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RogerH

Engine build query

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Hi Folks,

I am assembling my 4A engine and have come across an odd problem.

 

The liners are fitted and measure up OK.

The crank is in and rotates freely with 0.004"+ end float.

However I have noticed that the three front pistons have a slope on then - sloping forwards and downwards.

The rear one slopes backwards and downwards - by about 4 to 6 thou. It isn't much but does look noticeable.

 

I have checked the big-end area and all seems OK. There is apprx 3 thou clearance

 

Could it be that the con-rods are slightly bent - all four by apprx the same amount (most odd)

Is a slight incline from front to back of the piston of apprx 5 thou enough to worry about.

 

I intend taking them somewhere tomorrow to be measured - not sure where yet.

 

Annoyingly I didn't notice any of this sloppy stuff when I disassembled the engine.

 

What joy

 

Roger

 

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I'd trust your radar on something not being as it should, Roger. Your engineering background and experience with that engine are strong indicators.

 

You're sure the journals on the new crank have been machined in the right places?

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Roger,

Could it be caused by rocking on the wrist (gudgeon) pin? Considering the distance from the pin to the top of the piston,, 3 thousandths clearance might give you five thousandths rock at the crown. Can you change that angle by pushing down on the front and then rear of the piston causing it to rock on the pin? I have never seen wet sleeve Triumph pistons ever marked "front " so doubt installation is the problem. Good luck.

 

Dave

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Roger,

Looking at it as an error in angle it is 5 thou over 3,400 thou = 1 in 680 = 0.0015.

This is the tangent, so we get 0.086 degree. I would not worry.

Peter

 

Edit: 0.086deg would also be the degrees of bend in the conrod, or misalignment of gudgeon pin in either conrod or piston ( or abot of both). ) At roughly 0.1 degree it is surprising the error looks noticeable !

Edited by Peter Cobbold

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Sounds like bent rods, these are all 50 year old plus items now and whether you're bothered or not is down to how picky you want to be. If you were going to be stressing to get the maximum out of the engine or using 6000 revs regularly then you'd probably be bothered enough to try straightening or better replace with another set closer to vertical.

Just for the record the only "tilt" I'd expect from a 4 cylinder piston is across block as the piston hinges on the gudgeon pin, you can pressdown on the piston on the "high" edge with a wooden hammer shaft and equalise the measurement across the piston top, then measure it and that's your piston height. Any forwards to backwards tilt is fixed by the parallel (hopefully) alignment of the piston gudgeon pins and the crankshaft main and big end journals. Given the fine tolerances of the gudgeon pin to conrod bush and the crank journals I'd not expect more than 1 thou front to back tilt even when "prodding" it with the hammer shaft, and not surprisingly the 4 points of piston height from liner top should be the same within a thou when equalised.

The worry is that a rod with a 5 thou bend in it hasn't stopped bending, and whatever has caused it to bend has also implications regarding the metallurgical condition of them ie is it about to revert to putty like properties before breaking. I've only ever broken 1 TR standard conrod, after about 6 secs at full throttle on the long straight at Snetterton (from memory it's about 14 secs between bends). That rod was lightened balanced and shot peened and it broke clean across the oil "sqeazer" hole midway along the length. It had the typical granular appearance on one side showing the crack had been prevalent within the rod before it finally cried enough and the other side twisted and let go, they ventilate the block you know.

Check the crankshaft journals are parallel by strapping a generous straight rod (1") along the crankcase front to rear parallel to the crank (swing the crank and depth micrometer (or Vernier from outside diameters) onto it from the rod ensuring it's set parallel. Then do the same exercise at the front and rear of each journal, will give you a reasonable measurement within a thou to confirm you're not the victim of a rogue grind.

Given you want to give your crank experiment the best shot I would exchange the rods for straight items to ensure uneven stresses do not cause a variable factor in your judgement of the crank.

 

Mick Richards

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Hi Roger

Could you put the rods and piston in the bore the other may round and see if the tilt is the same to the front or the rear, that should prove if the rod is bent?

John

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Hi Folks,

thanks for the replies.

As mentioned by Pete - it isn;t a lot but It shouldn;t be there.

I could say bugger it and finish the build but I would always be wondering - what if!!

 

I don;t really want to fit new rods and would be happy to straighten these if they are bent.

 

What concerns me is the unknown - is it the rods, is it the crank.

 

I'l try to check the BE journals as suggested above

I could easily reverse the rod (if it will fit).

 

Roger

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Roger,

 

Did you fit new pistons? Could the piston crown have been machined incorrectly or the gugeon pin hole machined off spec? Seems strange they are all the same. You could fit an old piston and check the slope.

 

Cheers

Graeme

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Hi Folks,

I did as suggested by John and reversed the rod (do not drive like this). The slope followed the reversal - thus it is the rods that are slightly bent.

 

Hi Graeme,

I had the same thought this morning and check that the pistons are nigh on perfect.

 

My next question(s)

do I run the engine with a slight bend?

Can the rod be straightened ?

If I fit new rods will the crank require rebalancing (do they fit the rods and pistons when balancing.

 

getting closer

 

Roger

 

These look interesting http://www.ebay.co.uk/itm/For-Triumph-TR3-TR4-H-beam-conrod-con-rod-connecting-rod-rods-pleuel-bielle-ARP-/320903092122?hash=item4ab750979a:g:7lEAAOSwxYxU0t7y

Edited by RogerH

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40 years or so ago I bought two 18in lengths of precision ground 1in x 1/2in flat bar. They are oil-hardening grade steel.

These are for laying across things when using dial-indicators and so on.

 

At the time they would "ring together" indicating that the faces were flat and straight.

 

Now they have about the same amount of curvature as your rods. This is very surprising since they have seen no excess temperature nor any mechanical loading. They have simply crept into a banana shape.

 

Straightening rods was common practice in engine rebuild shops.

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Yes, rods can be straightened. My local, very old school, engine rebuilder has a simple but solid rig that can both check and straighten.

 

If you inspect your old pistons carefully you may find witness marks in the wear pattern reflecting the (slight) bend.

 

Nick

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Hi Neil,

the old small ends were in very good condition so I left them in. I had the same thiught that they could have been on the p*ss.

 

Hi Nick,

That's a good idea - I wonder where I put them !!

 

Roger

 

ps - the old pistons are in the skip in the sky.

Edited by RogerH

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Guest ntc

Roger

Looking in good nick does not mean they are.

Edit

Did the pin go straight in?

Edited by ntc

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Hi Neil,

quite right. There were no significant wear patterns on the small ends.

And when the pin was inserted it had a very smooth feel with very slight resistance - a very nice sliding fit.

 

Roger

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Its normal that rods are bent.

 

Either I fit new bronze small end bearings

and ream them to the proper size on the milling machine

that makes the bore hopefully parallel to big end

 

or have a long pin machined on the lathe

to fit into the small end and bend it by hand.

 

Its not a question of degrees in my opinion and not a question

of the revs but a question of the force put on the rod.

 

Normally the piston should push the rod downward

but with a bent rod some force to forward/backward is added

that pushes the piston to the wall and the worst pulls the rod to

the other side bringing bad forces on both small and big end bearing.

 

So its necessary to bent the rods back to normal and at that stage

also check the big ends for out of round condition what is also

common after several years of use.

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Guest ntc

Hi Neil,

quite right. There were no significant wear patterns on the small ends.

And when the pin was inserted it had a very smooth feel with very slight resistance - a very nice sliding fit.

 

Roger

Roger

If the new pistons was stone cold you may have found your answer.

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Roger, I look at this way. One side of the crown is 5 thou higher than the other 3.4inches away (the bore)

As the piston is square in section that means the bottom of the skirt on the same side will be 5 thou closer to the cylinder wall,assuming the height of the piston as a big as the diameter. ( What is piston height?)

At the gudgeon pin it will be about half of that, about 2.5 thou closer to the wall.

The skirt clearance for cast eutectic pistons - not forged- needs to be about 1 thou per inch of bore, or 3.4 thou for the TR, all round the circumference..

So I reckon the piston fits within tolerance, the skirt might be closer than perfect but it is lubricated.

Do you know the actual specified fit - piston diameter vs cylinder bore ?

The man thing is at the gudgeon pin you have plenty of spare clearance, and the above does not allow for the ovality machined into the pistons to allow for greater expansion along the pin bosses.

 

Peter

Edited by Peter Cobbold

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Hi Pete,

Thanks for that. Indeed we are talking about very small oddities.

 

Tomorrow I shall make a gauge to measure and straighten the rods.

It will make little difference but I will feel happier. Couple of hours work and then peace of mind.

 

One good thing about the rods; it took my mind off the engine and let me get on putting the car back together again.

 

Roger

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Hi Pete,

Thanks for that. Indeed we are talking about very small oddities.

 

Tomorrow I shall make a gauge to measure and straighten the rods.

It will make little difference but I will feel happier. Couple of hours work and then peace of mind.

 

One good thing about the rods; it took my mind off the engine and let me get on putting the car back together again.

 

Roger

Hi Roger, Fascinating - the go-to-it confidence of an engineer. Impressive. If I tried that I'd probably add another bend in a different place :)

 

Maybe I should pay more attention to rods in the 6 they are at 280k miles. But I dont do rpm and that's the real killer, the force reversals stretching the rods vastly exceeds any pressure, boosted or not, that combustion can put on them.

Peter

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Hi Roger, Fascinating - the go-to-it confidence of an engineer. Impressive. If I tried that I'd probably add another bend in a different place :)

 

They are very flexibel and need some force to bend.

From that I clamp them at the big end, have the mentioned

pin about 70cm long and bend the small end with it by hand.

After that I trial fit the piston/rod without rings and measure again

from the top of the block to the piston.

 

Always needed several attempts never bended too much.

I always had in mind that things became better than before

and even if its not fully perfect it will be a great progress.

 

I wonder if a engine company can do better without the piston and block at hand.

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If were doing this I would try to avoid applying force via a bar in the journal. There is some risk these will go oval.

 

This is how you straighten things if you have the gear:

 

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I love that guy's craftsman technique - using measurement AND the feel of it in his hands.

But that method won't do for a conrod.

 

This may be crude, but I think is more subtle than it looks.

Tell, Roger, is this going to be your method?

 

 

John

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