Jump to content

BMC 1.5 crankshaft pulley bolt.


GBW

Featured Posts

Be very careful.  The BMC 1.5 crankshaft pulley includes a torsional vibration damper.  Adding mass to the crankshaft, whether by modifying the existing pulley or by adding a separate pulley on a special bolt, will change the vibration frequency and may make the vibration damper ineffective, leading to a failed crankshaft.

 

The crank pulley bolt snd the crankshaft thread is also not meant for transmitting torque.  That is why the pulley has a a key. An extra pulley on the bolt alone will be placing all its inertia loads onto the threads, and, depending on the size of the pulley and the things it is driving, may well come loose.

 

N

Link to comment
Share on other sites

The extra pulley was added by the marinising engineer (Norris & Son).  I added a further twin groove pulley for an additional alternator.  The original extra and my additional one are all pinned together with dowels to take the torque.  (The thread is right hand anyway so a screw under load would undo.  There is/was an option for a starter dog).

 

The engine has run in this fashion for five years.The crankshaft is still intact (!) but I don't know if there is additional unbalance or not.

Link to comment
Share on other sites

The official marinisation of the 1.5 did have an additional SMALL pulley to drive the Jabsco raw water pump so that is normal. I am sort of with BEgo in that seeing how readily THE 1.5 crank snaps when the damper is damaged I would be wary. However, as long as any new pulley id driven by the original pulley hub and not the "flywheel" section I suspect it may not cause any damage in the shorter term but a heavy load on the front of the crankshaft must alter the shaft's natural vibration frequency. The OD of the starter dog (if it was ever fitted to a diesel as an actual starter dog rather than a big bolt already in stock) is not that much smaller than the additional pump pulley.

Link to comment
Share on other sites

I have a spare engine in rebuild.  That has a harmonically balanced pulley but that currently fitted to the boat has not.  Interesting.

Perhaps with low revs, <1500, the engine will be safe.

Link to comment
Share on other sites

33 minutes ago, GBW said:

(The thread is right hand anyway so a screw under load would undo.  T

If you have, say, a hefty alternator  being driven by the pulley and you shut the throttle smartly, the inertia of the alternator will, briefly,  cause a load reversal in the drive.  This would tend to undo the bolt.

 

Not a problem to the OP as the dowels will take the load.

 

 

Taking up Tony's point.  Most additional pulleys are added in pursuit of the modern fashion for being able to generate multiple kW.  That means big pulleys, with high mass and significant inertia.  The BMC 1.5 crank is known to be prone to  failure from TV induced fatigue.  I would want to keep well away from anything that might make it worse.  Not having failed yet does not mean that a fatigue crack is not developing.

As far as the effect of keeping the revs down, I do not know what the critical speed range or frequency of the BMC is.  TV is normally more of a problem in 6 pot motors, where the 6 x speed frequency can be critical.

 

N

Link to comment
Share on other sites

7 minutes ago, BEngo said:

As far as the effect of keeping the revs down, I do not know what the critical speed range or frequency of the BMC is.

 

All I can say is that one of ours snapped its shaft because of a damaged damper on an engine that had the maximum speed stop screwed well down so the boat could not exceed the river speed limit. None had rev counters so no idea about the actual speed but probably around 1500 rpm.

 

While Torsional Vibration and Torsional Oscillation are the same thing I feel Torsional Oscillation describes the phenomenon in a slightly easier to picture way.

Edited by Tony Brooks
Link to comment
Share on other sites

Its also common for added alternator/air conditioner load to cause failure of the front main bearing.....the loose main plus belt load causes crankshaft snout breakage................you dont need any torsional vibration to break a rotating shaft.,just bending load.

Link to comment
Share on other sites

I have fitted large extra pulleys on 1.5D BMC engines. They were specially turned in aluminium and as much weight drilled out of them as possible before balancing them.

They were dowelled and bolted. onto the centre of the existing pulley after checking that the rubber torsional damper was in perfect order.

The centre bolt was not used to fasten them.

 

I never experienced a crank fracture but did have problems with the weak undersize key and keyway in the pulley. We resorted to fixing them with new tighter hardened keys and with Loctite which made them very difficult to remove. The existing pulleys are cast steel and are not strong enough in the hub for much extra load.

 

Modify at your peril. Calcutt resorted to using the flywheel for drive to extra alternators possibly because of this known weakness but then belt replacements become very labour intensive.

Link to comment
Share on other sites

Assuming (!) the 1.8 litre petrol engine (as fitted to MGB) has the same crank, it seems the thread may be 5/8" X 16.  This is close to UNF (which is 5/8" X 18tpi).

Searching suggests UNS (5/8" X 16tpi).

Finding a die for this has so far proved impossible.

I will have to resort to thread cutting.

Edited by GBW
Link to comment
Share on other sites

1 minute ago, GBW said:

Assuming (!) the 1.8 litre petrol engine (as fitted to MGB) has the same crank, it seems the thread may be 5/8" X 16.  This is close to UNF (which is 5/8" X 18tpi).

Searching suggests UNS.

Finding a die for this has so far proved impossible.

I will have to resort to thread cutting.

It is not the same crank AFAIK.  5/8" UNF rings a bell, 30 MM AF but I could be mistaken. Its been awhile.....................

Its a hardened HT bolt, can you cut and harden it?

Link to comment
Share on other sites

20 minutes ago, GBW said:

Assuming (!) the 1.8 litre petrol engine (as fitted to MGB) has the same crank, it seems the thread may be 5/8" X 16.  This is close to UNF (which is 5/8" X 18tpi).

Searching suggests UNS (5/8" X 16tpi).

Finding a die for this has so far proved impossible.

I will have to resort to thread cutting.

 

You need to look at the 1500 CC B type engined  MGB or any other vehicle using the1500 CC petrol unit to find out the likely thread dimensions, not the 1800.

Link to comment
Share on other sites

I doubt that you will retain another pulley with the centre bolt alone. I would rely on fastening it directly to the existing pulley hub with 4 HT machine screws.

The thread in the crank end is not that great. The 1.5D had a reduced size pulley compared to the 1.8D, which had a habit of fretting keyways.

Link to comment
Share on other sites

Its not difficult to make your own tap from silver steel..........i made a 5/8x 10 LH Acme tap to make a new bronze nut for a lathe......someone like Tracy Tools probably have a tap and die maker available.

Link to comment
Share on other sites

Also the Tap and Die Company.  I think they have the means  to cut  every thread known to man.

 

That said, for 5/8 x 16 tpi  I would want to at least rough the thread out on a lathe before getting the final form with a die.  Too much like hard work to do it in one go with a die, and you are certain to avoid a drunken thread if it is screw cut first and die finished

N

 

Link to comment
Share on other sites

Tracy tools can supply the tap and Calcutt can supply the bolt.  I have asked the latter if they stock a long version.

 

Thanks everyone for your help.

18 hours ago, Tony Brooks said:

 

You need to look at the 1500 CC B type engined  MGB or any other vehicle using the1500 CC petrol unit to find out the likely thread dimensions, not the 1800.

MGOC have offered the information that the MGB bolt is indeed 5/8" UNS.  They can supply the damper as well.

Link to comment
Share on other sites

8 minutes ago, GBW said:

MGOC have offered the information that the MGB bolt is indeed 5/8" UNS.  They can supply the damper as well.

 

Are you basing that on the reply you gave some 18 hours ago where you said it was from an 1800CC MGB or is this specifically for the 1500 CC version? The 1.5 and 1.8 diesels have a number of differences so I would be far from confident that bits from one will definitely fit the other. The 1.8D has  a much beefier crank for a start, five bearing instead of three I believe.

 

 

Thanks everyone for your help.

Link to comment
Share on other sites

4 hours ago, Tony Brooks said:

 

Are you basing that on the reply you gave some 18 hours ago where you said it was from an 1800CC MGB or is this specifically for the 1500 CC version? The 1.5 and 1.8 diesels have a number of differences so I would be far from confident that bits from one will definitely fit the other. The 1.8D has  a much beefier crank for a start, five bearing instead of three I believe.

 

 

Thanks everyone for your help.

I've ordered from MG.  I'll let everyone know if it fits!

 

The Calcutt reply was "The Crank pulley bolt has a Unified Thread, I don’t recognise UNS".

Link to comment
Share on other sites

28 minutes ago, GBW said:

I've ordered from MG.  I'll let everyone know if it fits!

 

The Calcutt reply was "The Crank pulley bolt has a Unified Thread, I don’t recognise UNS".

 

Could it be a misheard/remembered UNF. Many threads on those engines were UNF but BMC always seemed to pop and odd one in now and again.

Link to comment
Share on other sites

3 minutes ago, Tony Brooks said:

 

Could it be a misheard/remembered UNF. Many threads on those engines were UNF but BMC always seemed to pop and odd one in now and again.

Easy to check if you have one in front of you!

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.