Does anyone have first-hand experience of having the pushrod tunnels either rebored with oversized followers or re-sleeved? (If that's possible) If they have then where did you get it done and was it worth it?
Norvil was offering this as a service fairly recently.
I see that Norvil do resleeving but do they do reboring to +.020" as they sell the oversize cam followers? I thought that RGM used to but can't find it on their website.
Restoring a 1973 850 mk1 Commando and have just had tunnels with local engineer rebored with +020 which I bought from andover. The cam followers were badly scored as were the tunnels probably grit left in there from previous rebuild shame as the cam follower tips were like new.
Does scoring really matter? They don't have much side thrust, and the oil from the head has to run down somehow.
You can recover scored undersized pistons by having them coated, worth keeping those followers in case you need another set.
Probably casting sand left in the followers from new (See NOC Service Notes). I can remember picking sand out from inside new followers with an old scriber...
I think it was definitely something like that.
...so why do they need to be replaced?
The alternative to casting sand is that someone decided that they had found a use for the coarse end of the grinding paste tin when lapping new followers in !
I had similar issues with mine years ago, caused by a PO leaving blasting media inside the crankcases, after having them blasted to look pretty. A wire brush can make them look just as good without the risk. It wasn't just the followers, but the bores, pistons, big end & main bearings, crankshaft journals, camshafts, camshaft bushes, valves & guides, oil pump; get the picture? If it is caused by casting sand, it would have been dragged through the whole engine, not just confined to the follower tunnels.
Why change them? The engine will be noisier with worn followers, it will rattle like a basket. An engine builder worth his salt would just not be happy putting those back in.
You will find if you check that the cam followers are perfectly ordinary cast iron , there is absolutely no case hardening applied
to them at all .
Fair comment. Post amended!