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RB25DET Valve lifter/Shim Clearance problem

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Hi Guys,

I just got an RB25Det Neo Head back from the machine shop and had it faced and also got 3 axis cut valve seats. I am about to install Tomei 260 cams but I now have no clearance at all between the lifter shim and the cam and I need 0.45mm for the intake and 0.38mm for the exhaust side. I very much doubt i can machine down the shims that much. Can anyone give me any idea what are the options I might have please ?

Mark

Is there enough material in the valve stem above the retainer to have the valves shortened the necessary amount?

https://www.hpacademy.com/forum/engine-building-fundamentals/show/adjusting-valve-stem-length

Hi David,

I will check this, thanks for the tip.

Mark

About to suggest the same thing. Something to bear in mind is because the distance between the spring seat and retainer is going to be larger, there will be a reduction in the force the spring is applying to the valve - this can result in loss of valve control and possible valve failure. There should be a spring seat 'pressure' and/or installed spring height specified but that can be difficult to find and/or measure, so if you can simply establish how much the spring length has increased, you can use spring shims on the head to correct it.

As implied above it sounds like the seats have been cut quite far into the head when they probably should have been replaced.

Taking material of the top of the valve stem above the retainer would give you your clearance back but take care not to overheat the valve stem as this would temper the steel changing the characteristics of the material.

Gord is quite correct and removing that much material form the valve seat will result in the the spring seat poundage reducing bringing on valve bounce earlier etc. Shimming to the same installed height as OEM is much better than nothing but ideally the head builder/machinist should be shimming to seat poundage using a proper spring tester or at least telling you how much they've had to remove from the valve seat so you can compensate. You also need to pay attention to not increasing the seat poundage too much as this will move the result in running closer to coil bound, broken springs and dropped valves.

I know that some of the Tomei cams for the RB26 have a bigger base circle that requires special Tomei lifters. On the top of my head, it's the pro-cam. Did you check if that's your case?

Thanks guys, yes the seatings were cut too deep in my opinion which resulted in creating these other problems. I think i will go to shortening the valves and will re shim the springs to try to get to the correct poundage.

Frank, I dont think its the case as the head was used before with the same 260 tomei cams but after it was desmantled it was taken to a machine shop for resurfacing and seat correction and came to me with this problem.