Changing Head on OS .91 VR-DF
 

I have just received my used Top Gun F-15 and even though it is setup for a Byron pusher fan, the engine has the small head. I have a new large heatsink Byron head for an OS .91 and want to change it over. Is there anything I should look out for or be careful of when pulling the old small head off the engine and swapping to the new heatsink head? Can I re-use the head bolts?

RE: Changing Head on OS .91 VR-DF
 

Since you're already taking the head off, I HIGHLY recommend getting a set of the hardened head bolts from JMP. These are a cheap and easy form of insurance against lean runs--especially with the Byron fan setup.

RE: Changing Head on OS .91 VR-DF
 

Either way you go, With new bolts or with out, Be sure and "Torque" the bolts like you would on a car. That is tighten them evenly then go back and Retorque them one at a time ACROSS from each other. Alternating from one side to the other until they are all torqued the same. I have found if you HOLD the torque and count to ten when you do the final tightening on EACH bolt they Will not come loose.[8D] ( I got this hint from an old Boat racer) ;)

RE: Changing Head on OS .91 VR-DF
 

I'm not sure I understand how different head bolts will stop a lean run. Can you please explain this?

RE: Changing Head on OS .91 VR-DF
 

Understood. Thanks Terry. I can't wait to get this baby in the air! :)

RE: Changing Head on OS .91 VR-DF
 

The factory head bolts are rather soft which renders them not as dimensionally stable as the hardened ones. Soft head bolts are prone to stretch as they are torqued. Heat cycles from running the engine, combined with the less dimensionally stable stetched bolts, contribute to the head bolts loosening. It was my practice and was told from several engine guys to NEVER LocTite the head bolts due to having false torque values. (You think the bolts are tight but aren't, contributing to a leaking head gasket. This, in turn, causes lean runs which destroys your engine.)

JMP's head bolts are harder and are less prone to bolt stretch when tightened. Also, being more dimensionally stable, the bolts do not expand and contract as much during heat cycles, which would contibute to them loosening. Another plus is that the hardened head bolts are not as easily rounded off with the hex key. Rounded off head bolts are a *itch to remove....[:@]

I hope this helps...

Kevin

RE: Changing Head on OS .91 VR-DF
 

Anyone know the www for JMP? I'd like to get me a set of these bolts. :)

thanks

Luis

RE: Changing Head on OS .91 VR-DF
 

One more thing -- you will need a new head shim. Don't reuse the old one if you can avoid it. K.

RE: Changing Head on OS .91 VR-DF
 

http://www.jetmodelproducts.com/

RE: Changing Head on OS .91 VR-DF
 

Thanks Ghosts...emailed them a request. Pretty cheap fix to a very expensive problem. Pilott28...whats wrong with using the old shim if its still in good condition? Just asking, I asked Tom Cook if he had extra shims for sale in the email I wrote.

Luis

3.5 X 15 MM
 

Or you could go to your local hobby shop and buy some Dubro 3.5 X 15 MM hex bolts. They come two per pack, are hardened and work well.

If they don't carry them any supply house that sells metric bolts should have them. My local hardware store carries some metric bolts but they don't stock M3.5 hex head, guess it's not a real common size.

Joe

RE: Changing Head on OS .91 VR-DF
 

Not a big deal, but the shim is brass and may not seal perfectly if re-aligned, particularly if it has seen a few removals already. New head shims are cheap. By the way, if you have trouble with idle, I have found that adding a second shim can help. Regards, Keith.

RE: Changing Head on OS .91 VR-DF
 

The new head comes with what I assume is a new shim. It is a brass ring that sits inside the line of holes that the bolts pass through but outside and around the curved section where the glowplug pokes through.

RE: Changing Head on OS .91 VR-DF
 

Sounds like new bolts are in order. Thanks for the tips. I will be doing the changeover this weekend if I get time.