Block line boring for camshaft

[john.r.davies]

Perfectly happy with the used (HiLift) camshaft I want to fit.

And I gave him the bearings I want fitted, so measuring them will be easy. 

The question remains - what should the bore diameter be?     The bearing diameter, clearly, but slightly less to allow an interference fit, without distorting the shell.     The question is, how much less?

There are equations to calculate the properties of an interference fit, but they are fierce, like this:

 for the "Inner Body Poisson Radial displacement"

and I don't know how to apply hem.    I hoped to find someone from the Engineering Faculty of TR University who would be my tutor to take me through them! 

The short cut is Rules of Thumb, and I have found an RoT of 0.7-1.5 thou per inch of diameter, suggesting 1.2-2.5 thou, whihc takes me back to exactly what the machinist said, "One, or Two thou?"   

John

PS RogerG,  Yes, my DoH!  I was on my mobile and missed that the decimal points were missing!

 

Edited July 17, 2021 by john.r.davies

[John L]

The original question arose when I fitted a new camshaft, there was some play on the front bearing, and a bit in the others, but didn't think it was that bad, the old cam had 2 shorter lobes was the reason to change it out, but the old oil pressure wasn't really that bad to require boring I thought, having fitted a new chain as well on the new cam the chain was a really good fit with no real deflection on the non drive side.  having now run the engine for some miles the oil pressure has dropped a lot at idle now down to about 15psi hot. Unfortunately I didn't think to measure the new cam to the old on the journals, but I did find out later that the Newman cams do have larger scrolls in the bearing surfaces.

So the question originally if there was some wear in the front bearing, and the timing chain was a really good fit so thinking the bearing could be oval and thinking the wear would be on the crank side of the journal how would they true up the alignment, and would now the new chain fit or will it be too tight?  Perhaps they use the rear bearing to align the machine, as there shouldn't be so much wear that end perhaps?

Still waiting to be able to bring the block to UK, as I'm really not sure that I can get a quality job done here unless any body here could recommend somebody, always worth looking into as would like to get the job done.

John

[Andy Moltu]

I always wondered what stopped the cam bearings from rotating when fitted to the 6 pot. Given that mine were not significantly worn, I resisted the temptation to fit them for the sake of it, just because the engine was apart.

The 4 cylinder TR engines have screws/pegs that pass through the block wall to hold the bearings to hold them. But if the shells were to rotate a few degrees the oilways would be obstructed with the inevitable consequences.

I’m not convinced too many machine shops would have the knowledge to know what size to machine to.  Likewise John’s equation might test the maths of the average machine shop operative! Let alone the Poisson required to do the calculation. (Spell checker effect I suspect)

[john.r.davies]

Thanks for that nugget of knowledge, Andy!    Never owned a TR 3/4, so ignorant of that, as so many things.

And as said, I don't have the knowledge to know what a "Inner Body Poisson Radial displacement" is,  but I do know that Siméon Poisson (1781–1840) was a great French scientist who made many contributions to statistics, physics and engineering.    I think his displacement equation concerns squeezing a hole from the outside, and may be applied to this problem, or a railway tunnel!     

So no missprint! After all, the UK had a great meteorologist, Michael Fish!

John