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Message |
    
doug11k
Member
Username: doug11k
Post Number: 4
Registered: 12-2003
| | Posted on Tuesday, July 19, 2005 - 08:58 am: | 
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Acquired another Dunn single recently. Found some interesting differences between the two engines(Andrew previously posted Dunn1 photos for me):Engines are physically the same size,both are 4" stroke, Dunn1 3.725" bore with piston rings hand fitted & varying in width from .415" to .392"; Dunn2 3.625" bore with .250" wide rings. Dunn1 con rod is bronze I-beam cross section, Dunn2 diamond cross section. Name tags are identical. Flywheels both 11 3/4" dia. but Dunn2 has four holes & is 1 3/8" instead of 1 1/8" thick. Water pumps and timer pieces near identical. Dunn1 base, cyl. & water jacket are a single iron casting; Dunn2 has a removeable cast aluminum water jacket outer sleeve... 4 rd. head screws secure jacket to thin flange on base /cyl. casting. Dunn2 2-bolt flanged intake valve cage was missing but is different than Dunn1's threaded cage. I've made a new cage using a raw casting of an Aermotor 8-cycle exhaust cage that I had on hand.... not exact but fairly close but intake port had to be 1" npt instead of original 3/4" npt. Also intake & exhaust ports point in same direction which will necessitate a street elbow on exhaust pipe to clear carb. The water jacket on Dunn2 appears to be either a factory repair or a method to salvage damaged or flawed castings. Engine was definitely fresh water run. |
thomas
Senior Member
Username: thomas
Post Number: 277
Registered: 07-2002
| | Posted on Friday, July 22, 2005 - 08:17 pm: |
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Nice find!! I believe that in an old thread on the Dunn someone (Richard?) posted the advert saying that Dunn had aluminum and bronze alloy
jackets and flywheel on the racing engines and also there was mention of an aluminum crankcase
so I assume yours was from that vintage. The casting you adapted should work ok. Were there already threaded holes in the head to mount the (missing original) intake valve casting? |
doug11k
Member
Username: doug11k
Post Number: 9
Registered: 12-2003
| | Posted on Saturday, July 23, 2005 - 06:15 am: |
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Yes, there were 3/8"-16 tapped holes & the actual opening for cage spigot in head is 1 1/2" dia. with no threads. I think I see flanged cages in some of the photos in Dunn section of multi cylinder engines.. My Dunn1 has the threaded style cage. Can't find 3/8" fillister head cap screws so will make a couple to replace the socket heads shown....Aermotor casting a little wide to make spacing between holes ... no room for hex heads. The 'ears' for the tapped holes on head casting appeared to me to be welded on but after bead blasting they are part of head casting. It looks like they modified the original head casting pattern at the foundry by adding those 'ears'.  |
doug11k
Member
Username: doug11k
Post Number: 10
Registered: 12-2003
| | Posted on Saturday, July 23, 2005 - 06:20 am: |
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Thomas,
I meant to ask you in previous post if the bore on your twin is a standard size. My Dunn1 was 3.725" & Dunn2 is 3.690". The ads say 3 3/4". Did all Dunns vary like that? |
thomas
Senior Member
Username: thomas
Post Number: 279
Registered: 07-2002
| | Posted on Saturday, July 23, 2005 - 03:44 pm: |
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Doug,
It looks like everything I recorded says my Dunn bore is 3-3/4" But that's going by the measurements I mailed Paul Weaver to get new rings. I must not have saved a copy of the sent letter but his 2 reply letters says I have a 3-3/4" bore. The old rings measure a little oval
but come out at 3-3/4". Just now I checked the documents section of my computer in case I may have saved a Word document but did not find one. |
thomas
Senior Member
Username: thomas
Post Number: 280
Registered: 07-2002
| | Posted on Saturday, July 23, 2005 - 03:56 pm: |
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You are correct Doug. "Mike",who posted a great bronze jacket three cylinder about March'05, sent some pictures that clearly show the "ear attached" intake cages. I printed and saved several views of his tripple but I missed the unusual cages until you mentioned yours. Maybe
there is some connection between the non-C/I
water jackets and the "ears"? |
