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Overfiled Ring gap 2nd ring TB42 Turbo build

Practical Engine Building

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Hoping someone can give me some honest advice or share experience

I am building a Turbo Nissan TB42, Ive had the block bored to the next oversize and Ive got forged pistons from 'spool' that came in a Carrillo box. I got a manual ring grinder and all was going well on 2/6 cylinders before I went a bit hard on one of the 2nd rings(much softer than the top) and have gone WAY over spec. I am trying to gap both top an bottom to 0.022" in line with the calculation provided by the manufacturer, this ones ended up at 0.032" and so I am wondering if it will be ok being the 2nd ring

Top/2nd Ring Bore x 0.0055 @ 15psi -- 3.799*0.0055= 0.021"


Top/2nd Ring Bore x 0.007 @ 30psi -- 3.799*0.007= 0.027"

Factory ring gap Top&2nd -- 0.0118 - 0.0177"

Now I plan to run 16psi at a maximum so figured 0.022" should be a good gap to aim for.

If it was a top ring I would get a new ring set no questions asked but being a 2nd ring I wonder if i could get away with this? percentage wise its ~%30 bigger than MY spec which sounds pretty scary.

The service manual for the TB42 says standard piston ring gap limit is 1.5mm and have read some similar posts on the form saying 0.003"over is quite negligible but another post read roughly 0.010" over get a new ring set. By that logic i am .005" over the 30psi spec which should be fine... right?

To add to the stress I waited 6 months for these forged pistons to be custom made and a search online hasn't come up with any Carrillo ring sets for my bore size.

Being Australia based I worry how long I will wait to order a new set and that's if I can even grab set off the shelf for my application. I want to do the build as best as I possible/practical but I don't want to wait another month before re assembling unless its an absolute MUST.

I would appreciate any honest advice or experience on the subject, I tend to think I can get away with it and might burn a little extra oil from allowing more oil into the top gap but I'm very worried about the longevity/reliability, blow by I assume will not be affected much if at all due to it being the 2nd ring.

At the end of the day its not a race car, its an old cast iron block with 2 valves per cylinder push rod motor, as much as I want to be anal about the build quality I need to be realistic about what difference such fine details will make.

Thanks for reading


Run them. It won't hurt to run other than some blowby.

i have many years of engine assembly and tuning have tried many things, one thing I hadn't until last year was running larger 2nd ring gaps than what I wanted ( I was forced by circumstances similar to your to run a race engine NoN turbo at 0.035 gap where I was only wanting 0.022 I was actually surprised I had no noticeable issues at all,

Regards Ross

Thanks Jstadelm and Ross, after plenty of research I did end up putting them in as is. I appreciate your extra info Ross, personal experience helps build this knowledge base for novices like myself.


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