Hey guys, I removed the head on my 1927 T and the head bolts measure 2 3/4" long. Those seem short- are those the numbers for low head bolts? My head itself is only about 2" on average when I measure the flattest spots around the spark plug holes.
Before I place an order for new head bolts, I wanted to ask.
3/4" of thread engagement is lots, more than enough. The basic rule is that the "minimum" thread engagement is the same as the diameter. With cast iron,probably nice to have a little more. You have 7/16" bolts
Les- so those are indeed low head bolts?
Simple; if you have a high head order high head bolts, if you have a low head order low head bolts. Just because it's a 27 does mean it couldn't have had a low head put on for a tiny bit more HP. Someone may have replaced the bolts with off the shelf length bolts. What head do you you have? 2 inches sounds like a low head, I would have to check for sure. If you are not sure post a photo or Google "mtfca; low head"
2-9/16 is the low head cylinder bolt...
2-3/4 is high head cylinder bolt length...
Do pick out the existing threads in the block, if you have a 7/16-14 bottom tap run it in...
Concern about bolt thread length in the block is confusing, but worked on 15,000,000 copies after the first 500. The thread length is NOT correct for 7/16-14 in iron...but that is the depth they had to work with as the first 500 cars were an experiment in 3/8 FINE threads and the block depth of actual thread was based on the 3/8".
I'm not sure. What I'm saying is that for the head you have they are a reasonable length. If the head you have is a low head, then I suppose they are. If you are installing a high head, then it would seem you need longer head bolts. When you have everything it is a good idea to trial fit the head WITHOUT a gasket and verify that the bolts thread ALL the way down to the head!! If they don't then you need to clean the threads using a "bottoming tap", and then recheck
26-27 head bolts were originally nickel plated, so if yours are not and originality matters, order some new stainless steel head bolts, or while your at it, order the complete engine set.
Measuring a high head bolt I get 3 5/8". That includes the head. Measuring from the bottom of a stock low head to the top of the bolt holes is 2 1/8". The same measurement on a stock high head is 2 5/8".
Correct 1927 head bolts are nickel plated with domed heads.
(Message edited by steve_jelf_parkerfield_ks on February 20, 2017)
Thanks to everyone, here are a few pics just to show you what I am working with.
After at least rinsing the threads in the engine with an awl and blowing out loose gunk, you can test mount your head without any gasket and see if you can tighten all 15 screws all the way down. Then you know they aren't too long.
Originally they were kind of short due to an early design mistake as George wrote above. Usually it'll work just fine if you don't tighten more than about 50 lb-ft.
I wanted to use all the available thread, so I used studs and nuts like a Model A Ford with my milled aluminum Prus head, and it works fine - just more work whenever I want to pull the head, I have to pull the studs too, since the firewall stops the head from being lifted straight up.. (Ouch, all the compromises in building a car..)
That's a low head. Bolts are measured in terms of the shank length. The head of the bolt doesn't count.
(Message edited by royce on February 20, 2017)
It's not only a low head, it looks like ¼" of it has been removed.
I have to learn that coming from memory isn't always as sharp as it use to be...and I should double check.
I stated a number above that is not right
Further, as Royce points out, maybe some do not know that bolts are never measured Over-All but rather as Royce points out, by shank length
So, here is a sketch with the proper info on it...
Thanks guys, I ordered new low head bolts. I'll test fit the head without the gasket and make sure the depthing works, Today I chased all the threads with a tap and blew them out clean.
I appreciate all the support!
Great drawing, George, I saved it to my computer with my other engine related pictures.