Big end clearances

[Dic Doretti]

I hope some of the engine gurus can help. Looking for a reason for low oil pressure I have been measuring the big end clearances using plastiguage, they are white metal bearings so not easy to see any wear (not a TR engine obviously). The clearances are between 0.002" and 0.003" but I do not know what they should be. While doing this I noticed one of the wet liners is seeping, typical of an old car there is always something else needs doing.

                   Cheers Richard

[RogerH]

Hi Richard,

                    what is the diameter of the bearing. The clearance is based on the diameter (I think)

 

Roger

[Waldi]

Richard,

as Roger indicates, it is related to the crank pin diameter, but also if used for racing.

Example: TR6 conrods measure 47 mm, say just under 2”. Nominal clearance: 0,04-0,06 mm. I measured before the engine overhaul, they averaged: 0,094 mm, so 1thou more than what you have. I had low oil pressure before the rebuild, and very good after, but did several other corrections too: new mains, oil pump, plunjer and spring for oil pressure valve.

main berarings have a lower clearance, for the TR6, around 0,04-0,05 mm, although the journal diameter is bigger (58 mm).

Regards,

Waldi

 

[Dick]

Would be interesting to know which car.

I have been chasing low oil pressure on a 1933 Talbot. Was main and cam shaft bearings.

Dick.

[Dic Doretti]

Hello again. The car is a 1949 Citroen Light 15 and the crankpin diameter is around 48mm. The engine was rebuilt for the last owner by a classic car specialist near Montrose in 2014 who noted play in the big ends and dealt with leaking liners, it was a substantial invoice. Then in 2016 another garage dealt with leaking core plugs and liners and another large bill. Now I find the liners are leaking again, obviously wet liner engines are not widely understood. Fortunately I have rebuilt several TR engines. As play was noted in the big ends that was my starting point in the search for the reason for the low oil pressure. I hope it is not the crankshaft bearings, I do not want to get involved with the dark art of white metalling.

                  Cheers Richard

[Ian Vincent]

FWIW the classic Alfa Romeo twin cam engines of the 60s and 70s were wet liner with a large rubber O ring at the bottom. Leakage was not an issue. Neither were head gaskets provided you followed their advice to warm the engine at 1500 rpm for a couple of minutes before driving off. 

Rgds Ian