PrecisionMike

That makes sense and a good transfer of experience based knowledge!

Not necessarily, although of course that would re instate the clearance. Numerous reface and cuts would recess (or pocket) the valves which would alter the compression ratio. In my case the seats were not pocketing the valves and I wanted to renew all followers anyway for two reasons...the original Lotus cast iron ones can crack and they have surprisingly low engagement in their bores, increasing the likelihood of them rocking particularly with high lift cams. Many people do bring a little off the top of the valve tip to solve the problem but caution must be used to ensure that you dont go through the hardened layer. Lotus publish the valve lengths in the workshop manual so measurement will determine if they tips have been ground. If they haven't, a few thou * is probably ok but I didnt want to take the chance . * But what's a few though I hear you ask as indeed I did. I've heard people suggest figures of up to .010 inch but that sounds to much to me but I do stand to be advised. If the situation was marginal a tip grind is the cheapest way out but I would recommend a hardness test after to be sure.

Prompted by ddroom, I'll briefly comment here but follow up with a spreadsheet which calculates required shim thickness and Valve spring pre load when closed and force when opened. I got into this heavily because after exhaust seat cut, valve grind and lap I discovered I'd need very thin shims which would not be reliable and indeed were outside the available range. The key parameters are *Valve stem protrusion above the cam carrier / head interface. (To my knowledge Lotus never published that dimension although the valve lengths were for both the 907 and 912 which are different) *Distance from the carrier/ head interface to the base circle of the cam. (A higher lift cam has a smaller base circle which calls for thicker shims) *The total thickness of of the pad in the cam follower. (There are four choices in my research, influencing the thickness of the required shim as well as reduced weight and increased stability by virtue of the long skirts) *Carrier to head sealant cured thickness A linear tolerance analysis needs to be made in order to determine the required valve stem protrusion referred to in this particular instance but this can only be calculated if the other dimensions are known. My spread sheet was a work in progress for me , primarily to determine the spring forces at fully open and closed as I was investigating valve acceleration and inertial mass in order to determine required valve spring rates for my application. It had the side benifit of calculating the required valve shim thickness for each valve. I will endeavor to post the spread sheet within a few days. My situation in which I required very thin unavailable shims was resolved by adopting thin pad light weight followers from Lotus Bits. Theses parts are lighter, stronger and long skirt and are beautifully made and rally proven. The thing that surprised me although obvious when I thought about the detail, was that it doesnt take many valve seat and valve face grinds plus lapping for the valves to suddenly be sticking up another 0.5 to 0.7 mm. And all our 907 / 912/ 910 engines are old now and could be on the third rebuild. I hope this helps and I'll do my best to post that spreadsheet.

Hi Kevin, I think there are no numpty comments here. All are valid and facilitate learning. I think we are talking of the same thing assuming your box is the citroen, but may be the manner of failure is different. I'm guessing yours broke and you lost drive? Anyway your bill sounds reasonable to me for the amount of work involved. My comments stem from a full dimensional analysis of the crank to gearbox interface on an early (1981 S2.2) in an attempt to understand why citroen sprung loaded the input shaft. It was a bit of an academic study I guess. I'm glad your car is sorted. I'm in the middle of a full gearbox rebuild at the moment due to horrible noises from it although it's perfectly functional!! Good luck with it all..

Sounds to me as if you should get another mechanic. If the retention circlip is in place at assembly then its assemble properly. If the nylatron washer is in place and not worn then the retention clip isnt even loaded. The crank thrust bearing reacts the constant spring preload. As the nylatron washer wears, the input shaft moves forward into the crank, resulting in two things.....reduction of spline engagement with the clutch friction plate and metal to metal contact between the shoulder of the splines and the spigot bearing. A very nasty situation requiring a bulky wallet to rectify.

Thanks guys for the tips . I'll try again and let you know how I get on.

Hi. Yep. I've locked the gears. Warmed the nut. WD 40 and two people. The tool held but it didnt budge. I'm in trouble🤔 Not sure what to do now other than open bottle of wine🍷

Thankyou for your rapid response. I had adopted a similar approach for the speedo nut but with a 46 mm socket which is a snug clearance fit over the speedo drive. When you say "normal clockwise" I assume you mean clockwise to tighten.......just to make sure. Please can I ask how you reacted the torque applied to the speedo nut. Many thanks and thanks for the link to your tools stuff.....I guess I'll be making those too!

Very impressive. I'm in the process of re building my early c35 box and am having great difficulty in undoing the speedo drive nut on the other end of the pinion shaft. Please can I ask.....Is that nut a normal direction thread ie anti clockwise to undo or is it a left hand thread. All help very much appreciated.

That "plug" blanks off the cross drilling that retain the spring and ball which forms the detention function in the selector mechanism. There are two, on on each side of the box. The manual talks of pricing the plugs out but not sure how you'd actually do that. My intention upon reassemble of my box is to use a medium strength bearing retainer loctite. Picture for reference.

Hi John, my understanding is that broadly speaking the 907 and 912 LC (low compression)are much the same in terms of gas flow but 912 valve stems are longer and the springs I think might be different. The valve spring retaining collets certainly are different but they are interchangeable and can be used to slightly change valve spring closed and open loads. Over time there were I believe slight changes to the head to improve cooling. The 912 has the heater take off at the rear of the head. The 907 took coolant from the inlet manifold. The real differences were in the 912 HC engine which is in the excel SE and Esprit HC which has bigger ports and larger valves. I might be missing something but I never understood the benefit of 1mm larger valves in a standard but re cut valve seat without modifying the seat bore or porting, IF in fact that was what was described. The lotus workshop manual does quote valve diameters and the Bell & Colvil web site has full exploded diagrams with all part numbers which I've found useful in finding differences and identifying part numbers. I'm just completing a full and "special " build myself and one of the many lessons I've learned, is that a with a valve seat cut, valve re face and longer stems you're likely to run out of valve shim range which is solved with new steel tappets of which there are various choices of pad thickness depending on supplier , so do budget for new steel long skirt followers which are a good idea anyway as they are stronger and offer increased stability. If you're interested in that part I do have as work in progress (not yet completed) a spread sheet which takes all relevant parameters and calculates required shim thickness as well as valve spring open and closed loads I'd recommend a call to either Lotusbits , PNM Engineering or Southwest lotus, all of whom have intimate factual knowledge and real current experience. The principle and very major lesson I've also learned is that there are usually knock on effects of any change which require engineering consideration and access to a thick wallet 😁 Good luck

Hi. I'm in the process doing lots of research and calculation looking at the differential thermal expansion at the liner to block seal interface as well as comparison of sealant properties. I would state that wellseal and hylomar (of which there are many varieties) are definitely NOT obsolete and remain excellent candidates for many places on the 9xx series engines. My focus is of course the liner seal for which the iron liner has the most thermal variation.

Not for me I'm afraid!

Yeah, I saw that article. Some people will like it but for me , if it doesn't go Vroom Vroom, I'm not sure if there's much point?

Any one considering ruining the lotus esprit experience by such a sinful modification should in my view seek either counselling or punishment 😁