The all-new Front Page News Feed is now live   Read the announcement
TRExp

TR4 & TR4A Forum

TR4A TR250 Vented Oil Cap OC-5 and PCV System

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

andy303 Avatar
andy303 Gold Member Andrew Blackley
Chardon, OH, USA   USA
Nobody asked me but....

I have been doing some research on the PCV system used in the TR4A which uses the Smiths "Mushroom" diaphram valve also found on other LBC cars during the early years of engine pollution control.
The function of the valve was to draw crankcase gases into the inlet manifold to be burnt instead of letting them escape onto the road using the draft tube fitted to the TR4 and earlier TRs.

How the valve functions is well explained here: http://mgaguru.com/mgtech/engine/cv103.htm

As Barney Gaylord explains it is a balancing valve that functions based on manifold vacuum. A critical component of this system is a properly vented oil cap. It has a vent hole of a precise size (about 7/64" or 0.11" ) to control the amount of fresh air drawn into the engine.The correct cap for TR4A engines is an AC part number OC-5, Triumph part# 143393, which is NLS (no longer stocked). The cap sold as a replacement is for the closed system and I do not believe it has a vent hole. This can lead to a rough idle or worse, as described in the link.

I recently bought an original cap on eBay, which I took apart to see how it functions. The pictures are posted below. It has metering hole through the cap (.11" dia.) that draws air in through a plug of steel wool. This is contained inside a cup on top of the cap, the cover of which can be easily pried off for cleaning. On this particular cap the steel wool was clean and dry, but a bit brittle. The very thin gasket on the bottom is hard and brittle and will have to be fixed with a new bit of rubber gasket material.

With the introduction of the TR250 it appears that Triumph went to the sealed cap that is now sold. Instead there was tee used on the hose connecting the valve cover to the inlet side of the valve,with another hose connected into the back of the air box. This early air box is different from those used on later TR6s in that it appears to have metering restriction inside the air box. All of which leads me to think that if one cannot find the correct cap then a similar tee and hose connected to an air filter but using with a proper restriction could substitute for the OC-5 cap.



Edited 2 time(s). Last edit at 2019-02-28 07:22 PM by andy303.

. Hide this ad & support a small business by becoming a Gold Supporting Member

Attachments:
IMG_2741.JPG    33.8 KB
IMG_2741.JPG

IMG_2739.JPG    29.6 KB
Sign In or Register to view this photo
IMG_2740.JPG    19.7 KB
Sign In or Register to view this photo
Was this post helpful or interesting?
Yes No Thank
andy303 Avatar
andy303 Gold Member Andrew Blackley
Chardon, OH, USA   USA
Here is a pic of the TR250 system and an early, correct air box with the hose fitting.

Piggott has a good picture of it in situ on page 78 of Original Triumph TR4 etc.


Attachments:
GRID200274.jpg    26.3 KB
GRID200274.jpg

156268-1-1000x750.jpg    24.2 KB
Sign In or Register to view this photo
156268-2-1000x750.jpg    34.4 KB
Sign In or Register to view this photo
Was this post helpful or interesting?
Yes No Thank
. Hide this ad & support a small business by becoming a Gold Supporting Member

To reply or ask your own question:




Registration is FREE and takes less than a minute


Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster