in the case of the ISX at least.While removing/replacing the head is also obviously going to take a bleedin great big crane with all the awkward fine location issues that creates when replacing the head.As opposed to just removing/replacing the rockers and the three seperate heads in the case of the N14.On that note I’m not aware of any issues caused by the seperate heads in that case nor others. 
Let alone Leyland’s engineers going one better than the ISX by saying let’s make it OHC and fixed head that’ll work. 
youtube.com/watch?v=3QODnuenNbw
youtube.com/watch?v=_dFg3-YnTh4
youtube.com/watch?v=Z0wcj9fE6pQ 1.49 - 2.03 3.01 - 3.15
Head gasket failure plagued most of the truck related engines back then, BMC/Ford/AEC/Leyland and others all had persistant problems and replacing head gaskets was a routine job and often associated with liner problems as well. Leyland obviously designed the 500 to eliminate that problem forever, the OHC design was brilliant and usually very reliable in most engines but alas other unforseen issues came to light in service. Maintenance/overhaul wouldn’t have been that difficult I imagine, especially with a tilt cab. Even Gardners up to the LXB required the cylinder block to be removed to replace a piston so most workshops could have handled block removal easily enough and a service exchange one dropped on in its place to save downtime?
Pete.
windrush:
Head gasket failure plagued most of the truck related engines back then, BMC/Ford/AEC/Leyland and others all had persistant problems and replacing head gaskets was a routine job and often associated with liner problems as well. Leyland obviously designed the 500 to eliminate that problem forever, the OHC design was brilliant and usually very reliable in most engines but alas other unforseen issues came to light in service. Maintenance/overhaul wouldn’t have been that difficult I imagine, especially with a tilt cab. Even Gardners up to the LXB required the cylinder block to be removed to replace a piston so most workshops could have handled block removal easily enough and a service exchange one dropped on in its place to save downtime?
It’s obvious that firstly to make best use of any potential higher cylinder pressures and reduce other stresses they needed a larger capacity.Which then created the problem of the extra height combined with the OHC design.Which leaves the question at what point does higher cylinder pressures no longer taking out head gaskets turn into destroying other more important and more expensive components instead.Together with the unarguable point that OHC let alone fixed head design just adds to the resulting problems regarding access for maintenance and resulting expense.IE you’ve just then ended up with a more costly liability than an ISX compared to an N14.To the point where it’s realistically just an expensive un repairable disposable grenade.Reducing the capacity to 8 litres thereby turning it into a high revving screamer obviously just making that situation worse.
The head gasket issues were mainly down to lack of liner protrusion and poor cooling.I have to question the engineering process and explain why.
The 680 was dry lined and the parent bore was left with a counter bore to keep the liners at the correct height.The TL11 which was basically a turbocharged 680 that had the counterbore removed and instead relied on an interference fit dry liner.In service number 6 liner would drop and destroy the fire ring on the gasket.Some engineers addressed this problem as being to the turbocharger located on the rear of the rear head and the added weight.
The options were open.Go the Volvo route of the TD 101 and 121 and make the liner locate in the head,or the Scania route of the gasket locate in the head,or the ■■■■■■■ route and drop the compression.I found it a bit odd.
Once the engine capacity exceeds about 5 litres all multi-pot cylinder heads are not easy to remove by hand unaided. Even the two-pot heads of the 855 ■■■■■■■ really required a pair of Tee handles to make life easier and they were unencumbered by studs and carbon. I certainly find that the Leyland 600/680 requires mechanical assistance to remove the heads. The ■■■■■■■ L10/M11 engines use a single head which is definitely too heavy for a one man lift.
Leyland had managed with their very first diesel engine to design an OHC cylinder head which did not require camshaft removal for access to the hold down nuts, nor for anything more involved than ensuring that the camshaft and engine timing marks were in line prior to removal. It however being one piece did require a second pair of hands. It incorporated, as did some AEC models, jacking nuts to free the bond between head and block.
The rear timing train was not something new, both Dennis with the 06 and Daimler with their CVD6 and CD650 engines had successful production engines of this format. Daimler of course being part of the BL group by the time of the 500 as were Bristol, another engine manufacturer of some ingenuity.
Typical of Leyland the 500 engine went through updates as the 510 and the 511 eventually becoming what it should have been on first release, only to be withdrawn from production. In many ways a shame because the last incarnation was a much better engine, IIRC a 511 powered Buffalo knocked Gardner off its efficiency perch in CM road tests.
railstaff:
The head gasket issues were mainly down to lack of liner protrusion and poor cooling.I have to question the engineering process and explain why.
The 680 was dry lined and the parent bore was left with a counter bore to keep the liners at the correct height.The TL11 which was basically a turbocharged 680 that had the counterbore removed and instead relied on an interference fit dry liner.In service number 6 liner would drop and destroy the fire ring on the gasket.Some engineers addressed this problem as being to the turbocharger located on the rear of the rear head and the added weight.
The options were open.Go the Volvo route of the TD 101 and 121 and make the liner locate in the head,or the Scania route of the gasket locate in the head,or the ■■■■■■■ route and drop the compression.I found it a bit odd.
I’ve often wondered why no one seems to have tried the idea of incorporating artillery gun breech design into engine design.IE no need for removable heads or gaskets to create a seal.Just bore out the centre of the breech to take the valves,valve seats and the guides and just turn it using whatever method to lock it or to open and remove it from the block and cylinders.Job done.That would probably create the best compromise between the fixed head and removable head design in that the removable breech would contain all the required service items like valves and allow access to the cylinders while still allowing the advantages of monobloc design in the form of block rigidity and removal of head gaskets. 
Carryfast:
railstaff:
Carryfast:
While I was only referring to the example of the opposite extremes of fixed head OHC v pushrods and seperate heads regarding maintenance.While single head or not major engine work is going to take more time and aggro in the case of single head and OHC/CIH than even single head and pushrods.While going for a fixed head OHC design effectively makes the engine uneconomic to overhaul which seems to have been confirmed in the case of the 500.Which notwithstanding the issues of OHC/CIH v pushrod design is probably why no other manufacturer has gone for the fixed head option.Its a known fact a one piece head stiffens the block face,this reduces flexing and liner fretting,thats why nearly all are now single piece.As for maintenance every OHC engine ive been involved with(D20,D13,Cursor,ISX)the camshaft has been left in to remove the head.Valve clearance set up is easier because the inner base is viewable on the camshaft.In certain cases less head bolts,and to top it all off ease of removing the camshaft unlike an OHV set up.In essence the 500 had a good spec but the technology to produce the block wasn’t available.
Blimey I’ve never known an OHC motor that didn’t mean removing the camshaft/s to access the head bolts and remove the head nor removing the cam drive from the cam shaft let alone disturbing the cam drive train and timing in one way or another which seems to hold true in the case of the ISX at least.While removing/replacing the head is also obviously going to take a bleedin great big crane with all the awkward fine location issues that creates when replacing the head.As opposed to just removing/replacing the rockers and the three seperate heads in the case of the N14.On that note I’m not aware of any issues caused by the seperate heads in that case nor others.
Let alone Leyland’s engineers going one better than the ISX by saying let’s make it OHC and fixed head that’ll work.
youtube.com/watch?v=3QODnuenNbw
youtube.com/watch?v=_dFg3-YnTh4
youtube.com/watch?v=Z0wcj9fE6pQ 1.49 - 2.03 3.01 - 3.15
So please enlighten me,what actual heavy duty overhead cam designs have you been involved with because I’m missing something in a big way?
Everything OHC that ive listed comes off with the cam in place.
railstaff:
…Scanias latest 13litre suffers badly with liners fretting in the block,rendering the block scrap in certain cases.It also suffers with the block face creeping away as the heads move around.This is mainly due to not having a one piece head as support to the block.Again it suffers from oil leaks at the bottom of the block were it cracks in line with any 7 of the main housings…
I’m surprised that separate heads are worse with regard to the integrity of the block/liner joint. I guess that the problems are due to differential thermal expansion of the head versus block (and the top of the block versus the bottom of it). I would have thought that separate heads would reduce shear at the head/block interface, helping the gasket do its job. Secondly, a large, relatively massive object (the head) getting hotter than the block seems more likely to expand or distort the bores locating the liners. Thirdly, unless the block is subject to significant bending stress in its longitudinal vertical plane, I would not expect things happening at the top affecting the stress around the bearing caps. Again, separating the heads would help reduce the bending moment on the block.
The above are just my thoughts as a generalist- I am not a base engine designer. However, I am always intrigued to read the opinions of those with more knowledge.
When Daf released the 530 version of the 12.6 XEC engine the block material changed to Grafite steel against the lower carbon grade for the lower hp versions.Watch a clip of an engine with single cylinder heads,clearly seen is the head lifting during the firing stroke.For an example take a look at either an MAN D28 or Scania DC12 or 13 with single heads.Basically the strength stops at the face of the block or more precisely the deck inbetween the liner counterbores which on a DC13 is about 20mm.Major flex happens in this area,thermal imaging shows it.Both of these engines share a common problem,the block face frets away over time due to the heads moving.Now bolt a piece of steel across it at 100mm deep(the head) and it stiffens the block,no more fretting of the face,it doesn’t lift on the firing stoke because its clamped either side.Really speaking the block is weak,most crankcases are only 7mm thick,a wet linered block contains nothing until half way down the block.
railstaff:
Carryfast:
Blimey I’ve never known an OHC motor that didn’t mean removing the camshaft/s to access the head bolts and remove the head nor removing the cam drive from the cam shaft let alone disturbing the cam drive train and timing in one way or another which seems to hold true in the case of the ISX at least.While removing/replacing the head is also obviously going to take a bleedin great big crane with all the awkward fine location issues that creates when replacing the head.As opposed to just removing/replacing the rockers and the three seperate heads in the case of the N14.On that note I’m not aware of any issues caused by the seperate heads in that case nor others.Let alone Leyland’s engineers going one better than the ISX by saying let’s make it OHC and fixed head that’ll work.
youtube.com/watch?v=3QODnuenNbw
youtube.com/watch?v=_dFg3-YnTh4
youtube.com/watch?v=Z0wcj9fE6pQ 1.49 - 2.03 3.01 - 3.15
So please enlighten me,what actual heavy duty overhead cam designs have you been involved with because I’m missing something in a big way?
Everything OHC that ive listed comes off with the cam in place.
I’ve never even seen an OHC/CIH truck engine personally.However what I know of things in general is that a head removal job will at the very least involve disturbing the cam drive and timing and usually removal of the cam shaft/s to access the head securing bolts/nuts.Which at least seems to have been confirmed in the case of the ISX example which I posted.Any or all of that, combined with the single piece head,obviously unarguably makes a head removal and refitting job far easier and quicker in the case of the N14 regardless.
While I’d be interested to see any confirmation that the N14 suffers with any of the unreliability issues which you’ve described regarding seperate heads.Or for that matter examples showing head removal/refitting of OHC/CIH type heads without first removing he camshaft/s and/or at least disconnecting/stripping the cam drive gear.
A hard worked MAN D28 will eventually start to crack along the line of the camshaft. The ■■■■■■■ 855 frequently bubbled at head gashet line, in the short to medium term gasket renewal would suffice but the liner height measurement comparisons would be a clue that something was happenning which would require a counterbore repair and deck resurface at major overhaul. There were also changes in liner location arrangements in the BC series developments; upper and lower press fit liners being introduced along with further machining and recovery work available for reclamation.
Carryfast:
railstaff:
Carryfast:
Blimey I’ve never known an OHC motor that didn’t mean removing the camshaft/s to access the head bolts and remove the head nor removing the cam drive from the cam shaft let alone disturbing the cam drive train and timing in one way or another which seems to hold true in the case of the ISX at least.While removing/replacing the head is also obviously going to take a bleedin great big crane with all the awkward fine location issues that creates when replacing the head.As opposed to just removing/replacing the rockers and the three seperate heads in the case of the N14.On that note I’m not aware of any issues caused by the seperate heads in that case nor others.Let alone Leyland’s engineers going one better than the ISX by saying let’s make it OHC and fixed head that’ll work.
youtube.com/watch?v=3QODnuenNbw
youtube.com/watch?v=_dFg3-YnTh4
youtube.com/watch?v=Z0wcj9fE6pQ 1.49 - 2.03 3.01 - 3.15
So please enlighten me,what actual heavy duty overhead cam designs have you been involved with because I’m missing something in a big way?
Everything OHC that ive listed comes off with the cam in place.
I’ve never even seen an OHC/CIH truck engine personally.However what I know of things in general is that a head removal job will at the very least involve disturbing the cam drive and timing and usually removal of the cam shaft/s to access the head securing bolts/nuts.Which at least seems to have been confirmed in the case of the ISX example which I posted.Any or all of that, combined with the single piece head,obviously unarguably makes a head removal and refitting job far easier and quicker in the case of the N14 regardless.
While I’d be interested to see any confirmation that the N14 suffers with any of the unreliability issues which you’ve described regarding seperate heads.Or for that matter examples showing head removal/refitting of OHC/CIH type heads without first removing he camshaft/s and/or at least disconnecting/stripping the cam drive gear.
A truck engine gets quite an easy life,its main issue as such is rapid heating and rapid cooling,so fluctuating temperatures is its main enermy.Take a look at a marine application.Admittedly cooling temperature stays constant.Full load for 24hrs day in day out.The averge 855 lasts 3 years.Again take a look at G drive,constant load at one engine speed were they are designed to run.Red hot temps in the cabinet.Oil and snot spewing out of everywere in both examples.I think you live a sheltered life.
cav551:
A hard worked MAN D28 will eventually start to crack along the line of the camshaft. The ■■■■■■■ 855 frequently bubbled at head gashet line, in the short to medium term gasket renewal would suffice but the liner height measurement comparisons would be a clue that something was happenning which would require a counterbore repair and deck resurface at major overhaul. There were also changes in liner location arrangements in the BC series developments; upper and lower press fit liners being introduced along with further machining and recovery work available for reclamation.
Just to clarify, the D28 has separate heads, while the 855 has a one-piece one. The D28, IIRC, is based upon the same engine as the Mercedes OM 42- series, which became the straight-six engine powering the Axor, and the Detroit 55(?) series. As far as I know, all of those engines have good reputaions for durability. As usual, please correct me if I am wrong.
Thanks to everyone for contributing their knowledge to these discussions- even the one whose con rod never gets any tension.
855 has three heads,one head for two cylinders.Without doubt a brilliant masterpiece but still susceptible to problems.
The D28 was shared with MB but in different configurations and only when in two valve per cylinder form.I have heard it said that as you say the Axor engine was based on a D28,but I’m not sure on that point.
[zb]
anorak:
Thanks to everyone for contributing their knowledge to these discussions- even the one whose con rod never gets any tension.
To be fair I’m still trying to work out how the equivalent amount of torque at the crank regardless of whether it was made by more force x less distance or less force by more distance on the ‘power’ stroke,can possibly apply more tension to the con rod on the ‘induction’ stroke bearing in mind that it’s the same amount of force applied by the ‘crank’ to the con rod in either case.
IE the premise would only work if the stroke on the induction cylinder was longer than the stroke on the power cylinder.Without that you just ain’t got any multiplication of the force that went into the crank on the power stroke v the amount of force that came out of it on the induction stroke.
railstaff:
A truck engine gets quite an easy life,its main issue as such is rapid heating and rapid cooling,so fluctuating temperatures is its main enermy.Take a look at a marine application.Admittedly cooling temperature stays constant.Full load for 24hrs day in day out.The averge 855 lasts 3 years.Again take a look at G drive,constant load at one engine speed were they are designed to run.Red hot temps in the cabinet.Oil and snot spewing out of everywere in both examples.I think you live a sheltered life.
So are you saying that MTU’s are zb because if I’ve got it right they are seperate heads ?.
When I went to work for Spillers Milling in 1981 virtually the entire artic fleet (total about 130 tractor units) was Leyland Buffalo based with a mixture of TL11 and L12 engines. There were a handful of Guy Big J’s and elderly AEC Mandators still dotted about the country at some mills. To be fair the TL11 engines didn’t give much bother in service, but their work was mainly short haul with, of course, blowing hours daily. They weren’t a particularly high mileage engine, however. Spillers had gone through the fixed head 500 engine traumas in the early '70s.
gingerfold:
When I went to work for Spillers Milling in 1981 virtually the entire artic fleet (total about 130 tractor units) was Leyland Buffalo based with a mixture of TL11 and L12 engines. There were a handful of Guy Big J’s and elderly AEC Mandators still dotted about the country at some mills. To be fair the TL11 engines didn’t give much bother in service, but their work was mainly short haul with, of course, blowing hours daily. They weren’t a particularly high mileage engine, however. Spillers had gone through the fixed head 500 engine traumas in the early '70s.
How did the L12 compare Graham?
The Lorry engine does get a fairly easy life in comparison with the same basic engine’s alternative applications. However for continuous duty the output and speed is usually reduced. As said thermal cycling is the main enemy, it is noticeable that city bus engines do not achieve the same mileage which lorry engines manage. This being because of the constant cycling between full load acceleration and idling at stops, the latter compounded by saloon heating using engine coolant so that following a period of idling the engine receives a rush of colder water just as it comes onto full load. The severest application being high speed rail traction viz the problems encountered with the Paxman Valenta, much of which was related to auxiliaries. The fixed head 500 series engine which was also intended in its horizontal version for citybus use offered the tantalising prospect of eliminating the eliminating the primary area affected by this heat cycling. It would be wrong to dismiss the fixed head engine idea as a complete failure; Daimler Benz managed to produce successful aero engines of this pattern during WW2. One cannot blame Leyland’s design engineers for investigating the possibilities. If they had incorporated DB’s method of liner and block location then they might have overcome one of the 500 series engine’s other failings.