I'm having a hard time understanding the TIS systems specs on new vs. wear tolerance for cylinder bore diameter and piston diameter. To be up front, this is in an R60, but it's an N18 engine and this section of the forum seems more lively. Hope I don't offend anyone by posting here.

Using telescope gages and a micrometer, my cylinder bores are between 77.051mm and 77.06mm (converting from inches (3.0335"-3.034"), my native unit). I should mention I'm a toolmaker by trade and know how to use aforementioned tools (hopefully). According to a TIS document I'll have below, the maximum permissible cylinder bore size is 77.016mm, it does not specify that is for a new engine block, implying it is maximum period. If that's the case, I'm oversized a bit (before honing). That same document goes on to say that the "piston running clearance is .033mm-.067mm. BUT, it then says max permissible wear tolerance between piston and cylinder is 0.1mm. Those last two specs seem to contradict each other. .067mm is smaller than .1mm, after all. Why not make the "running clearance" spec .033mm-.1mm? It just makes me think I'm misunderstanding the spec. Wish it was a blueprint.

I don't have the pistons out yet to measure them, but they visibly rock in the bore by pressing on the top of the pistons, and I can see the top piston ring when any piston is pushed one way or another around the piston wrist pin. Makes me think there is at least .015" (.381mm) of gap between the piston and cylinder wall at present, admittedly, guessing by eye on that one.

At this point, I'm debating between the RIGHT way, of having the block bored over, or calling it close enough to reuse the block as is with a hone job (expecting that the hone removes another .001" max of material). It's only .0025" of excess in gap, and the engine has been running fine except for one cylinder, with 160-180psi of compression across all. I THINK the engine has a lot of life left in it without a bore job. BUT, I'd feel a lot better knowing how to read these specifications for certain. If the pistons are worn, then new pistons and rings will only help, obviously. I say obviously because of the previous statement of the pistons being visibly wobbly in the cylinder bore. I should mention there isn't obvious piston slap noise.

The engine has 146k on it or thereabouts.

I should also mention I am here because of an all-at-once continuous misfire on cylinder number 4. I took the head off expecting a valve seat or carbon build up holding the valve slightly open. This cylinder had the lowest compression of the four. After pulling the cylinder head I'm posting these measurements, with the engine in the timing position (mid-stroke), measurements taken above the pistons.

I'm used to V8s and Chrysler inline-6. .003" of piston to wall clearance is fine there. Supposedly, it's not for the N18, but then again, compression as ok and it ran well (excluding misfire), making me think the specs are overtight and reassembling the engine with new pistons and rings is a reasonable way forward.


Sorry for the long post, and thanks for any assistance.

 

I understand your frustration. from your first example, cylinder bore diameter, that's an easy one. The bore exceeds the max limit. From your measurements posted, you don't have below the max; you have exceeded the max wear limit.

The manufacturer won't make the running clearance .033 - .01 because the .033 - .067 is use at the factory during assembly to reject parts supplied by the vendor. The 0.1 limit is for a wear limit during rebuild. For example if you were taking the engine apart to replace another component and the tech has to make a call as to either reuse or replace an associated component. The new and service limits have to be different.

If you have the cylinder head off, get all the valve seats replaced with the deeper versions. If you don't you'll be right back in a few miles pulling the cylinder head again.

My head is in the shop right now getting all new valve seats. On these engines, dropped valve seats are a big cause of cylinder misfires and engine damage.

He're the difference between the OEM and aftermarket valve seats.

 

Ok, perfect. Thank you for the response, makes me feel as though I do understand what that spec was trying to say.

I am over spec on bore. The right thing to do then is bore the cylinders .25mm over and proceed. Sadly, I am dumb, and have already ordered standard sized pistons. I guess I'll measure those when they come in, remeasure bore diameter after honing, and compare the gap to the .1mm max permissible wear limit, and make a decision. As the compression was still well above the service limit, I think I am likely to proceed without boring the cylinders .25mm over, only if I'm within wear limit. At least with your response I'm not making that decision lightly, I can make an informed decision. Even if that decision is not the best one, lots of factors go into it. I can measure ring end gap in the cylinder also to help me make a choice.

Valve seats... yeah I saw I believe it was your post discussing that. I have to take the valves out anyway for wicked carbon fouling. Planned on hand lapping the valves to the existing seats. If I get new seats pressed in, should I get new valves? Or would this hand lapping still work with the old valves?

Thanks.

 

Yes! Always get new valves (all 16) when servicing a N series cylinder head; if you grind them, there won't be enough margin left on the valve and they will burn through (that's from my machinist), and get those deeper seats installed (again all 16); if you don't, you will regret it.

 

Biscuits. Already have $1400 parts into this. OK, thanks for the advice, it is appreciated.