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Taper tolerances of main journal bores when measuring for bearing clearances

Practical Engine Building

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I've had an align bore performed on my Honda B18B block since I'll be running an aftermarket girdle and arp main studs to ensure minimal distortion.

With the main caps installed and no bearings fitted, I've measured the main journal bore to size up the main bearings correctly. When measuring both sides of the ID of each main cap (ie each side of the oil supply gallery in the block) I've found taper between each side by as much as 0.8 thou.

Each engine will have its own set of limits in terms of taper and out of round but what is would be a rule of thumb tolerance in this instance? After an align bore it should be pretty close to perfect I would have thought. Should I take it back to the machinist?

If you've had the main bearing journal line honed then it should be perfect. A taper of 0.8 thou is massive and certainly not something I'd expect or accept from a freshly honed bore. This sort of variation will affect both the crush on the bearing shells (which is essential for proper shell retention), and correct and accurate control of oil clearances. I'd be talking to your machinist.

Great, thanks Andre!

Hi Andre and everyone

If everyone doesn't mind I thought I'd add on to this thread as I've got a similar issue as above and thought it would be useful to keep this info in one place .

I have a honda B18C block I'm building up for 9000 rpm NA use.

The issue is torque on the ARP main studs. When receiving the engine from the machinists, they have changed the torque on the main studs from ARP setting 80ftlb to 50ftlb. They say the reason is 80ftlb distorts the block too much. I would have thought this shouldn't matter as the line hone should have removed any distortion?

The honda factory torque is 49ftlb for the center 3 mains with the girdle and 56ftlb for the number 1 and 5 main caps. So the new torque is less than factory.

I'm now in a predicament as to what to torque these to. I'm worried I'll distort the holes if the machine shop line honed at 50ftlb and I go higher but if I only torque to 50ftlb they may come loose and the extra clamping benefits of the fastener will be wasted? Will it need extra clamping than factory? Power goals are around 170kw and roughly 9000rpm.

I've only plastigauged at this stage as I can't get to measurement tools for another couple of weeks however after much mucking around have used factory honda bearings to get 0.0014" clearances on all of the mains except the front. The front is 0.0015" mostly but tapers toward the very front of that journal to around 0.0020".

Using all same bearings before adjusting to achieve equal clearances the taper through all of the main journals front to rear seems to be 0.001". Is that acceptable accuracy for a block that's just been freshly line honed?

I'm a little hesitant to hit the machine shop up because I'm not an experienced car engine builder and not sure what's acceptable. Also worried they aren't going to take me seriously with just the plastiguage measurements at this stage. All measurements have been done with the crank and bearings dry as per your plastiguaging video. Crankshaft has be orientated in the same position for every measurement. I've not plastiguaged with the higher torque yet but even if I had It won't show me if the mains are still round?

Will higher torque on the mains distort the roundness of the piston cyclinder bores given the aluminum open deck design?

Sorry for the novel, just not sure where to go from here.

Cheers Andy.

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