Lotus Elan

Removal and rebuilding of a Twincam

PostPost by: rgh0 » Sun May 03, 2020 12:27 pm

Donels wrote:Rohan,
I have seen this type of wear before on a friends Triumph Stag shortly after he had lapped the valves. It was on all tappets he had lapped. We concluded it was inadequate cleaning after lapping leaving debis in the bores.
I suspect the hourglass shape (like a cooling tower) is from debris being trapped in the middle, whereas it gets pushed out at the ends.

Hmm, looking at you picture again I see the wear is on the horizontal axis and not the vertical, so the above may be not applicable.

Dave



The sleeve wear is oriented transversal (at 90 degrees) to the cam centre line due to the side thrust of the cam on the follower. The sleeve has worn onto an hour glass shape vertically with the measurements taken transversal to the cam centre line. This is the normal wear pattern for the sleeves but it just does not normally happen as quickly and normally it happens evenly across all sleeves in a head just not one sleeve,

If abrasive material had been left in this one sleeve bore I would have expected some wear on the follower also different from the other followers but there was none.

So i struggle to see anything but a soft or somehow otherwise faulty sleeve.

In going back through my engine build records tonight. I had this head converted by McCoy from a Stromberg head and race ported in 2009 ( I had two heads done at that time). At that time new un-honed sleeves were fitted and I recorded a rough patch on the No.4 inlet sleeve in my notes. I had the sleeves finish honed to factory tolerances locally and asked the machine shop to replace the No.4 inlet sleeve if the honing did not remove the roughness. Unfortunately my records don't show whether it was replaced or not. The head was not used until 2017 when I used it to build a new race engine and at that time all my notes show no issues with sleeve bore or follower fit. So maybe something has been wrong with this sleeve... either it was replaced and soft or was not replaced but had hidden fault under the surface after honing ??

Some testing after its removed will tell me more I hope

cheers
Rohan
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PostPost by: Foxie » Sun May 03, 2020 1:27 pm

rgh0 wrote:Continuing with cleaning and closer inspection of the cylinder head


to be continued .........

cheers
Rohan


Hello Rohan,

Am I looking at a bolt-on inlet manifold ??? :shock:

That's a new one on me !
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PostPost by: rgh0 » Sun May 03, 2020 1:31 pm

Yes it a McCoy Stromberg conversion. The Stromberg inlets are machined off and the Weber style inlets bolted on.

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PostPost by: rgh0 » Tue May 12, 2020 11:49 am

Continuing with the next stage which is sorting out all the bits needed for rebuilding

I spent the weekend carefully cleaning and measuring all the engine components to see what needed replacing due to wear or damage. Takes a while as the key dimensions on every component need to be checked.

IMG_8484 (1).jpg and


IMG_8482 (1).jpg and


then need to check my stock of spares to see what I had versus whats need, here is my stock of thrust bearings behind it you will see the gauge I use to check the spring loads versus length

IMG_8485 (1).jpg and



The outcome was not to bad and as you can see form the list below that given shipping costs to Australia it is cost effective to try to order what you may need in advance for future builds !

# The crank No3 rod journal had about 5 thou wear and the crank is off to my local crank specialist ( Crankshaft Rebuilders) for a regrind to 20 thou under on the Rod journals and a polish on the mains. The crank is a converted Datson L16 steel forging and this ends up with 10 thou undersize to get the standard stroke when first machined so next size is 20 thou undersize on the journals. I had a spare set of std mains and 20 thou under Rod bearings so right for crank bearings and I have plenty of spare thrust bearings !

# The No.3 Carillo Rod big end has distorted slightly due to overheating and is not usable so ordered 5 rods as i had no spares from Tony Ingram at Lotus7.com who is a great source of racing engine parts in the US.

# Pistons and block are all OK and can go back together without issues

# The No. 4 inlet cam follower was distorted due to the sleeve wear I have some spares but of different pad thickness so ordered 9 more from Tony Ingram of the 140 thou pad who supplied the current ones in the engine. All the other followers were perfect.

# The head has gone to my local head specialist ( HSD, Head Stud Developments) to have the sleeve in No. 4 inlet valve bore replaced and honed to standard factory tolerances. He will also hardness test the removed worn sleeve to see if that explains why it wore so quickly. The rest of the head was fine with no wear or damage on seats, guides or the other 7 follower sleeves.

# All the springs were fine and within original tolerances on load versus compression length

# The valve spring seat in No.4 inlet had been damaged but I have plenty of spares of these in various thicknesses to get the right spring seat load on assembly

# The valve tip, collet groove, titanium retainer and collets on the No 4 inlet had some damage from the follower rocking in the worn bore. So new ones of these have also been order from Tony Ingram. I got 9 so I have a spare set for my next race engine build also. Also ordered spare set of inlet and exhaust valves as he now sells one piece forged stainless valves with a black nitrided surface finish and i wanted to see what they looked like for

# Going through the rest of what was needed. I ordered a race inlet valve to replace the damaged one as the others in this engine were also from QED and some gaskets and other bits and pieces such as the lock washers on the layshaft that I was running low on stock of and another set of race springs from them. I use the QED red race springs often as these best suit the typical competition engine builds I do in combination with the Tony Ingram titanium retainers and I had no spares of these left. I have spare sets of race springs from all the suppliers and they all have slightly different characteristics in terms of install height and spring rate but I find the QED ones most flexible in their application so I end up using these the most.

So having stressed my credit card I will now finish cleaning everything over the next few weeks while waiting for the parts to arrive and for the machining work to be done. I will go through the engine bay looking for issues as plenty of room with the engine out and I need to check for cracks from racing stresses around engine mounts and roll bar mounts. I will remove the gear box so I can check the prop shaft U/J as I have not had this out since I rebuilt the car in 1980! I also need to have the gear box out so I can get the jammed and rounded off fill plug out. I also have a set of adjustable front wishbones to fit if I run out of other things to do and then I can use my new electronic camber and caster gauge to do the front setup .. nothing like new toys to play with and never any shortage of work to do to keep an old race car on the track and reliable :lol:

I will continue when the the parts arrive and the rebuild begins

cheers
Rohan
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PostPost by: alan.barker » Tue May 12, 2020 2:47 pm

4.png and

3.png and

This was a McCoy Head with a few problems with porosity. Had to mill metal away then weld in alloy plate.
Alan
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PostPost by: david.g.chapman » Tue May 12, 2020 5:54 pm

A fascinating thread Rohan.

I have had trouble in the past with broken top piston rings, possibly due to the groove to ring clearance being 6 thou when I fitted new rings some years ago. The ring manufacturers spec. showed this was on the top limit but OK. The rings broke again after just 15000 miles, and this time I fitted new pistons and rings with a 2 to 3 thou gap. There have been no problems since.

So I have a theory that once the gap gets too big, then wear begins to accelerate and the rings fail soon after.

From your inspection it looks as if your pistons are fine, including the ring to groove clearance? Do you find that ring to groove clearances are stable below 3-4 thou?

Dave Chapman.
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PostPost by: benymazz » Tue May 12, 2020 6:23 pm

david.g.chapman wrote:A fascinating thread Rohan.

I have had trouble in the past with broken top piston rings, possibly due to the groove to ring clearance being 6 thou when I fitted new rings some years ago. The ring manufacturers spec. showed this was on the top limit but OK. The rings broke again after just 15000 miles, and this time I fitted new pistons and rings with a 2 to 3 thou gap. There have been no problems since.

So I have a theory that once the gap gets too big, then wear begins to accelerate and the rings fail soon after.

From your inspection it looks as if your pistons are fine, including the ring to groove clearance? Do you find that ring to groove clearances are stable below 3-4 thou?

Dave Chapman.


Dave,

I know your comment was directed at Rohan but I’ll share my experience. Last year I tore down a race engine that was almost identical to a Cosworth Mk. XII in terms of components used and performance, the only major difference being bore was kept as stock at 82.55mm so displacement was 1557cc not 1594. It was used in the late 60’s and through the 70s to power a Legrand Mk. IVb SCCA B/SR car that my dad raced. The car was sold in 1980 but my dad kept the engine and trans. When I got to the engine, it had been sitting since the car was sold. Upon disassembly I found that the top ring lands on the pistons were totally smashed out. They looked fine visually but when clearances were measured the top ring to groove clearance was between 10 and 12 thou on all pistons.

Seems like this was a common problem on race engines of the period - when running at high revs for extended periods of time, the ring lands get totally wallered out.

When disassembling the engine, one of the top compression rings came out in two pieces although I will never know if that was because of my less-than-delicate approach to piston ring disassembly, or if it had broken inside the engine. Given your experience with broken rings I’m just as likely to believe either scenario.

The pistons that I removed were AE Hepolites. Can’t remember the number offhand - 16100 or 16400 maybe? I believe these pistons were cast and not forged which could explain why this was a common problem back in the day.

FWIW I also observed some “stretching” of some of the wristpin bores on those pistons - the wristpin was a proper fit in the bore, but it would not clear the part of the bore outside the circlip groove and I had to beat it out with a hammer and wooden drift - indicating to me that the piston had deformed and stretched by a very small amount in the vertical axis due to the extreme forces encountered when running at 7000+ rpm.

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PostPost by: alan.barker » Tue May 12, 2020 7:19 pm

In my David Vizard book ""Tuning twin cam Fords" he says .004" thou maximum clearance.
When i bought my Sprint with 68,000 miles on the clock the first thing i did was strip Engine. All the Ring grooves were badly worn and a few Rings broken. I have found Twinks need a rebuild at around 60,000 miles before there is big damage.
I just deglazed the Bores and fitted a set of NOS standard Pistons £100 for complet set of 4.
Alan
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PostPost by: 2cams70 » Wed May 13, 2020 12:55 am

Excessive ring groove wear is most likely caused by ring flutter at high RPM - i.e the rings reach a harmonic resonance at a certain point and start bouncing around in the grooves. Later aftermarket pistons probably have thinner rings for reduced ring weight to alleviate the problem. Ford introduced a change to their regular crossflows in this regard. In the uprated 1971 onwards engines the ring width was decreased. They probably didn't bother about the Twin Cam engine for reasons of "Low volume, low profit, it's only going into a Lotus and we don't wear the warranty claims!"
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PostPost by: rgh0 » Wed May 13, 2020 10:39 am

david.g.chapman wrote:A fascinating thread Rohan.

I have had trouble in the past with broken top piston rings, possibly due to the groove to ring clearance being 6 thou when I fitted new rings some years ago. The ring manufacturers spec. showed this was on the top limit but OK. The rings broke again after just 15000 miles, and this time I fitted new pistons and rings with a 2 to 3 thou gap. There have been no problems since.

So I have a theory that once the gap gets too big, then wear begins to accelerate and the rings fail soon after.

From your inspection it looks as if your pistons are fine, including the ring to groove clearance? Do you find that ring to groove clearances are stable below 3-4 thou?

Dave Chapman.


The forged JE pistons I use have a thin light weight set of rings based on modern racing and motor cycle technology where revs are the name of the game and ring land wear and broken rings has never been a problem.

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PostPost by: david.g.chapman » Wed May 13, 2020 11:47 am

Thanks Rohan. That's something I will bear in mind if I rebuild my engine again.
Dave.
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PostPost by: rgh0 » Wed May 13, 2020 12:02 pm

alan.barker wrote:
4.png

3.png

This was a McCoy Head with a few problems with porosity. Had to mill metal away then weld in alloy plate.
Alan


When doing his stage 4 race conversion core shift and porosity can be a problem as you are pushing the limits (but not as much as Hart did in his 416B heads :lol: ) . John has been great and has source new used Stromberg heads for me when one of then heads i have sent him has not worked out and he has broken through when doing the race porting on a converted head.

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Rohan
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PostPost by: promotor » Wed May 13, 2020 12:08 pm

rgh0 wrote:
alan.barker wrote:
4.png

3.png

This was a McCoy Head with a few problems with porosity. Had to mill metal away then weld in alloy plate.
Alan


When doing his stage 4 race conversion core shift and porosity can be a problem as you are pushing the limits (but not as much as Hart did in his 416B heads :lol: ) . John has been great and has source new used Stromberg heads for me when one of then heads i have sent him has not worked out and he has broken through when doing the race porting on a converted head.

cheers
Rohan


It does make me wonder about the strength / rigidity of the head once the big chamber is machined off the side of the head - particularly around number 4 and possibly number 3 inlet sleeves. I would want to look at the head once the sleeve on no.4 is taken out to see if the material has been compromised a little. Hopefully it hasn't been though.
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PostPost by: rgh0 » Wed May 13, 2020 12:47 pm

The oil knock out chamber was an add on during the design development. If you look at the casting you can see how it was added. Removing it does not seem to affect the head integrity with the McCoy conversion. The large flange bolted to the machined face along the full length of the head probably makes them stiffer. Certainly 13.5 :1 comp ratio and 8500+ rpm and 200 hp from a 1600 cc twink works with them :lol:

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PostPost by: promotor » Wed May 13, 2020 2:07 pm

rgh0 wrote:The oil knock out chamber was an add on during the design development. If you look at the casting you can see how it was added. Removing it does not seem to affect the head integrity with the McCoy conversion. The large flange bolted to the machined face along the full length of the head probably makes them stiffer. Certainly 13.5 :1 comp ratio and 8500+ rpm and 200 hp from a 1600 cc twink works with them :lol:

cheers
Rohan


For the sake of looking at the parent bore of the sleeve before a new one is put in it would surely be worth it, it doesn't cost to look! It would be a great result if just the sleeve was faulty. Let's hope so.

Regarding follower sleeves I have seen an issue where the original follower sleeve bores from factory were not machined at exactly the same angle as the original valve guide bores (replacement guides and sleeves were fitted for this job) - this caused an issue with the new sleeves that are available.
The new replacement sleeves have minimal material in them available for the final machining operation and due to the angles not being the same the sleeve bore did not finish round as the cutter couldn't cut material that wasn't there due to the minimal material and angle change as the cutter went down the bore. They were perfectly round and on size at the top, but tapered gradually from top to bottom by approx 0.008" (perhaps there is some maths that could be used there to work out the angle difference) when I measured the finished sleeve bores with a proper bore gauge. The machinist (a so-called specialist of Twincams) didn't own a bore gauge to measure them when finished and told me he had never actually measured one which if true is a major fail!! He relied on setting the cutter to the correct cutting size and then trying a follower in the hole once the hole was cut!!
Of the issue I saw perhaps other Lotus heads have had similar issues from the factory - I have seen other machining tolerances / alignments from un-messed-with heads that would make you question what Lotus were doing! Line-boring not being in a horizontal plane to the head being one of them!
Hopefully you won't see this issue, just thought I'd raise it even if it is something to keep in mind for other jobs on other heads.

I'm certainly not trying to "throw shade" (if it seems I was implying it) on Omnitech as I have plenty of respect for what he knows and can do.
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