Joined: Dec 2019 Posts: 34 Threads: 14
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Location: Somerset, England
Car type: Austin 7 Opal 1936
I need some guidance please. I have dismantled our 3B engine to renew main bearings. (Crank crack tested, all OK). Those familiar with this engine will know that each con rod is offset on the big end bearing, presumably to accommodate amendments to the crankshaft to make space for the centre bearing while using the original bore spacing. On our engine the rods do not line up with the centre line of the cylinder bore and I'm wondering if the rods have been fitted the wrong way round. Viewed from below, currently for cylinders 1 & 4 the big end cap bolts are furthest from the centre bearing. For cylinders 2 & 3 the cap bolts are closest to the centre bearing. Is this right? Any help gratefully received...
Cheers from Chris and Hilary
Joined: Mar 2015 Posts: 5,481 Threads: 231
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Location: Scotchland
The thinner side of the big end goes towards the front, centre and rear main bearings respectively.
Joined: Dec 2019 Posts: 34 Threads: 14
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Location: Somerset, England
Car type: Austin 7 Opal 1936
Thanks for the quick response, amazing. The photo from Chris KC is what we have. Is that what you described Ruairidh?... Thanks again both..
Cheers from Chris and Hilary
Joined: Mar 2015 Posts: 5,481 Threads: 231
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Location: Scotchland
It is - a picture saves a thousand words, thanks Chris.
Joined: Aug 2017 Posts: 3,436 Threads: 108
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Location: Darkest Bedfordshire
Ladybird you don't say by how much your rods are off bore centre line - but if it makes you feel better I run a 3-bearing with Reliant crankshaft and that results in significantly more offset without any issues - not for sporting road use at any rate.
Joined: Dec 2017 Posts: 1,160 Threads: 68
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Location: Nottinghamshire. Robin Hood County
Car type: Austin Ruby Mk1 1935
Chris, Could these mean Ladybird also has a reliant crank fitted.
John Mason.
Would you believe it "Her who must be obeyed" refers to my Ruby as the toy.
Joined: Dec 2019 Posts: 34 Threads: 14
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Location: Somerset, England
Car type: Austin 7 Opal 1936
My crank looks exactly like the one in the picture above so assume it to be Austin. I can’t measure the offset easily now but noticed the issue during dismantling. Viewed from below I could see that the (clamp on type) little end was showing about 10mm of gudgeon pin on one side and virtually none on the other. As the pin is relieved centrally for the clamping bolt I feared that would mean that the pin was scoring the bore but this wan’t the case, the pin seems short enough to accommodate the offset. Still looks odd though...
Cheers from Chris and Hilary
Joined: Aug 2017 Posts: 3,436 Threads: 108
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Location: Darkest Bedfordshire
13-11-2020, 07:09 PM
(This post was last modified: 13-11-2020, 07:09 PM by Chris KC.)
Good thought John.
The Reliant crank is usually fitted together with (slightly modified) Reliant rods.
It has been done with A7 rods though, notably the 10,000 mile record car (I think for the record attempt it ran with custom made rods from Paul Bonewell but it later reverted to modified A7 rods i.e. with some added offset).
The presence of an Austin part number (as above) I guess is the quickest way for the unfamiliar to confirm.
Joined: Aug 2017 Posts: 2,748 Threads: 31
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Location: Auckland, NZ
It seems highly desirable that the pistons are centred. Otherwise the rods are subject to a bending action not intended for. However many have been wrong due confusion about front bearings, and alsp due poor machining of remetalled rods.
Incidentally much is made of the right and wrong way around for early conrods but as these later have gudgeon bolts facing both ways presumably it does not really matter.
