D4 7U Precup

Note the change in the pre-com chamber retainer. This allowed using the standard pre-com in the larger hole head but with the retainer removed you have the correct diameter for the one piece glow plug chamber.

Also recently verified that the earlier heads can be bored out for the larger pre-coms

Thanks for the info. Previous reply failed to post.

I did not know there was a precup retainer. Never worked on anything above 17K. Now I can take another look at a parts machine with a replacement head. At first I thought it had a 1H head but after measuring the top of one of its precups concluded it was a 5F. Hope I was wrong. Couldn't find a casting number and concluded it was behind the water pump.

There was a machine shop in Watsonville or Freedom whose owner was reputed to be an expert when it came to boring heads for the larger precups. Its been a few years and I do not remember his name. He was young enough to still be in business. From what I remember he worked on a lot of coastal crawlers.

I have a friend who has turned dozens of 1S-1970 precups to fit the 5F heads. The oring groove gets similar treatment. He resorted to this to decrease the cost of conversion to electric start and has never had a failure. I would have thought the precup would get a little thin between the glow plug and the oring but maybe all the pressure and force is down below and the top of the chamber does no more than serve to center and seal the precup in the head.

Thanks for the help.

All of them have a retainer (Cat part name)....probably not the best name as they don't retain the pre-com but do retain the water😉

Seems like you are referring to Moreno's Machine Shop. Les Moreno (spelling might be off) was the go to guy for all the old iron in the valley but sadly, Les has been retired for a long time.....not even sure he is still alive....and the kids and clan have taken over the business and it is not the same anymore.