Loose fit main bg cap in Fiat TC block

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Piotrek125p
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Loose fit main bg cap in Fiat TC block

Post by Piotrek125p » Wed Mar 02, 2016 2:33 pm

Hi.

Yesterday I inspected one of my 2L TC blocks and I notice that some of main bg caps are loose fit, whether it is a cause for concern ?

Sincerely Piotrek.
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Spider 1969
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Re: Loose fit main bg cap in Fiat TC block

Post by Spider 1969 » Wed Mar 02, 2016 7:16 pm

Hi Piotrek,

What do you mean by "loose fit"?

Please explain so I can be of help.

Sincerely,

Charles
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Guy Croft
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Re: Loose fit main bg cap in Fiat TC block

Post by Guy Croft » Wed Mar 02, 2016 7:52 pm

they can be, but making them a tight fit is pretty complicated.

Here is how to fit them.

1. Make sure their numbers correspond to the number stamped on the block!
2. Lubricate the bearings with engine oil and smear a small amount of molybdenum grease into the oil. It is a superb lubricant of itself and will help to retain the oil. Don't use too much - it can dry out and block the oil holes!
3. Fit the crank and caps and tighten the caps lightly with a wrench in the position they naturally sit, do not torque them up..
4. Loosen the caps and spin the crank several times - this will 'settle' the caps in the best orientation to the crank of itself!
5. Torque the caps one-at-a-time and check the crank still turns freely with no tighteness at all.

If the crank 'locks-up" at any point, either the crank is bent or the block needs to be line-bored and honed straight!

I have had to get several blocks line bored in the last two years and I have had to straighten many cranks! The limit on center-main runout is no more than 0.05mm..!

Before a TC block can be line bored the main bearing caps have to be nickel-welded and ground to a tight fit. Then the block can be line-bored and then HONED!

In two cases I have done recently the block was twisted. Hard to believe but yes, the whole thing moves in use, you would not believe how much. It is not a solid thing, it is completely elastic when under power and permanent distortion can develop. The most recent one I did was the original block used by Michelle Mouton in a Gp4 131 in the 1970s, if you can imagine. I did this for Rallysport Development. They were reluctant to do it but I insisted. And when the job was finished the crank turned like a dream

It is expensive.

If you follow my regime the block will be OK, I hope!

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Piotrek125p
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Re: Loose fit main bg cap in Fiat TC block

Post by Piotrek125p » Thu Mar 03, 2016 8:49 am

Loose fit-I mean, when You fitting a maing bg cap You need to press it eg. by hammer to fit, in 2pcs. of my blocks I don't need it, I only put it and I don't need use ANY force. In my opinion this is scrap and need rework or replacement. I remember that You wasn't a fan of line boring TC blocks, me too, so I thought that I have two scraps of block.
Your regime is very good know by me, You wrote about it in Your books and I working the same way.
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Re: Loose fit main bg cap in Fiat TC block

Post by Guy Croft » Thu Mar 03, 2016 12:20 pm

Here is one at my works, recently line bored...

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Piotrek125p
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Re: Loose fit main bg cap in Fiat TC block

Post by Piotrek125p » Thu Mar 03, 2016 3:36 pm

GC, could You explain me why M.M 131 gp.4 engine block was twisted ? It has a bearing failure or what ?
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Re: Loose fit main bg cap in Fiat TC block

Post by Guy Croft » Thu Mar 03, 2016 3:42 pm

I cannot say whether it was distorted or not, but I did not like the appearance of the main bearing housings at all. Somebody had worked on them with stones and polishing equipment.

So I did it as a precaution.

In my long experience the TC crankcases are stiffer than almost any other and distortion is very rare. But it can happen. The only reason that crankshafts can bend - is distortion of the crankcase. In most cases the distortion is within the elastic limit of the crankcase material - the block returns to its original shape but the crank undergoes plastic deformation. Of course the crank can be pressed straight......!

But not the block (crankcase)...

Hard to imagine - but completely true...

G
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Re: Loose fit main bg cap in Fiat TC block

Post by Rallyroller » Sat Mar 05, 2016 1:38 pm

Hi Guy

I assume blcok distorion is the reason why most ( if not all) modern engines have bearing ladders, not individual caps. With the reduction in weight of modern engines, improved casting techniques etc the blocks are thinner, lighter, so the bearing ladder improves strength. ( and makes for quicker automated assembly.) Just shows your enviable skills when you make the older technology and design powerfull and strong.

Re Rallysport Developments- are they your engines in the 131's seen in the British Historic rally champinonship?

N.
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Re: Loose fit main bg cap in Fiat TC block

Post by Guy Croft » Sun Mar 06, 2016 3:07 pm

RSD do their own engines..

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