Does anyone know what the head/combustion chamber volume of a stock 75/76 DT400 is? I am looking to possibly replace my 76 DT400 head (500-11111-00-00) with a 75 MX / 76 YZ 400 head (510-11111-01-00). The MX/YZ400 head has an extra spark plug hole that I would like to use for a manual compression release for starting. I know the DT400 has an automatic compression release connected to the starter, but I have seen pictures on this forum of what some people's legs looked like when the engine kicks back. I want to avoid that kind of damage to my leg.

I notice the two heads have very different combustion chamber shapes, but in eyeballing them in pictures on the internet they don't look to have a huge volume difference. As I read somewhere, one good measurement is worth a thousand expert opinions! So, does anyone know what the head/combustion chamber volume of a stock 75/76 DT400 is?

In my experience, ignition timing is the major reason for kick back.

With the prevalence of cheap heads on ebay, if you wanted to add an extra compression release I'd consider buying a DT400 head and having a machine shop install one of the Harley or chain saw compression releases right into the head... thread is only 10mm and could probably be done without too much trouble.

Agree with you about the cheap inexpensive heads on ebay. The 76 DT400 head even has a boss cast in on the left hand side for the second plug. I even saw two on ebay that were modified with the second plug hole. They were in bad condition for one reason or another. Someone on this forum posted a picture of a DT400 head with the second plug hole facing forward or aft, I couldn't tell from the picture. I don't have access to a machine shop to do the mod.

I have never used one of the Harley or chain saw type compression releases. How do they work?

44 years ago I installed a manual type compression release (CR) on a 350 alpina and it still works great. I have the coordination down of holding the CR open, kicking through with one motion but 1/2 to 3/4 through the kick, closing the CR. No clutch slip (on the Bultaco) and no kick back because of all the momentum of the engine internals.

I guess I might have to look in the yellow pages for a machine shop.

You're definitely skilled using the old-fashioned compression release for starting!

The Harley or chain saw type have a button you push down every time you start it. They close automatically. Really easy to use. I think they are 10mm x 1.0 thread. Search ebay for "chain saw compression release" and you'll find hundreds of them. Many people here have used them. They work great.

A NAPA store with auto machine shop or other automotive place would likely have the "machining" capability... just need to drill and tap a hole... maybe file down or grind a fin a little.

I did machine and add a second 14mm threaded hole in the head of my DT400C. To me the casting looked like there was intentions for a second hole so I added one. I wanted a second hole for a manual compression release, just like I had on every bike I had (with second hole) in my early riding days. Mostly for braking especially down hill with the Dt400 "nature of the beast" surging. Wish I still had pictures. There is or was pictures over on the other enduro site. I do still have the fixture I made to clamp the head on on my milling machine.

A follow up to answer my original question:
Machine ________________________Part Number_________Combustion Chamber Volume in CC_____Weight of head in pounds
75 MX400, 76 IT400, 76 YZ400___510-11111-01-00______42.0_____________________________________4.1
75/76 DT400____________________500-11111-00-00_______54.7_____________________________________5.0
75/76 DT400____________________Webco #1310__________49.7_____________________________________6.7

I had to use underscores to get the columns to line up in preview mode.

Stock head is 6.4:1 compression ratio, Webco is 7.04:1, MX/YZ would be 8.33:1 if it was put on a DT400.

One thing to remember is for some reason the Japanese decided to measure compression ratio from "exhaust port closing"... not "bottom dead center" like the rest of the world did (and does).

So a Yamaha MX bike with say 7:1 compression would be more like 12.5:1 or more if calculated the normal way from BDC... these are not "low compression" engines! Don't let the misleadingly low numbers fool you...

I have a stock 1975 DT400 and a modified 1975 DT400 with a 1975 MX400B top end on it and a 1976 IT400C 38 mm Mikuni on it. both bikes are nowhere as bad on kickback that my 1970 RT1M and 1971 RT1B are. the RT1s can be downright brutal on the kickback and they are known to break bones and or parts.

The DT400 has a much larger chamber in the head than the MX400 has. the MX400B and YZ400 have small chambers compared to the DT400. the IT400C is somewhere in the middle of the DT400 and MX400 as it has a milder tune than the MX400 and YZ400.

And for a 74/75 DT175:
Stock head 443-11111-00 Combustion chamber volume 25.1 CC, weight 2.9 pounds
Webco head #268 Combustion chamber volume 19.3 CC, weight 4.3 pounds

I give the weight for those on a diet, and it gives some idea of the heat dissipation ability.