A Quick Word About.......DX Valves

One of the main changes between the DX2 (introduced for the 1969 model year in September 1968) and the DX engine that came before it, is different diameter inlet valves. Up until September 1968 they were 47mm. After that, they were 49mm. This change, possibly together with an uprated carburettor, is one of the factors that boosted the 109 bhp of the DX engine to a whopping 115 bhp on the DX2.....

Having pulled the valves from my several cylinder heads, I started to notice a number of other differences and, as a consequence spent long nights trying to get my head around the subtle changes to DX valves circa 1967/ 1968. It was something of a crossover period......

The valves on a 1968 DX engine are different to those used on the 1969 DX2 engine and thereafter. In fact they are also different to the valves used on the DX engine before May 1967. What I'm calling 'early' and 'late' DX. I needed to (wanted to?) get my head around all of this before I could crack on with fitting valves.

From my parts and the various workshop parts books and repair manuals, the first thing I discovered was that up until May 1967, the inlet valves didn't use valve seals as we would know them. Instead, they had a small plastic ring at the top of the stem - up by the cap and collars. The cap and collars were also different: on a late DX, and on DX2 inlet valve, the collars are 'ribbed' and match to grooves around the end of the stem. On an early DX valve, the collars are 'plain'.

Early and Late DX Inlet Valves: Plastic Ring. Different Collars

To accomodate a conventional valve seal, the valve guides on late DX and then DX2 engines have a shoulder - against which the valve seal is seated. As an early DX inlet valve (pre- May 1967) doesn't have a conventional valve seal, the valve guide doesn't have a shoulder.......From May 1967, the valve guide changed to ones with shoulders.

The area around the base of the valve guide is also different - with the early DX head having a distinctive step. As a consequence, the thrust washers that sit on these - and the inner valve springs - are different to those on a late DX and DX2 head. That's a bit of a minefield with several changes during 1967. Best not to go there......

Early DX Inlet Valve Guide - Double-Stepped base. No Shoulder on Guide

Late DX Inlet Valve Guide: Single Base Step. Guide has a Shoulder for a Seal

Exhaust valves: unlike the inlet valves, both the early and the late DX exhaust valves did have 'ribbed' collars that match to grooves around the end of the stem. In this respect they are similar to the subsequent DX2 exhaust valves. But not the same........A DX exhaust valve is 9mm along the full length - including the collar area. On a DX2 exhaust valve, the stem is 9mm diameter along most of its length, but ribbed collar section reduces to 8mm. That change seems to have happened for September 1968. So a DX2 cap and collar won't fit a DX exhaust valve properly. Remember that if you lose your DX collar and need to buy replacements - they will almost certainly be for a DX2.

DX and DX2 Exhaust Valves: stem end is smaller

Still with me? Okay, nearly finished.

DX exhaust valve guides: Prior to May 1967, the guide had no shoulder and valve seals weren't fitted - at all - not even a plastic collar on the end of the valve stem. Exhaust valves seals seem to have been introduced in May 1967, along with a change on the valve guides - presumably to guides with a shoulder. Seals are described in the relevant parts book but not shown in the parts picture. Because of the stepped base, thrust washers and inner springs before May 1967 are different to those on late cars.

Early DX Exhaust Valve Guide - Base is Double-Stepped

Late DX Exhaust Valve Guide - Flat Base. Shoulder for a Seal.

I'd already bought a set of replacement valve guides from Citroen Classics back in summer 2015 without even considering they might not be standard for all engines. With the vast majority of reproduction parts geared to DX2 engines, my conclusions over the implications for my 1968 DS21 were as follows:

DX2 valve guides - good thing
DX2 valve seals - good thing.
DX2 valves - bad thing.

I could replace the valve guides and fit new valve seals, but I should really try to press my valves back into service.

Technical details about replacing valve guides can be found in Operation D.112-3 in Section 2 of Volume 2 of manual 814, however it's a specialised job if done right.