Lotus Elan

Crankshaft Stiff to Rotate with Pistons

PostPost by: dlb123 » Tue Sep 05, 2017 9:02 pm

Currently building up the bottom end of the twink. Please excuse the writing style, but I've tried to be clear with the steps ive taken.

Crank reground and straight. Spins easily from the snout when the main bearings are torqued down. End float good.

Pistons are new QED type with new rings, all gapped between 9-14thou (average 11thou)

Block is also new, and has been bored/honed. Piston to bore clearance of 3-4thou for all cylinders.

Installed pistons, rotating each time to check for increasing tightness working along 1 through to number 4. Seemed to require a fair push to get them to move down the bores. All rod caps torqued up correctly and correct sequence.

Once assembled, the short block required about 15-18 ft-lb on the wrench to break the stiction and perhaps a little less to keep rotation. Particularly tight when mid-stroke. This seems a lot to me. (furious kicking of tools, this engine has been a nightmare). I was told less than 10 lb-ft is sensible.

Suspect the stiction is coming from the pistons, rather than the bearings. Removed all the pistons, and the rings (broke one bollocks) Re-inserted all pistons minus rings and torqued down rods again. Rotating assembly is seemless with no tight spots anywhere. Can rotate the crank by the snout, ruling out tight big end bearings or mains.

So, breathe, my question is. WTF is going on? Everything checks out, so should I just build it and go? Also, since ive installed and removed the new rings (and I was disturbed that one of the compression rings broke), do I need to junk those rings and replace them all? I know they are brittle and dont take kindly to being man-handled, and I would hate to have a ring let go for the sake of ?100.

This engine is taking me to dark places . :x

Thanks for any input
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PostPost by: Chancer » Tue Sep 05, 2017 10:06 pm

Perfectly normal.

It will be harder to break the stiction mid stroke because there is no dwell period to build up a little momentum also the piston velocity is at its highest when the crankpin is at 90?
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PostPost by: Davidb » Tue Sep 05, 2017 10:46 pm

If old style pistons/rings then the rings have quite a bit of drag-especially mid stroke as pointed out. The ring gaps seem a little tight to me?

New style piston rings are very thin (1mm or so) and offer very little drag.

The engine will loosen up quickly. Put very little oil in the bores would be my suggestion. I use only a quick spray of WD40 and have no problem. When I used to use copious quantities of oil in the bores I had problems getting the rings to bed in.
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PostPost by: 512BB » Wed Sep 06, 2017 9:12 pm

'broke one bollocks'
Well at least you didn't break one bollock, so thats something.

' The ring gaps seem a little tight to me?'
But not to me. Hepolite state 3 thou gap per inch of bore...............3 inch bore = 9 thou gap. Man has 9 - 11 thou.

I would have no problem refitting the rings you have removed, provided you have not bent them out of shape, and if you had have done, you would most probably have broken them. In which case, stamp on your other ball.

Where are you going to buy 1 ring from DLB, Thorntons?

Post up some pictures, love to see an engine coming together.

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PostPost by: Davidb » Thu Sep 07, 2017 1:01 am

Re. ring gap: The latest set I fitted-to my Elan-were from CP pistons (Formerly JE I believe?) and the suggested ring gap is 16 thou. That is with modern, very thin, gas nitrided top ring-2nd ring is greater. With older style rings the gap is about ten thou at 'our' bore size.
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PostPost by: Gray » Thu Sep 07, 2017 8:27 am

Bent conrods can cause problems, although would probably result in a lot more friction. It might be worthwhile checking one piston at a time to see if any create more resistance. I did have a torque figure for short engine rotation for blueprinted Formula Ford engines many years ago which I used to check the last twincam I rebuilt. I'll see if I can find it.
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PostPost by: steveh » Thu Sep 07, 2017 6:23 pm

Is there any scuffing on the pistons ? .Have the small end bearings been changed ? , if so check they are square to the con rod . You can check by threading 2 rods onto 1 gudgeon pin then turning 1 over to see if they sit parallel , other than that how rough is the honing ?
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PostPost by: pharriso » Thu Sep 07, 2017 9:48 pm

Can you assemble 1 piston & rod in the engine & measure the turning torque? Do for #1 then #2 only, #3 only & #4 only to identify if it's one assembly that's giving you issues.
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PostPost by: dlb123 » Wed Sep 20, 2017 11:24 am

Hello again,

Thanks very much for all the contributions. I rebuilt the block minus the piston rings to check for binding in the bearings, or issues that might suggest a bent con rod. Without the rings, I was able to rotate the assembly by spinning the snout between thumb and forefinger, which is good news I guess. While the block was apart, I re-checked all the ring gaps and p to b clearances, all were found to be in tolerance.

There is no scuffing on any of the pistons, or the bores, although the hone is fairly rough.

I have since rebuilt the block (and my bollocks), one piston at a time to check for a culprit. The torque required to turn the assembly increases in a linear fashion as each piston is added. Now I reassembled this time with light oiling in the bores (thanks for the suggestion). Once the crank is turning, the wrench seems to crack at about 12nm (8-9 ft.lbs), which seems a little on the tight side, but ok. This of course occurs at mid-stroke. I ferried the block down to the machine shop- he agreed that it was a little tight, but nothing extraordinary.

Hence I can only chalk it up to drag from the rings, which I'm confident will bed in. I don't think there is anything else to check on that front!

I digress, Ive added some photographs, perhaps I'll convert this to a build thread.

Ive replaced the timing cover back-plate because the old one was quite warped. I've now discovered the timing cover itself is slightly bowed, as per photos. Frustratingly, Ive already replaced the waterpump into this old cover. it was my intention to make up some .25mm gaskets to aid with sealing. Would I be wrong in thinking that torquing it down should pull it flush with the back plate? If not, then I reluctantly will have to call QED again :(.

As always, your input is greatly appreciated.

Dave
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PostPost by: pharriso » Wed Sep 20, 2017 1:39 pm

dlb123 wrote:The torque required to turn the assembly increases in a linear fashion as each piston is added. Now I reassembled this time with light oiling in the bores (thanks for the suggestion).
Dave


Bores definitely need to be lubricated when you assemble the pistons into them!
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PostPost by: 2cams70 » Wed Sep 20, 2017 2:10 pm

My engine is currently at a similar build stage. Pistons are presently fitted without rings after having set the camshaft timing and doing a clay test for clearances. I'll be fitting the rings and pistons for final assembly sometime over the next week or two. After I do so I can measure the "breakaway" torque figure I achieve and advise for comparison if you can wait that long. My engine has the same QED pistons and rings as yours. I doubt you have a problem though.
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PostPost by: dlb123 » Wed Sep 20, 2017 2:33 pm

pharriso wrote:
dlb123 wrote:The torque required to turn the assembly increases in a linear fashion as each piston is added. Now I reassembled this time with light oiling in the bores (thanks for the suggestion).
Dave


Bores definitely need to be lubricated when you assemble the pistons into them!


I probably wasn't very clear. I think I over-oiled them last time, it was pooling up in the valve pockets!!
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PostPost by: elj221c » Thu Sep 21, 2017 10:46 am

dlb123 wrote:Once the crank is turning, the wrench seems to crack at about 12nm (8-9 ft.lbs), which seems a little on the tight side, but ok.

Dave


David Vizard in his book, Tuning Twin Cam Fords, mentions 8lb/ft. The ring gaps are about right according to him. 10 to 14 thou. He does mention 'stoning' the ring ends to stop scoring. However, it seems that your piston clearance might be a little tight. He suggests that 0.0045 is about right. Maybe a light hone would ease things a little.

On the other hand, Miles Wilkins would say that your clearances are OK for a standard build, according to his book. Compression ring gap 9 to 14, oil ring 10 to 20. I don't remember a quote for torque in his book.

None of my timing chain covers have been totally straight when not bolted up..... :wink:
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