MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - crossdrilling the crank

Could I please have some comments on the need to cross drill a 5 brg B crank. The car is a reasonably serious racing B. 125 BHP I hope on this rebuild. Would love more but am just beginning. The crank that came out [now bent] was crossdrilled but the new engine builder has never seen the need to do this,The BHP is at the wheels,I read of 200 plus and shake my head in disbelief.....surely not out a 5 port head ??
R F Murray

Hiya

200 at the wheels seems rather a lot, we know what was 150 on ours showed 190 on someone else who does a lot of MGs, we have now seen 160 at the wheels with improvements in terms of chokes at 42mm and redesigned ex manifold and system. Someone in Holland is running roller followers and is very likely to be up above 180 on our rollers!

With regard to crossdrilling, some of the pre 71 forged cranks were crossdrilled but not blanked off on the outer hole so maybe superfluous to x-drill? The idea of x-drilling is to prevent centrifuging of the oil supply at high rpms, ie feed from inside of the journal not the outside. We have seen no signs of oil starvation and do not x-drill the cranks. We have had a few 10,000 rpm mini engines x-drilled!

Take care

Peter
peter burgess

Renton, I think you should be looking at a lot more than 125!! What with your new steel rods as well!

We were getting 115 out of a 1600, but unfortunately only for about 10 minutes!!!

I am just doing 2 1800 race engines, 1840 FIA spec, and 1950 screamer, steel crank and rods, forged pistons etc. Looking at 170+. If you are not getting these figures you will need to get a Burgess head!

As far as cross drilling, I am lead to believe by the experts that it is not necessary.

Colin
Colin Parkinson

Renton, I am definitely popping over to see this car when you get it done!

Simon
Simon Jansen

Huffaker always believed that the bearing problems they had observed on engines run above about 6500 were due to oil starvation, and the ones that used the crossdrilled cranks didn't experience that, so......
Bill Spohn

Thanks for the comments so far. The car is BRU 467 that some in the may remember from the early 90's. Run by Neville Marriner and prepared I believe by McCarthy.
When we pulled the engine apart after running the bearing we found the cam was timed at 99 degrees inlet C/line,probably retarded in interests of road use. It has forged pistons and is 1950.Running about 12.5 to 1 comp.It's all nice inside,polished rockers and lightened pushrods,MGC followers and vernier cam sprocket.It runs a 45 Weber and LCB exhaust. What else do I do.??????
PS the engine builders down here in NZ reckon the horses must be a different breed up there in the Northern Hemisphere.
R F Murray

I forgot to mention the cam is a Kent 719 Scatter Pattern
Perhaps that is where some of the missing horses are, any ideas ??
R F Murray

Hi RF

What size inlet ports have you got? The 719 is maybe a little old fashioned in terms of bhp, what size weber, what size chokes?

All I can say is we have around 20 race engines out there all pulling high rpms, I have never seen a B fail from oil starvation. Pump failure, mechanical overrevving but not oil starvation.

With regard to bhp, it is easy to be sceptical, what you need is bhps from cars we can all relate to, then we can sort of cross correlate. Colin should be in the 135-145 range at the wheels.

When I first worked on TR4s in the late 80s we won a championship with 148 at the wheels on SUs, at the time this car was overtaking some of the full race TRs, we discovered the webered cars were only making about 130 at the wheels when people started using my rolling road ( at first people thought I must be able to work some kind of magic on the rollers!)all the extra power was in the head work, tr4s are now in the high 160s at the wheels with webers and shoving out 130-140 for road use, progress.

Peter

Peter
peter burgess

This thread was discussed between 28/05/2009 and 29/05/2009

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now