Lotus Elan

Twincam Rebuild Saga

PostPost by: type26owner » Mon Nov 08, 2004 2:09 am

Took me a full day to finally install the three new jackshaft bearings in the block and have it turn by twisting it with my bare hands. Turns out the center bearing bore in the block is undersized by a few thou and it's centerline is skew to either end by about a .06" offset in the other bores. The skewing is only apparent when the center bearing insert is the only bearing installed and the clearance is a just fit. Inspecting the bore in the block tells the story. It has severe chatter marks and was done poorly. I finally got it to all loosen up enough by reducing the OD on the bearing shell not so much it would fall through the bore but the interference fit was minimal so to be safe I glued it in with sleeve retainer adhesive. Luckily I had some NOS bearings as spares which were not the split type otherwise reducing the OD would have been nearly impossible to do. The bearings are STD size so this problem was there on day one of it being assembled at the factory. It's worked like this ok for 179K miles so I'm hoping it will be good to go for like amount of miles again. Beginning to suspect they ran low of good blocks so it was ordered that a clever chap should go and 'fix' some of the rejected ones.

This is a gotcha for me. Wasn't paying that much attention I guess when disassembling the engine because I only noticed this wear upon trying to put it back together. Doh! Go here and look in the folder 'Twincam_Rebuild'.
<a href='http://briefcase.yahoo.com/[email protected]' target='_blank'>http://briefcase.yahoo.com/[email protected]</a>
I've gently persuaded the arm on the fuel pump to bend over to the lefthand side but I'm having anxiety bouts by doing so. Just trying to be cheap. :(
-Keith
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PostPost by: type36lotus » Mon Nov 08, 2004 3:49 pm

How about an electric fule pump. Not very costly. That surface is really scary, I can see your concern.
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PostPost by: type26owner » Mon Nov 08, 2004 4:21 pm

Nah, I'll stick with the mechanical fuel pump to keep it simple and safe. Luckily brand new jackshafts are priced reasonably at $59 so my old worn one is outta there.

My next big purchase is a safety related item. It's a slightly oversized fuel cell which is an exact match for the Elan fuel tank's shape. I talked it over with Bob Booth of BellMotorSports and he said he'd make me a deal I could not match anywhere else. Need to 3D model and produce up a detailed drawing of the tank next and send it to him for a quote.
-Keith
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PostPost by: type26owner » Mon Nov 15, 2004 4:17 am

Replaced the Hepolite rings which were worn out on the pistons that have 100k miles on them but are still well within the wear tolerance and found a problem that was not obvious at all. The bottom of all the ring grooves were clean except for a small amount of build-up of soft gooey carbon under some of the bottom oil scraper rings. Upon closer examination found the four drain holes though the piston underneath the oil scraper rings were plugged up solid on ALL the pistons. WOT for extended periods of time burns the oil on the underside of the pistons and clogs up the drain holes. Looks like I need to routinely pull the pistons out every 50K miles and clean out the oil drain holes, replace the waterpump and fit new bearings. Hope I can remember this in another 3 years from now!

With a mill smooth file I was able to finish the area around the cylinders on the engine block were the crush ring on the headgasket mates against to better than a 10 microinch finish no problem. Trick is to remove all the tool cutter marks which are aligned across the sealing surface.
-Keith
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PostPost by: type26owner » Tue Nov 16, 2004 3:35 pm

Pondering the burnt synthetic oil deposit on the underside of the pistons and the more I consider this situation, the more I get concerned the piston is close to overheating there. IIRC, that oil cracks at 450F+. I've already maximized the flow (throughput) of the oil circulating through the oil galleries by going to 10-30W oil and by doing so have minimized the thermal gradients most likely. This keeps the oil pressure always below the level so the by-pass valve does not dump any excess back into the pan. So it boils down to would enlarging the tiny oil orifice in the bigend of the conrod so it directs more oil up into that area to enhance the amount of cooling a good idea? Anyone tread here before me? Don't think there is a real possibly of bleeding off too much oil into the orifice and causing the bearing to run short of oil itself. I understand the trade-off of possibly flooding the oil control ring and causing the engine to consume oil past the rings. Wonder if the oil coming out of the tiny orifice is a pressurized stream or it dribbles out and is flung up there. There is a cavity which fills with oil behind the orifice and if the oil is flung out by centrifical force then enlarging that cavity also would be prudent. Wonder how much is flung off the conrod's bigend bearing? I guess the answer is not enough otherwise they would not have added the orifice. Am I overlooking something here? Gee, maybe I should install the new high capacity, high pressure oil pump I've got just laying around right now because I've been waiting for the other one to die. Ideally there should be an extra array of oil fed nozzles which only drenches the undersides of the piston at the bottom part of the stroke. B)

Pretty sure this is only relevant for a car being flogged out on the racetrack so you cruisers out there don't fret about it.
-Keith
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PostPost by: type26owner » Tue Nov 16, 2004 4:08 pm

My air/fuel mixture at WOT is at 11:1 now. Suspect there is a little more hp to extract if I lean that up a bit more but that most likely would also increase the heatflow through the piston. Gee, maybe I should also clean off the grungy burnt oil deposit already stuck on the piston since it could act as thermal insulator also. Yikes!
-Keith
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PostPost by: type26owner » Tue Nov 16, 2004 8:23 pm

Had a chat with Gary at JE pistons and the conclusion was more oil to cool the pistons is mostly likely a good move. He thought the deposit was from the dinosaur oil I used to use for the first 25k miles and the oil control drain holes in the pistons have been clogged up ever since. If it was caused by the synthetic oil then most likely the pistons have been overheated and they will have been annealed. He recommended I Brinell hardness test the pistons to make sure they are okay. Let's see I can do that by dropping a ball bearing from a common height first on a sample of known hardness and then do the same to the piston and measure the diameter size of the indentations. Geez, I don't have one of those at home.... Learned lots of other stuff I didn't know too. Like the recommended procedure to break-in and seat the rings that it's done in stages over the course of just four laps on the track. First lap you keep it WOT for 1/4 the length of the longest straight and on the second lap it's 1/2 the length and so on...... Also why the rings wear out in thickness from rotating around due to the lead angle of the honing surface finish when the mating grooves in the pistons stay virtually perfect. Urged him to post this info on their website eventually.
-Keith
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PostPost by: rgh0 » Wed Nov 17, 2004 10:33 am

Keith

I run my engines as hard as anyone with lots of track miles and with synthetic oil I have never had any deposits build up under the piston or in the oil drain holes behind the oil rings. Pre synthetic oil days the engines were full of carbonised crud everywhere. I also use Carrillo rods that dont have the oil spray hole either so I doubt if you need to increase that flow. I suspect you either have to much ignition advance for the fuel you are using or the deposits are from your pre synthetic oil days.

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PostPost by: type26owner » Wed Nov 17, 2004 3:32 pm

Hi Rohan,
Suppose I could have caused a thermal excursion by my recent testing of the two-phase flow effect on the Webers. Had the mixture above 20:1 a few times for a short period (not more than 25 miles if I recall correctly) when it did a major domain transition. Oops!
-Keith
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PostPost by: type26owner » Fri Nov 19, 2004 3:32 pm

Rohan,
It occured to me the actual force exerted on the oil which transits the front oil fed bearing on the jackshaft on it's way up to the head is centrifical. Up until now I kept thinking it was that the oil was somewhat compressible because it had some air frothing up some of the volume. Also the sideways force exerted from the timing chain was somewhat canceled by the location and highest pressure of the oil film presumably of the supply oil passageway. That argument falls to bits when the other two bearings and the forces on them are considered though. I wonder what kind of oil pressure the OHC actually recieves. Is that value published anywhere? I'm still waiting for Walmart to send me Miles' book everybody raves about. They list it for $14 but they are very SLOW to ship it.

Ever see any info on the actual oil pressure the bigend bearings of the conrod recieves because similiar centrifical forces must be present there too?
-Keith
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PostPost by: rgh0 » Fri Nov 19, 2004 11:56 pm

Keith

I have never seen any articles or actual measurement of the oil pressure the big end journal actually receives. However your question prompted me to do a quick calculation to detemine the order of magnitude of the pressure boost due to the centrifugal pumping action of the crankshaft oil passages.

At 6000 rpm the speed of the journal is given by the equation

V=2 x pi x r / T

pi=3.1412
r= half the stroke length = .036 metre
T= time to complete a revolution= .01 sec

Thus V= 22.6 metres per second

The centripetal acceleration force at the journal is given by the equation

accel=V x V /r

Thus acceleration is 14187 metres per second squared

Average acceleration of the column of oil from the main bearing to the big end bearing is half of this as the acceleration increases linearily with the radius from zero at the main bearing to maximum at the big end bearing

Average acceleration is 7094 metres per sec sqared

This is equivalent to 722 times the gravitational acceleration of 9.82 metres per second squared

A 10 metre column of oil under gravitational accelration will have a pressure at its base of approximately 15 psi

Thus the .036 metre column of oil in the crank shaft under an accelration of 722 times gravity has a pressure at the big end of approximately 39 psi.

If you add this 39 psi to the supply pressure of around 35 psi in a standard engine at the main bearings you get a supply pressure to the big end journal of around 74 psi.

A lot of other more detailed factors will affect how well the crankshaft works as a centrifugal pump but the calculation gives you an idea of the maximum sort of big end bearing supply pressure to expect.

The actual pressures inside the bearing will be something different again due to the hydrodymanic oil wedge created inside the bearing as it rotates.

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Rohan
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PostPost by: type26owner » Sat Nov 20, 2004 3:45 pm

Thanks Rohan for running the numbers. Factor of two is what I'd guessed beforehand. Can't add the pressure contribution from the oilpump onto the supply for the cams though.

Time to reassemble my twincam today and press it back into service for another 50k miles. This time it should hold up to being flogged on the track without springing a leak somewhere. :)

It appears NSK makes a better waterpump integral bearing which has a roller bearing array on the pulley side that can be kludged into the stock timing chain cover. It would be nice to double the waterpump time interval till failure that the stock unit suffers from. The bore of the pocket in the cover which fits the bearing must be enlarged though. Going to wait to incorporate this modification until the rebuild cycle though.
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PostPost by: rgh0 » Sun Nov 21, 2004 12:20 am

Keith

Dont know if the bearing is the same but the later Ford pumps used a larger diameter bearing also. Fitting this bigger bearing by boring out the casing is a common modification in Australia.

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PostPost by: type26owner » Tue Nov 23, 2004 3:53 pm

Rohan,
I'll look for a Ford integral bearing with a 1/2" shaft? Fords come in different flavors down under in OZ so it may not be available here in the USA. Is this correct?

It appears that a simple failure of the plastic slinger between the bearing and the pump seal is common and could shorten the bearing lifetime somewhat. The supplied slinger splits and allows water to run down the shaft right to the bearing's seal. Made my own slinger out of delrin this time and made it substantial so it would not split apart over time from being under constant stress from the slight interference fit.

Also modified the dipstick to seal oil tight by adding a section which carries dual o-rings into the tube. Damn thing broke in two pieces so was useless anyway.

My baby runs again! :) Debating whether or not to retorque the headgasket. Fel-Pro states this is not necessary except in some rare cases of high performance imports with alloy heads. Called them and they don't have a specific list of those imports. Go figure.
-Keith
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PostPost by: rgh0 » Sat Nov 27, 2004 1:53 am

Keith

Its an integral bearing with a half inch shaft for the water pump end and a larger dia shaft for the pulley end. The bearing itself is both longer and bigger in diameter and the shafts are also longer.

You need to bore out the housing to accomodate the bigger bearing and cut the shafts to the correct length. You then need to bore the pulley adaptor or turn down the larger diameter shaft to mount the pump pulley adaptor. The locating groove in the bearing for the little wire clip is in a different location but leaving the clip out has never been a problem.

I have some old bearings in the garage and will post you a photo when I get a chance. The bearing is from SKF so a search of their catalogue should find it. It was used on the later Kent engine waterpumps in Oz from my understanding but I am not certain of this.

The Elan Factory does the modification here and I am sure Steve Talyor could supply you with more information

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