CMB 91HR head screws??

[CConrad]

Hello,

i need a little help:

I bought two new CMB 91HR engines and as ever there are steel headscrews mounted :angry: . Now as usually i have to change these to stainless steel screws, but surprisingly these are not metric! :huh: Could anyone tell me please the exact measurement of these screws??

Thank you in advance

Curt

 

[Ken Retallick]

I think they are 8/32 Curt!

Seems CMB for what ever reason has gone back to imperial threads for many bolts.

i also think there are something like 3 different sized bolts used on that engine alone :huh:

 

[CConrad]

I think they are 8/32 Curt!

Seems CMB for what ever reason has gone back to imperial threads for many bolts.

i also think there are something like 3 different sized bolts used on that engine alone :huh:

8/32?? this is 1/4 = 6,35mm ?? These screws are something about 3,6mm ?? Could be 9/64, but i don't know, if there are such screws existing??

You are right they use metric 3mm, metric 4mm, the water intakes are metric 5mm and the bolts for the case and the head are inch.

 

[Ken Retallick]

Curt, 8/32 is number 8 wire gauge 32 threads per inch.

 

[Bob Finn]

The 91HR head bolts are the same thread as the 67 and 80. They are 6-32 x 3/4" long.

 

[HendricX]

[QUOTE='Bob Finn]They are 6-32 x 3/4" long.
[/QUOTE]As Curt (and I) are not used to the Imperial sizes ........ wire gauge and so are mysterious.

But I found a good explanation here : http://www.gizmology.net/nutsbolts.htm

So 6-32 means size 6 = 0,1380Inch(3,5179mm) with 32 threads and is not the same as 6/32"or 3/16" (4,7625mm).

OK ... now it's clear :lol:

 

[Ken Retallick]

Oops!! your correct 6-32. Yeah the imperial sizes can be confusing!

 

[CConrad]

Oops!! your correct 6-32. Yeah the imperial sizes can be confusing!

ok, the correct size is 6-32 x 3/4"

but here in Germany i can't buy these screws in ss! :angry:

Is there anybody of you willing to send me a set of 30 stainless allen screws this size?

Paypal + 3% would be fine. Give me a PM please!

Thank you all for your help

Curt

 

[tnrcboatracer]

I probably have one or 2 (hundred) in hte garage. Be glad to. PM me if you still need them.

 

[Charles Perdue]

Stainless steel screws are not a very good idea for head bolts as they stetch and do not hold their torque. They will cause a mysterious power loss as the head will leak, but only while running under a load. Just my opinion.

 

[CConrad]

Stainless steel screws are not a very good idea for head bolts as they stetch and do not hold their torque. They will cause a mysterious power loss as the head will leak, but only while running under a load. Just my opinion.

since more than 20 years i always use stainless head bolts on every marine engine i had and never had a leaking head or power loss. Here in Germany most stainless screws are available in two qualities, A2 the standart quality and A4 is the extreme hard one. Most of the small allan screws ( 1-5mm) are generally A4.

 

[Piston1]

Try Tower Hobbies. http://www2.towerhobbies.com/cgi-bin/wti00...=LXD601&P=7

 

[Charles Perdue]

Stainless steel screws are not a very good idea for head bolts as they stetch and do not hold their torque. They will cause a mysterious power loss as the head will leak, but only while running under a load. Just my opinion.

since more than 20 years i always use stainless head bolts on every marine engine i had and never had a leaking head or power loss. Here in Germany most stainless screws are available in two qualities, A2 the standart quality and A4 is the extreme hard one. Most of the small allan screws ( 1-5mm) are generally A4.

Torque was the wrong word. Tension is the correct word. Too early in the morning..........Everyone to their own.

 

[Chris Wood]

Stainless steel screws are not a very good idea for head bolts as they stetch and do not hold their torque. They will cause a mysterious power loss as the head will leak, but only while running under a load. Just my opinion.

since more than 20 years i always use stainless head bolts on every marine engine i had and never had a leaking head or power loss. Here in Germany most stainless screws are available in two qualities, A2 the standart quality and A4 is the extreme hard one. Most of the small allan screws ( 1-5mm) are generally A4.

Torque was the wrong word. Tension is the correct word. Too early in the morning..........Everyone to their own.

I would have to agree with charlie.. i would NEVER use s.s screws on the head of a motor.. regular black allen heads screws are equil to or greater than a gr-8 bolt..

chris

 

[Bob Finn]

For years, Tidewater sold SS head bolts to replace the ones on the Picco motors. We used them very successfully. Also ran them in the old CMB 90's. Sure saves the rusted up bolts in the water jackets. I agree in theory that SS is not as strong as the std bolts, but !!!!!!!????????

 

[tnrcboatracer]

I use ANSI spec stainless screws that are rated at 100ksi. Standard GR8 SHCS are typically 180ksi, so yes, they are harder, but they also rust.

 

[Charles Perdue]

Years ago I was running an engine on the dyno, using shiney SS head bolts, and during a hard dyno pull a halo of smoke developed around the engine, almost like a smoke ring. Stopped the pull still letting the engine run and the smoke went away. Retightened the head bolts using more torque. Made another pull on the dyno and no smoke, but the engine did not run as well. Overtightening the head bolts to stop the head from leaking warped the sleeve. The head WAS lapped to the sleeve. Just my experience. <_< <_< <_<