Main and Big End Bolts

[Rob72CP]

Another question from me, this time about the use of thread lock on mains and big end bolts.

 

My engine originally had no lock washers or indeed any type of washers on these bolts. The consensus appears to be that a bit of thread lock is advisable on the big end bolts but not required on the mains. Do you chaps agree?

 

Is is OK to lightly oil the main bearing bolts before fitting? (hope so as I've already done so!)

 

I've just successfully set the crank endfloat at 5 thou using a mix and match pair of new thrust washers and will be fitting the pistons and rods tomorrow evening all being well, so it would be useful to get some views on whether to use the threadlock on these bolts or not.

 

Many thanks!

 

Rob

[charlie74]

i replaced my main and big ends recently and was told by the roadster factory tech help gent that the big end bolts had to be replaced with new (mains should be also but i reused mine), torqued in stages of 25 lbs/ft at a time and assembly lube should be used.

he did not mention anything about thread lock and i did not use any. it sounded to me that the torque was the important thing.

i don't beleive there are any lock washers, mine did not have any and i haven't seen any in illustrated parts catalogues etc...

[RogerH]

Hi Rob,

the early 4 cylinder engines had lock washers on the big-ends (not sure about the mains).

The later four pots did not have the lock washers but had bolts that would stretch (ever so sligtly) under the torque load to the correct tightness. These required no thread lock etc.

 

I believe the 6 cylinder engines use the same stretchy bolts.

 

Roger

[88V8]

My engine originally had no lock washers or indeed any type of washers on these bolts. The consensus appears to be that a bit of thread lock is advisable on the big end bolts but not required on the mains. Do you chaps agree?

 

Is is OK to lightly oil the main bearing bolts before fitting? (hope so as I've already done so!)

 

 

It's OK to lightly oil the main and conrod bolt threads, but keep it to a minimum just to ensure that the threads are free-running. Don't get lube on the back of the shells or on the mating faces of the caps. One should always be cautious about putting oil into blind holes as hydraulic pressure can split the housing.

 

As you know, threadlock can only be used on dry (unoiled) threads.

The bolts (which should be new) will not undo if they are torqued to the correct value. As Roger says, the specified torque will slightly stretch them and it is this stretch that locks them in place, so threadlock isn't needed. Confusingly, Haynes refers to 'locking tabs' although the BB does not and there are none in the parts book.

 

Ivor

[Steve Knight]

Thread lock is not required - I guess it would do no harm. No washers is OE and correct, but not sure why, as opposed to earlier spring washers. I know the correct advise is to use new bolts; but re-fitting old bolts on Triumphs is perfectly okay. Not anywhere have I seen they are considered stretch bolts. Normal S / 8.8 high tensile bolts do not stretch. It is only on some modern engine applications; the actual mechanical reason? I would have to research; could be to get the exact torque setting - whereas a wrench reading can vary due to oil or thread drag. Do not oil up the threads, especially in blind holes, as said, hydro-locking could be a big issue! Just make sure the bolts are not completely dry. Head studs are another blind hole installation, and some say a fine cut up the thread is advisory. Assembly lube is just for the Main / Big end bearing surfaces, which will give extra protection during first start up. Also used on the camshaft and followers, before a good flow of oil is assured.

[Rob72CP]

Thanks everyone, as ever this forum has provided all the required answers, info and opinions. It is greatly appreciated.