The MX5 / Miata engines used two types of lifter system, hydraulic and solid. Hydraulic lifters are a great option for a road car as they keen the valve train quiet, they keep the gap between the cam lobe and the lifter tight, but not to tight.
Why is that important? Well, if that isn’t in control then the valve train gets noisy as it slaps into the lifter taking up the gap quickly as the lobe comes around – not great for wear let alone being very annoying.
Hydraulic lifters work by pumping themselves up via the engines oil pressure. If you have good clean oil and good oil pressure across the whole rev range, the system will be working efficiently. If not, there’s a risk you might not be getting the maximum designed lift – allowing less valve lift, less air into the cylinder and less power.
With the 99-05 engines (BP4W / BP6D aka VVT and the NB 1.6 engine) Mazda moved to a solid lifer system, which doesn’t rely on hydraulic pressure to keep the lash in check – but a coin shaped shim of a specific size.
The valve train grows and shrinks with time, the valve bashing into the seat over time will mean the lifter goes ‘up’ closer to the cam, making the lash tighter. Over time the lash can get so tight that the valve never properly closes.
It’s important, if you’re building an engine for any sort of power, to check the lash on the intake and exhaust valves.
Greg at The Car Passion Channel has documented in great detail the process of checking and fixing valve lash issues – solving a cold running issue on his 11k mile freshly rebuilt engine, highlighting how much the lash changes over time.
So, who said checking the lash on your cams was hard?
Some additional food for thought here, the MX5 / Miata uses 27mm valve shims / lifters, when moving to move aggressive camshafts (like the cat cams units we offer) where applicable some require a larger lifter as the lobe of the cam is physically larger and with enough RPM will pop the shim out of the lifter sending it flinging through the head! (A common problem on RB engines interestingly).
A good solution is to move to a shim under bucket system.
More detail here; https://www.thecarpassionchannel.com/lash
Get your shims here;