530/531 Head Oil Gallery Plug Sizes

I've searched multiple threads for about 2 hours and I can't find what the sizes are for the front and rear oil galley plugs on a 530/531 head. I have 2 heads, a 530 and a 531. The 531 came with no plugs and the 530 head has them but when I tried removing the front plug the allen head stripped the plug.... i don't even want to try to remove the rear plug.

Any ideas?

1/8” npt

Much thanks!

I'm curious...did your 531 have a plug in the hole under the #4 intake runner, in the water jacket? Having flashbacks to plugging this hole the night before I was all set to install the head...used the plug from a 530 head, but the marine 531 head didn't have any threads in the hole, so I used the plug to cut the threads and then RTV'd it just in case. What a disaster...but it sealed and worked out fine.

The hole in the coolant jacket under port no. 4 on the 531 is a hangover from that heads use in the European market B230ET.
There was an extra sensor (a thermal time switch) in that hole for the Motronic ECU. It uses a M14x1.5mm thread. Or none at all on the marine engine I guess to save money!

towerymt said:

I'm curious...did your 531 have a plug in the hole under the #4 intake runner, in the water jacket? Having flashbacks to plugging this hole the night before I was all set to install the head...used the plug from a 530 head, but the marine 531 head didn't have any threads in the hole, so I used the plug to cut the threads and then RTV'd it just in case. What a disaster...but it sealed and worked out fine.

Click to expand...

It did not, however thanks to a thread you and a couple of other members were on i was able to determine what size tap and npt plug i would be able to use. The thread was difficult to find though. I also had to to drill and tap the boss for the ECU temp as this was a marine 531.

crogthomas said:

The hole in the coolant jacket under port no. 4 on the 531 is a hangover from that heads use in the European market B230ET.
There was an extra sensor (a thermal time switch) in that hole for the Motronic ECU. It uses a M14x1.5mm thread. Or none at all on the marine engine I guess to save money!

Click to expand...

This was a marine 531 head and the coolant passage was smooth bore which I had to tap.

Cool. The marine head would have had coolant flowing into the intake manifold, I believe....so that's why it wasn't threaded...it wasn't capped off in marine applications.

I think mine had the right size hole for the ECU temp sensor, but the sensor for the gauge was different. it was an adapter essentially, with a 1/8npt sensor screwed into the adapter. I had another super fun time hogging out the adapter so I could thread my new temp sensor in further. The marine sensor was extra long. The sensor that came with my new VDO water temp gauge was not extra long.







Looks like you're all set.