The SR20DET is truly a formidable engine. With its 2.0 liters of displacement and turbocharged from the factory, it has earned its place in automotive history. As people have pushed the SR20 to its limits, weak points in the engine’s design have shown themselves and some of those weaknesses have proven to have been remedied in aftermarket support for the engine.
A good pair of aftermarket camshafts such as BC SR20 cams, Tomei ProCams or Tomei PonCams along with this tricky little mod detailed below is a great way to increase power and create a reliable solution to the “rocker arm and shim problem”.
One of the biggest weaknesses the SR20 has is the valve train design. Surely the engineers at Nissan never anticipated the SR20 would turn into what it has, being the go to engine swap for drifters and circuit racers looking for an affordable factory turbocharged 4-cylinder engine.
The valve train consists of dual overhead camshafts that ride on top of a rocker arm with any slack between the cam and the rocker arm being taken out by a hydraulic lifter fed by oil pressure.
This isn’t a terrible design for a car that spends its life being driven conservatively and results in a quiet, and smooth operating valve train. Nissan didn’t design the SR20 valve train to be used at its limit consistently for minutes on end as most track cars do.
The rocker arm comes from the factory with a single guide shim that sits on top of the valve stem and holds the rocker arm in place from sliding side to side. The other valve that the rocker arm actuates only has a flat shim that is used to level the rocker arm between the two valves so that the rocker arm is not sitting crooked on the two valves it is opening. When RPMs increase the rocker arm moves at an incredible speed up and down.
Because the hydraulic lifter isn’t the perfect component for a high RPM engine the rocker arm can experience instances where it is not being held tight against the camshaft and shims. When this happens the flat shim can be thrown out of the retainer it sits in and then the rocker arm is being pressed down crooked by the camshaft and is no longer level.
This can either result in a broken rocker arm or the rocker arm getting dislodged entirely from its position. Below is a picture of the OEM rocker arm and shim setup.
One solution to this predicament is modifying the rocker arms to accept dual guide shims. This modification will help keep the rocker arm in its place in those instances where the hydraulic lifter is not able to keep up with the high RPMs and the opening and the up and down movement of the rocker arm.
The dual guide modification is relatively straight forward and can be done with the engine still in the car. To prepare for the installation, the only part you will need are 8 dual guide shims available from Nissan (part number 13218-53J00) and a tool to measure the valve stem heights with the shims in place to make sure the two are sitting flat.
We won’t go over the specific steps to remove the camshafts because that has been covered numerous times, but that is ultimately your first step.
Once the camshafts have been removed then the rocker arms are free and can be removed as well.
Be sure to inspect your rocker arms for any serious wear and replace them if necessary.
With the rocker arms removed you can see the guide shim and the flat shim. Remove the flat shims and keep track of which valve the shim came from (this will come in handy later if you don’t have access to the special tool for measuring the final height of the shims to ensure they are flat, see notes at the end of this article).
With the rocker arm removed you can begin the modification process to the rocker arm. You can see how the rocker arm has been specially machined on the side that uses the guide shim, this is what you are trying to replicate.
There are a few ways to go about this but the best way is to first make a mark on the rocker arm as pictured below and get out a new sharp cut-off wheel and a rotary tool. Start slowly removing material from the line down to where the rocker arm sits on the shim. You want this as flat as possible without any curvature in the cut. Cut slowly and measure often. You want the width of area on the rocker arm you are removing material from to be no thicker than 5mm and flat all the way across the face where you removed material.
When you feel you have the rocker modified to fit the guide shim, place the arm of the rocker arm in the shim and make sure it fits. The reason you don’t want any curvature in the area you cut is as the rocker moves around a little bit in the shim it’s possible that the curvature of the cut could press against the side of the guide shim and break it off.
With the rocker arms all modified, place the new guide shims into the area of the retainer where you removed the flat shims. This is where the special tool comes into play. Nissan used to offer this tool but has since been discontinued (and was $400). We simply sacrificed a hydraulic lifter and welded a rod into it that allows us to attach a dial gauge to it.
As mentioned previously, you need to make sure the rocker arm sits flat across the two shims. The only way to properly do this is to measure the height of one guide shim, zero the dial gauge out, move it over to the other guide shim and you will see the difference between the final height of the shims.
Per the Nissan SR20 Factory Service Manual, the difference in height between the two shims should be no greater than + or – .025mm or .0010”.
Depending on your results you may need to remove material from the shims to get the correct specs. All of the guide shims are the same thickness of 3mm. The best way to remove material is to butt the shim up against the side of a grinding wheel and press the shim into the side of the wheel (Be careful. They get HOT quick. Have a cup of water on hand you can drop them into when they get too hot).
You can see on the valve above that the shim is sitting +.008″ higher than the other valve that rocker arm actuates. If we just left it like this, the rocker arm would not sit flat on the two valves and the rocker arm could break due to uneven stresses. So we simply take that guide shim out and remove .008″ of material from the bottom of the shim. Remeasure and verify that you are within spec. For reference here is the section out of the Nissan factory service manual for completing the above steps.
With the shims set, your hydraulic lifters freshly bled (see notes below), and the rocker arms placed on top of the shims and lifters, you can begin reinstallation of the cams.
In the case of this engine we did the dual rocker guide conversion to, we took the head off to surface it, and are installing some Tomei Powered PonCams and a set of BC Racing springs and Ti retainers. When installing the cams, put a healthy amount of engine grease (we use the Joe Gibbs Driven Assembly Grease) on top of the rocker arms and in the cam journals where the camshaft sits. Follow the factory service manual for reassembly.
This engine also got a nice little Tomei Oversized Oil Pan as well
Once you’re done sit back and know that you should feel much more comfortable driving your SR20 powered machine to the limit.
Choosing The Best Cams for your build:
There are a large range or cam specs available for the SR20DET ranging from 258 duration with 11.5mm of lift all the way up to over 28x duration and 12.5mm of lift for high rpm builds. BC and Tomei have both proven time and time again to make GREAT cams for the SR20DET that deliver big gains and when matched with some mild head work/port n polish we see huge gains throughout the entire RPM range as well as a faster spool time from the turbo.
We have also noticed the “center line” on the Tomei cams is VERY close to stock which means you can actually bolt these cams in with stock cam gears and see good results. Many other aftermarket cams will require a good amount more adjustment compared to the Tomei cams to “degree the cams” into spec.
BC 264 degree SR20det cam hydraulic lifter: These cams are good for a mid-large turbo build looking strong power from 3500-7800 rpm. **aftermarket springs and retainers recommended**
Tomei 260 ProCam hydraulic lifter: These cams are good for a small turbo build looking for decent idle and strong power from 3500-7000 rpm. **This cam has been discontinued, please contact us for other 260 duration cam options**
BC 272 degree SR20det cam hydraulic lifter: These cams are good for a mid-large turbo build looking strong power from 4000-8000 rpm. **aftermarket springs and retainers must be used**
This is the SR20 Brian Crower valve spring and Titanium retainer kit we highly recommend for any SR20DET build. These springs and retainers are a great match for an aggressive cam shaft 2 key parts in preventing valve float at high RPM when using a large cam with lots of lift.
**The alternative option if you don’t have access to the dial gauge tool is to take the flat shims out, measure their thickness and modify the guide shim to that thickness (make sure you put the same thickness guide shim on the valve you took the flat shim out of!!!). This isn’t ideal as you can’t tell if the rocker is perfectly flat because it’s possible the shims have worn over time and weren’t perfect when you pulled them out. This will get you incredibly close however and it is better than just tossing the guide shims in without doing any measurements.
Please note we don’t advise this and is a last resort. If you want it done right and can remove the head, we are more then happy to help you convert your SR20 to dual guide shims and any other machine or head work you may need done in the process including porting and polishing, surfacing, valve seat cutting, etc!** ** Now is a great time to bleed your lifters and make sure they are still in good shape. Here is the info from the FSM on how to do that!**
If you’re planning to build your cylinder head give us a call or send an email and speak to one of our experienced engine builders, we build everything from full blown race heads for 9500rpm+ to mild cylinder head builds for daily drivers looking for an increase in power throughout the RPM range.