Thread Rating:
  • 0 Votes - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Crankshaft clearances
Hi All

Sorry for posting several threads on my engine build but new questions come up as I continue. I have the 1 1/2 crank and Nippy rods assembled (trial assembly) in the case and I think everything turns without hitting anything.  I will fit trial fit the block and pistons in due course for a more accurate check.

But by how much should everything clear things.  For example in another thread Parazine suggests 3/32” as a gap between the crank and middle cam bearing.  Currently I have 50 thou!!

I have even less between the button head big end caps and the camshaft but this is more difficult to measure until I get the block and pistons in place.

I was originally working on the assumption that the oft quoted 20 thou for crank whip would be a good start (my 1 1/2 crank shouldn’t whip by this much!)

Advice please


No real answers from me I'm afraid Howard but I'm watching this post with interest.

From where I sit I would heartily agree with 0.020" for crank whip (we all miss a gear now and then); and I'd hope for another 0.010-0.020" at the big end in case I should ever run a bearing at speed.

p.s. I would have measured it for you on my Reliant set up (you may recall I mentioned it is bolted into helicoils from below for exactly the same reason, bolt head vs camshaft) - but I'm afraid I just stripped it all down again last weekend.
With a big crank the challenge is ensuring that the conrods don't hit the cam lobes. This means that axial position as well as whip needs to be considered since the rod may go in between lobes.
On my car I needed to do work on the No3 rod to give clearance. You really can't test it until you have the rods running in bores since the rod needs to move in the right plane.
Also I've discovered that Austin didn't machine the cam bearings very accurately. I've seen mag crankcases with the rear cam bearing 20thou nearer to the crank centreline than the front. This really doesn't help!

I work on 50 thou clearance. The most difficult being to the cam lobes because you have to assemble the engine to find out. There are surprising differences between crankcases - some go together ok, some need quite a bit of work.
Alan Fairless
Years ago when I fitted my 1.1/2 Phoenix into a magy crankcase, the con rods touched the side where the magy indentation was.
I faxed them and they thought 40 thou would be enough.  This I removed off the inside of the case at the offending point.
The motor is in pieces for another reason and there is no evidence of any touching. I have done about 12,000 miles. 
I do drive the car "enthusiastically". Some friends use the word "flog".

When checking  conrod crankcase clearance , I used corrogated cardboard, oiled, instead of the nice new bearings on the crank  in the big end. 
 Cheers, Peter
Hi All

Thanks for all the replies. 

Charles, I’d already cottoned on to the need to check longitudinal clearances. There is not too much space between the lobes on the cam so positioning is important.   What would happen if the crank lip broke is anyone’s guess!

Seems 40 to 50 thou is a reasonable figure.  I should get my pistons this week and can trial fit them and the block to check cam to big end clearance.

It’s all good fun but I am feeling like being in the “fools rush in where angels fear to tread” territory!

Thanks again

For what it's worth Howard I think slow and thoughtful is a very wise approach here. It's all about the details.

Curious to know do all these 1.5" set ups result in big end nestling between the cam lobes? Or is there any overlap? I'm wondering whether any of the rods (Hadley for example) would survive coming into alignment with the cam.

n.b. I've heard of big ends hitting cam lobes, but as far as I'm aware only on 'special' high lift camshafts
The cam catches the rod at this point (see photo) with a high lift cam, it appeared to have enough clearance on assy cases vary quite a lot. If there is not enough clearance a small half moon shape is ground out of the rod.  Terry.

Attached File(s)
.jpg   conrod Renault (2).JPG (Size: 39.51 KB / Downloads: 195)
Thanks for that Terry
There’s a very distinctive tinkling sound as the revs increase when you get it wrong. Don’t ask how I know that.
Alan Fairless

Forum Jump:

Users browsing this thread: 1 Guest(s)