loose pre cup in redecked head. Is this a problem?

[m1garand_man]

I had a head redecked recently and I noticed one of the pre cups is loose and can be removed with my fingers. I'm guessing they put them back in the head in no particular order and one ended up in a different hole. I'm not thinking this will be a problem as its still sitting about .002" above the deck surface and will be sealed by the head gasket. The TM doesn't mention anything about loose precups only damaged ones.

Will this be a problem? When they re decked they also milled the cups down separately to have proper clearance so I am thinking that installing one from another head will only cause more problems than good.

 

[cucvrus]

A machine shop with the right equipment for the older IDI stuff will have the proper equipment to adjust the depth of the precup seats after the heads are (presumably) milled. Make sure they mill the heads with the precups out and then adjust the depth of the seat to properly fit the cups - if your chosen shop wants to mill the heads with the precups in place, run away screaming.

As for making them fit tight, if the machining is done properly and the cups aren't beat (and were of proper quality to begin with) they should fit just snug - some people reportedly need to knurl them a little to get them snug if they're fitting too loose for comfort.

This is a note I had from googling this issue a few years back. I hope that helps. I would say they should be tight because they would vibrate and turn in the head. Causing wear as the 2 pieces rub together. Remember if its loose and not supposed to be loose tighten it. Easier now then when the head is on and in place. That is my spin on the issue. I would not install a head with a loose pre-chamber. Do as you wish.

 

[m1garand_man]

A machine shop with the right equipment for the older IDI stuff will have the proper equipment to adjust the depth of the precup seats after the heads are (presumably) milled. Make sure they mill the heads with the precups out and then adjust the depth of the seat to properly

fit​

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the cups - if your chosen shop wants to mill the heads with the precups in place, run away screaming.

As for making them fit tight, if the machining is done properly and the cups aren't beat (and were of proper quality to begin with) they should fit just snug - some people reportedly need to knurl them a little to get them snug if they're fitting too loose for comfort.

This is a note I had from googling this issue a few years back. I hope that helps. I would say they should be tight because they would vibrate and turn in the head. Causing wear as the 2 pieces rub together. Remember if its loose and not supposed to be loose tighten it. Easier now then when the head is on and in place. That is my spin on the issue. I would not install a head with a loose pre-chamber. Do as you wish.

Yeah these guys machined the cups separately from the heads. I'll call them tomorrow and see if the can knurl the cup in question. I get what you're saying about them moving which I could imagine may eventually cause a blown head gasket. I would attempt to peen the cup but am worried that I might cause microscopic cracks if I did it with a center punch unless you guys on here think they are ductile enough that this isn't a problem. I do understand that they are hardened however.

 

[llong66]

While having some issues with a damaged precup recently, one of the manuals, I can't recall which one allows for 0.002" protrusion above the deck surface and zero below. I am am guessing that the protrusion is 1 all the distance even a "crushed" fire ring will seal and 2. anything more than this will keep the head from seating correctly. Part of the precup is inside and part outside the cylinder. I can see where ANY distance below the deck surface will allow for compression loss. I can see the same issue with a loose cup, all JMHO