Head Buttons?

Head Buttons?
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Head Buttons? - 3/16/2006 2:49:01 AM

AllanU

Posts: 27
Joined: 7/19/2002
From: St.Albert, AB, CANADA
Status: offline

Hi Bob

I have been reading in this forum and on the Jett Web site, and I would like to better understand some of the things being discussed about head buttons and nelson plugs.

I am familiar with motors that have conventional glow plugs and I am looking to buy a Jett SS40 with a red-jett carb for sport racing. I am reading about button heads ,button type head chambers and nelson plugs but do not know what they are all about. How or what is the difference from say an OS motor and is this something I need to specify when I order the motor.

Al

< Message edited by umdadum -- 3/16/2006 2:50:14 AM >

Post #: 1

RE: Head Buttons? - 3/16/2006 4:53:14 PM

bob27s

Posts: 4637
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline

quote:

ORIGINAL: umdadum

Hi Bob

I have been reading in this forum and on the Jett Web site, and I would like to better understand some of the things being discussed about head buttons and nelson plugs.

I am familiar with motors that have conventional glow plugs and I am looking to buy a Jett SS40 with a red-jett carb for sport racing. I am reading about button heads ,button type head chambers and nelson plugs but do not know what they are all about. How or what is the difference from say an OS motor and is this something I need to specify when I order the motor.

Al

Hi Al,

Thanks for writing.

The Nelson plug being discussed is a differnt thread size than a conventional glow plug. Most engines, including the vast majority of the Jett engines, use a 1/4-32 thread glow plug. Your standard K&B, Fox, OS, et all ...... plugs are this size.

The Nelson plug is a 5/16" thread size. The hex size is larger as well (you can not use most glow driver devices). The nelson plugs are designed for racing applications, which includes a superior plug seal, and usually a heavy-duty element. The nelson plug also does not use a crush-gasket - relies on a tapered seal.

The head button "design", by enlarge, is the same for the SJ-40, the SS-40 and QJ-40 ....... the only difference is the plug type you select - the head will be drilled and tapped to accomodate that plug.

The QJ-40 and QM-40 engines always are set up for the Nelson plugs. The SS-40 is usually provided set up for a standard glow plug. However, we do offer the option on the SS-40 for a nelson-type head button. Some of the club racing rules permit this, and some folks desire using the Nelson plug.

The Nelson plug will not afford a good idle or transition. If the engine will be run wide open most of the time (racing setup) then this is not an issue. If you are using the engine as a sport engine, and wish for it to idle and transition well, you will want to use a conventional sport plug.

From my experience, the SS-40 runs exactly the same with either a Nelson head/plug, or a standard head and a K&B HP or K&B 1-L plug installed. The difference is that the K&B 1-L plug will usually have a shorter life span if you are running the engine very hard. If you are using a bubble-jett fuel tank, where you have low risk of the engine going lean at the end of a run, there is practically no difference at all in plug life.

My suggestion here, when in doubt, and if you are just starting out in some club racing, order the standard head.

I hope this is helpful

Bob Brassell

_____________________________

Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host

(in reply to AllanU)

Post #: 2

RE: Head Buttons? - 3/22/2006 2:26:13 AM

AllanU

Posts: 27
Joined: 7/19/2002
From: St.Albert, AB, CANADA
Status: offline

Thanks Bob.

I think I understand the difference in the plugs but what does this mean "Replaceable button-type head chamber for lower maintenance". It is written in the SJ-40 features on the Jett Website.

Al

(in reply to bob27s)

Post #: 3

RE: Head Buttons? - 3/22/2006 3:22:41 PM

bob27s

Posts: 4637
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline

The head button (combustion chamber) and the finned external head are two pieces. Not a single piece like with OS, Webra, etc.

We refer to the finned section as the head clamp.

you can see what I mean here

Item #15 is the head button

Item #16 is the head clamp

The head button can be replaced by itself if perhaps you strip or cross thread the glow plug threads. It is an inexpensive part.

It is a fitted part, so in some cases folks will send the engine back in for replacement

Usually if you were rebuilding an engine like this, you would purchase a piston/liner/button combination as a drop-in set. That way all of the parts are properly fitted right off the bat.

Bob

_____________________________

Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host

(in reply to AllanU)

Post #: 4

RE: Head Buttons? - 3/23/2006 2:46:23 AM

AllanU

Posts: 27
Joined: 7/19/2002
From: St.Albert, AB, CANADA
Status: offline

Awsome. I now get it.

Thanks a lot

Al

(in reply to bob27s)

Post #: 5

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